Anotherpower.com Forum
Renewable Energy Questions/Discussion => Automation, Controls, Inverters, MPPT, etc => Topic started by: ChrisOlson on May 11, 2012, 09:04:09 pm
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After making more parallel connections in my battery bank (24 Rolls T12-250's) and less series connections, I've had much better luck keeping all 24 batteries balanced within a few hundreths of a volt.
Yesterday I got a wild hair and decided to take it one step further. I wired my bank up in two parallel strings with 12 batteries in each string. Then I put two series bars between the two strings, at the ends of each string, made of two 1/4" thick x 1" wide copper bars.
Bank 1 is the positive string, Bank 2 is the negative string, and testing between any of the positive posts on Bank 1 to any of the negative posts on Bank 2 gives me 24 volt nominal. I had six #2 positive cables, and six #2 negative cables, from the bank to the DC bus. I hooked those existing cables to the positive and negative strings, "tapping" the strings to the bus every two batteries.
I tested the whole setup with the inverters at 8 kW load and it worked good.
As a final touch I put in a battery bank balance meter to monitor the two halves of the bank:
[attachimg=1]
These are analog meters that I got at Radio Shack and they can be calibrated. I measured the actual voltage of each bank with a digital meter and calibrated the analog to read exactly what the digital said for both banks. These analog meters are pretty sensitive and a needle's width is .05 volts. To test this, last night after I got this all done I unhooked one battery in Bank 1 with about 1,700 watts of load on the inverters. Within 5 minutes the analog meter for Bank 1 already showed a needle's width lower than Bank 2. Putting the disconnected battery back online brought the two analog meters dead even again.
I'm going to try this for the rest of the summer and see how it works. With 24 batteries if a person ever had one go bad you'd never know it until it was too late with the way I had them wired previously. I think this new setup will show me instantly if a battery would ever develop a problem so it could be dealt with before it causes damage to other batteries.
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Chris
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How will you know when the rest of the paralleled batteries (if they are good) keep the voltages up.
I can only see it who a problem if there is a cell shorted and drawing the power down, and then only if the banks are disconnected from one another for a reasonable time period.
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What I considered when I did this was that as long as the voltage of the two banks stays perfectly the same all the time I am never going to wreck any batteries from one going bad. Say I did have a battery with a cell that goes tits up. Typically what happens is that a 12V battery with a dead cell will cause the other 5 cells in that battery to run at excessively high voltage, boiling them dry. But that battery will still operate at the same nominal voltage as the other good batteries it's in parallel with. In this parallel configuration it will still not hurt any other batteries in the string if one develops a dead cell.
Now, still considering that I have a battery with a dead cell, that battery is low on amp-hours as compared to the other 11 that it's in parallel with. That's going to cause one of the meters to read low when there's no incoming power, telling me I have a problem with a battery in that string.
On the other hand, consider two 12 volt batteries in series for 24 volt and one develops a dead cell. The internal resistance of the two batteries is different and they're going to charge at different voltages which will wreck both of them eventually - usually not discovered until it's too late and they're both shot. The good battery in that series string could've been saved if you had a monitoring system to tell you that something was out of whack. But how many people got that? I've seen lots of 6 volt batteries in series get wrecked because one went tits up and it wasn't discovered until the others were also junk. And when that happens you have no options except to replace the entire string because you will never get a new battery to match a used one when they're in series.
With parallel strings you can mix and match and replace a bad battery without affecting the others. It's done in heavy trucks and heavy equipment that use 12 volt batteries all the time. A typical semi tractor has four Group 31's in parallel. When the truck is serviced and batteries tested, if you find a bad one you just replace that one. You don't put all four in just because one tests bad.
I set my bank up the same way as that semi tractor. And I got a monitoring system on the two halves of it. As long as those meters stay even it will never wreck good batteries from being over or under charged. If they don't agree I got a problem that needs attention. That's why I did it.
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Chris
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Ok that makes sense, let me know how it all goes, I might try it later this year if all seems to go well.
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Well, like I said, I'm going to try this for the rest of the summer. My batteries are all pretty new (April 2011) so I expect no problems with any of them any time soon. But with time, with 24 batteries, you have 24x more chances of one going tits up than you do with just a single battery. It WILL happen. It's a matter of when because my batteries get worked hard every single day. And when that happens I want to know about it immediately, without being taken by surprise, so I can evaluate the situation and decide what to do about it.
Removing the one battery from Bank 1, and seeing a difference in only five minutes under a light load, showed me that this monitoring system will work. At 8 kW load on the inverters (360 amps) I had .55 volts drop from the bank to the inverter studs. That showed me this method of wiring the bank up will deliver the power OK.
Admittedly, it's a rather unusual way to configure an RE battery bank. But nothing much that I have ever done was done "by the book". Most of these wild hair ideas I've tried comes from observing things in other applications that I see works well, putting two and two and two together and going, "since this is a time-proven method for application 'x', why won't it work for application 'y'?"
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Chris
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Chris, I'm still attempting to understand how you are doing this new configuration. You did another post not too long ago with a similar configuration, I think.
You are paralleling 6 volt batteries (positive to positive) first, then using them in series to get 24 volts.
The first attachment illustrates the new configuration. The second is the old configuration I have been using.
Nice work, BTW.
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(But that battery will still operate at the same nominal voltage as the other good batteries it's in parallel with)
No it will not, as you also stated the other batteries in that string will be at a higher voltage.
I like your approach, but there is and never will be any alternative to diligent monitoring.
Allan
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You are paralleling 6 volt batteries (positive to positive) first, then using them in series to get 24 volts.
My batteries are 12 volt. Rolls T12-250's.
Unfortunately, I cannot open your image files here.
What I did was parallel 24 12V batteries in two strings of 12 batteries in each string. Then connect the two strings in series. So, basically, consider it to be two huge 12 volt 2,400 amp-hour batteries with a series connection between them.
Previously I had them arranged in "groups" of four with two batteries in parallel on the positive side and two in parallel on the negative side with a series connection. Then that "group" of four had positive and negative wires going to the bus.
Before that I had them in series pairs with the series pairs parallel'd. That configuration was horrible and I had some that were up to .25 volts different than others when being charged at absorb voltage.
No two batteries are ever identical. But I've found that using more parallel connections when you have a lot of batteries tends to minimize the slight differences between them.
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Chris
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No it will not, as you also stated the other batteries in that string will be at a higher voltage.
No they won't. Batteries in parallel will always be at the same voltage no matter what. A six cell 12 volt battery can have one dead cell and that will cause the other 5 cells in it to be at excessively high voltage and eventually wreck them from constant overcharge. But if that battery is connected parallel to another one the voltage of both batteries will be the same when tested at the posts.
If one battery in a parallel string develops an internal short, or goes open, it will pull the other batteries in the string down, or in the event of an open will not contribute anything to the string. But they will still all be the same voltage when tested at the posts until you disconnect them to find which one has zero volts.
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Chris
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Could you please scratch out on a piece of paper what you have achieved and post it, I for one can not understand what you have done/achieved.
Not to mention trying to understand why anybody would have 24 x 12volt batteries, do you buy them as a job lot?
Oh and I stand by my statement until it is clear what you have done, at the moment I suspect you have over engineered a battery bank. None of your word descriptions make sense to me, then maybe I am going senile.
You write about fractions of a volt but choose analogue meters, I am in awe of your eye sight. "These analog meters are pretty sensitive and a needle's width is .05 volt".
Allan more confused than ever.
OK since posting I have thunk about this, let me put it simply, if I had 4 x 12 Volt batteries connected series/parallel for 24 Volt, then positive to negative will always be the same voltage, BUT positive OR negative to mid point may be different by a few hundredths of a volt (your comment) or even more. Is this what you are trying to convey?
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Allan more confused than ever...... it's that jungle juice you have up there. :o
I am going to be playing with 40 12v batteries next week, and will also be making parallel 12v strings of 10 batteries each. These 4 12v banks will then be seriesed for 48v. Same kind of thing as Chris has I'm afraid.
...............oztules
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So apart from standing out in the roaring forties which makes iugkyjtfrxtfghj what are you saying.
Be that as it may it begs the question WHY????????????????????????
Persons who have attained the level of 'listen to" still have to explain what and why they are doing, remembering that many readers are just new comers to all of this renewable energy stuff.
Do not say I am so great that you should believe me, document why!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
Confused Allan
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Be that as it may it begs the question WHY????????????????????????
I'm not certain that I've "got" it, but let me see - I'm sure Chris will correct me if I'm way off beam here!
He has a substantial number of batteries. ok, that's a given.
He has a 24V system. That's historical fact.
He has established that connecting cells in parallel at the lowest level practical for the cells/batteries in use helps.
Sooo... break the "substantial" number of batteries into two equal groups.
Call them bank1 and bank2.
Connect all bank1 cells in parallel.
Connect all bank2 cells in parallel.
He already had a "battery bus" positive and negative rail.
He's now introduced a third rail. Lets call it "midpoint".
Bank1 connects from "battery bus (negative)" to "midpoint".
Bank2 connects from "midpoint" to "battery bus (positive)".
We now still have the full 24V as before, but we also have a nice central reference.
Adding two identical voltmeters allows measuring bank1 and comparing to bank2.
Since the two banks are in series, any current into or out of one must be identical to the current into our out of the other, and since the two banks are identical numbers of "identical" and same age batteries, they should stay at an equal SoC.
If one cell in one battery in one bank should fail - open or short - the capacity of that entire bank will be reduced - not by a lot, but by a measurable amount. And given the constant and substantial load the banks get in a day, should result in a detectable difference fairly quickly (if anyone is there to watch).
I was thinking that the meters don't really give enough precision, and that I'd like to see a centre-zero meter and a simple opamp driver to show imbalance, but thats a lot of extra complexity compared to what chris has built - and heck, at the end of the season we should have a good idea if it's working as he hopes!
So, Chris - how far wide of the mark am I?
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So, Chris - how far wide of the mark am I?
You're right on. I'd like to add that I don't want anybody to think this is the cat's meow. I'm trying it to see how it works long term. I think it will, but with 24 batteries you never know.
Those batteries cost us 400 bucks apiece. I'm trying this because I want to take care of them, and monitor them to keep them healthy so they last us 10 years. I've been very happy with them because they pack one hell of a lot of punch.
But the way I had them wired previous to this, even though it worked well, made me nervous. I had no way to tell if one would develop a problem without unhooking them and testing every single one. Plus it was a royal PITA to service them because I had wires strung all over the bank. This configuration got rid of a bunch of wire on the bank and it's very simple and straight-forward - and it makes it easy to service them because I don't have to move any batteries out of the cases to do it.
As far as the analog meters, they are very sensitive meters and they have very fine thin needles that makes it relatively easy to see a change of only .05 volts. I selected them specifically because they can be calibrated. With digital meters, you can buy two identical meters and they might read .05 volts difference testing the same battery. I've never seen two digital meters that are exactly the same, and while I suppose it's possible to calibrate digital meters I don't know how to do it. These analog meters can be checked and corrected for accuracy quite easily during bank service, and they were relatively inexpensive (15 bucks apiece).
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Chris
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Well, I'm happy to say that it appears to be working fine. We had a very poor power day yesterday and our bank never even got to float stage. Today it's absorbing at present, at about 180-200 amps charge rate from solar and wind. Both sides of the bank are dead on 14.7 volts with almost 6 kW input to the bus from the solar panels and wind turbines.
So I think it's going to be OK. In the mean time my wife and I got some fishing to get done 8)
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Chris
Edit, I made a little video demonstrating what the meters do with a little 2 amp battery maintainer hooked up to one bank. Due to camera angle the meters look different initially (you have to view them direct on), but they're not. After I made the video I saw that the needles were casting shadows too, which makes it harder to see in the video. In real life, looking at the meters, it's instantly apparent that the bank is out of balance.
But I think you get the idea when you see what hooking up a little battery maintainer to one side of the bank does to the bank balance.
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Chris/Ross I know what it is about, every power station I have worked in had a similar single center zero meter that showed any earth leakage between positive and negative buzz. And I do know how important it is because I blacked out Port Hedland in western Australia once when my largest engine 12.5 MW (diesel power station) shut down because I caused a 50 volt DC earth leakage on another unit. The large unit had a known earth fault, my introduced fault caused the fuse to blow on that (largest) unit faster than I could react despite seeing the magic smoke.
In those days although Port Hedland was a stand alone system we had to notify any outages to the powers to be in Perth, the states capital. It is a long story but Country Undertakings 2 IC (the mob I worked for) rang once he was awake, and his opening comment was "and what were you doing this time Allan". I can explain but I don't want to hijack Chris's thread.
What I fail to understand is the number or 12 volt batteries Chris has, to me it's a bit like buying trouble.
I had eight banks of 2 volt 650 AH cells as a 24 volt system because they were the right price, and in the 9 years they were mine cells did fail, but 0.05 volt difference?
Chris is a cleaver person, when he posts I sometimes read, but in the absence of a diagram I don't want to know, how the hell are other members of this board meant to understand? Go on put your hand up people, who understands what Chris has done.
grumpy Allan
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With digital meters, you can buy two identical meters and they might read .05 volts difference testing the same battery. I've never seen two digital meters that are exactly the same, and while I suppose it's possible to calibrate digital meters I don't know how to do it. These analog meters can be checked and corrected for accuracy quite easily during bank service, and they were relatively inexpensive (15 bucks apiece).
Chris, if it were me (it's not me!), I'd definately use either a centre-zero meter and an amplifier, or I'd use two digital meters. Digital meters are generally very easily calibrated. Even without that, a meter that reads to 999 or 9999 is really cheap and easy to get nowdays. I'd use two of them and two 10 or 20 turn trimpots. I'd call them "battery PERCENTAGE meters". rather than voltmeters.
Connect the two up to the same reference source, adjust the reference to (say) 15.000 volts, and then tweek the trimmers until both meters read 999 or 9999. Ie, near enough to 100%.
The reason for digital is that it completely eliminates the guesswork, and of course the extra precision.
Something like http://tinyurl.com/d2ox2p4 (ebay item 250894831268) looks about perfect. $5.20 each, free shipping, digital panel meter, 7.5 to 20 volts display. Powered directly off the input so no mucking about required.
AND it has a trimpot on the back that I reckon will be used to 'calibrate' it.
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"So apart from standing out in the roaring forties which makes iugkyjtfrxtfghj what are you saying.
Be that as it may it begs the question WHY????????????????????????
Persons who have attained the level of 'listen to" still have to explain what and why they are doing, remembering that many readers are just new comers to all of this renewable energy stuff.
Do not say I am so great that you should believe me, document why!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! "
The explain part goes like this:
1. 40 batteries ... because over here we work with what we have. his bloke has gotten 40 batteries from somewhere, and wants to hook em up.... so we work with that 40 batts.
2. Knowing 1., we then need to wire them up as best we can. In this case I think that it would be best if every cell had equal access to the load and the supply.
3. If we parallel things, all those things in parallel will see the same emf.
4. If we series things, all those things in series will see the same current ... in or out.
5. A cell plate pair is the most basic of the redox reaction devices in the unit. They are arranged into groups within physical containers called batteries. But ......the cells in each battery can share more surface area for more current carrying capability.
To this end, they arrange more plates in parallel within a cell structure. Each plate adds to the surface area of the plate beside it, and we can go on doing this until we build a very big cell.... which when we add more cells to it, becomes a very big battery.
Note, all additional plates added into a cell are paralleled. They do not series string the plates from each cell and join them at the + and- posts.
They could if they chose to, but build convenience, and equal emf distribution across the entire surface area of the lead is best served by paralleling them into a single cell, and then series the cells into a battery
6. If it's good practice to build batteries where at even the plate level we parallel the outputs, then we can follow this outside of the physical battery, into our battery bank.... all the same rules apply
.... and they are:
A. All cells supposed to be at the same emf, should be paralleled, so all cells at that emf behave as a single cell, just as in a battery cell made from numbers of plates.....They parallel the plates to make a bigger cell .... in our upcoming example there will be about 100 plates paralleled to make each effective cell... but the nearest we can get physically to them is at the battery level (every 6 series cells for 12v),,, so the 12v battery will be made from 6 x 100 plate cells. That makes for a very big 12v battery. Yes there will be variances within the battery /s that we can't do anything about, but hopefully they are matched well enough to keep the differences down (more hope than belief)... at least the terminal voltage will be the same.
B. Once A is achieved, simply string the new super battery in series to make your bank the EMF you want.
So that is how and why I chose to do this. If battery manufacture could produce a better battery by making the individual plates isolated, (messsy maybe) then the best manufacturers would do it.... but they dont.
On a more theoretical rather than follow the battery makers lead, then I would point to Chris's remarks, where he has tested extensively for cell differentiation on a regular basis, and this shows that series strings of batteries parallel with other series strings go out of kilter.
It is thought that equalising is the answer to this, and that may be true with wet cells.... but with gell cells (we will be using these apparently next week) it is less likely that equalisation is particularly safe.
Boiling wet cells to use up allow the current to make H2 and O2 in the fully charged cells, and then allowing any non-charged series cells to catch up by using series current to reverse the redox reaction, is only good if you can replace the water. If you overdo it with gell types, it can be sad, as the gas will escape past the valves, and not be available for re-assimilation into the matrix..... they dry out.
So the best we can hope for is to fix all same level EMF batteries to the SAME EMF, and hope this mitigates to some extent the variability of unbalanced series strings. You can see that particularly for gels, it is necessary to not let some cells get too far ahead of the rest.
The unbalance within the battery may make this less than satisfactory, but it's the best we can get without using independent 2v cells where we can balance the lot.
Does that help at all?
And some of us southerners will wonder at the translation of this iugkyjtfrxtfghj ..... I figure it is some sort of jungle speak after the bar is closed for the night.
....................oztules
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Was thinking about my post, and the digital displays - and went looking for something with a little better accuracy and precision.
Ebay 160655632497 (http://tinyurl.com/6tkboln) is a little more expensive - but at around $24 delivered it's still a very tempting device. Salient points:
5 digits display, so you could show 0-20V with 1 millivolt resolution.
Power supply: DC5V or DC6V~15V±5%
Measuring precision: ±0.05%FS
Overall dimensions:73mm X 26mm X 22
Intteresting note is that it will read +/-, so a single SPDT pushbutton or switch would let you read from bank1/2, the sign (+ or -) would be your meter indication of which bank is which - and you have no calibration issues since it's the same meter!
Just press the button/flick the switch to see the difference.... (and one meter is cheaper than two)
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tules, say what!
now I do know I am suffering from oldsimers, but that is alright just hit the old fellow about the head some more.
BUT what no one has done is post a diagram so all members of this board can understand what Chris has done, I for one am still confused even after reading your explanation of er er well I am not sure what, but at least I have an excuse.
A new member of this board may read this and believe that using 100 X 12 Volt batteries to build a 12 system is a good way to go.
Allan in 28.5 degree C in Far North Queensland not in the Bass straight.
Oh and up here we work with what ever we can get also, albeit we can drive to the nearest town, with the only water section being the ferry to cross the Daintree river and we don't have mains power even though yours is diesel generated. ;D
Have Redox batteries made it to the Bass straight?
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"Have Redox batteries made it to the Bass straight?"
.... hmm... all batteries use redox reactions as oxidation reduction reactions.... however I think your talking about the vanadium version.
Answer is yes. King Island used them for a few "years" actually months I think. They came from Canada, and for various reasons were patently unsuccessful. I don't know what happened
here: http://www.kingislandrenewableenergy.com.au/project-information/vrbr
So they made it to 40 south, but not known as working successfully.
................oztules
pictures later
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BUT what no one has done is post a diagram so all members of this board can understand what Chris has done, I for one am still confused even after reading your explanation of er er well I am not sure what, but at least I have an excuse.
Attached is a simple diagram for reference. The diagram just shows 8 batteries and I have 24 of them. But anybody should be able to figure that out - with more batteries you just keep adding them in parallel with the two strings.
The diagram is in PDF format.
[attachimg=1]
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Chris
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Chris, if it were me (it's not me!), I'd definately use either a centre-zero meter and an amplifier, or I'd use two digital meters.
Ross, I agree that digital meters could be used as well. I've always liked analog stuff - kinda' like a speedometer in a car, I guess. I still prefer the good old needle over digital numbers on the dash. When I saw those volt meters on the rack at Radio Shack, and saw they have jeweled movements, can be calibrated, have +/- .5% accuracy full scale, and are quite sensitive, I liked them instantly. That's why I got them. :)
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Chris
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Chris, looking at your diagram, it looks to me like the batteries closest to the buss bars will take most of the load and most of the charge. That buss bar to rear battery connection lead looks as though it might have just a bit more resistance than the closer leads. Are the wire sizes front and rear sized accordingly or all the same. Thanks.
BTW, I tried this sorta configuration once. I clamped an ammeter on each battery and found a bit of difference. I guess my problem could have been connections. I don't remember for sure so I will leave it at that. At any rate, I will be watching to see how you progress.
BUT what no one has done is post a diagram so all members of this board can understand what Chris has done, I for one am still confused even after reading your explanation of er er well I am not sure what, but at least I have an excuse.
Attached is a simple diagram for reference. The diagram just shows 8 batteries and I have 24 of them. But anybody should be able to figure that out - with more batteries you just keep adding them in parallel with the two strings.
The diagram is in PDF format.
(Attachment Link)
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Chris
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Thanks for that now I understand why I was confused, you have two parallel sets of 12 volt batteries, with a common mid point connection and you draw off each end because you have so many the cable losses are a problem for you. ::)
What you need is all your 12V batteries set out in two's nose to tail (24v) and a positive and negative bus bar of suitable cross sectional area that is as long as your string of batteries and a common mid point connection. Yea I know same as what you have done, sort of.
I can see it now you have 12v batteries because you can write them off against tax, they are for your trucks etc after all, seriously if one needs a bank of batteries for renewable energy one buys 2V cells of adequate amp hour capacity. :-X
Allan
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What I fail to understand is the number or 12 volt batteries Chris has, to me it's a bit like buying trouble.
Allan, the batteries we bought have the same warranty as any of Rolls-Surrette's RE battery lineup - 7 years
We got them because they were recommended to us by the dealer we got our inverters from. The T12's are 2.4 kW @ the 20 hour rate. The S-1380 2V cells are 2.1 kW @ 20 hr rate.
The T12's are quite a bit heavier than the 2 volt cells and they're a bitch to move around. They're also more expensive. But 24 of the S-1380's was 2,100 amp-hours (1,050 ah per battery @ 20 hr rate). 24 of the T12's was 2,400 amp-hours (200 ah per battery @ 20 hr rate). We use 25-30 kWh/day in our home. 24 of the S-1380 2V cells would've been a little too small for us, with no reserve, and they would've gotten deep cycled more than the T12's. So the guy recommended the T12-250's because we need a 30 kWh (usable) bank.
The other option was to get 12 of the Rolls 12CS11P's. But those batteries weigh 375 lbs apiece. The ones we got only weigh 155 lbs apiece so they're easier to move around.
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Chris
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Thanks for the explanations. I do understand what you are doing. ;)
It is easier to monitor two battery banks versus one large one. I suppose you could add just one battery to one bank without disrupting the electrical output. That banks amp-hour output would change, but would not change the overall health of the system.
The same parallel-series system could be used for any voltage.
I'll try a pdf format next time.
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Chris I still like my idea but it works for you, what depth of discharge do you reach overnight, I know it depends on wind but on the average?
Allan
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Allan, with zero incoming power it takes 24-30 hours to cycle them down to 50%. Overnight they rarely go below 70-80% SoC.
Zero incoming power rarely happens, and neither does cycling them down to 50%. With several poor days in a row of no sun and light winds our automatic start generator comes online and recovers the bank before it gets below 60% SoC. We use our bank hard every day because we got 8 kW of inverter power, and we use the full 8 kW on a regular basis. But at the same time, I take care of the bank and have the system set up so it doesn't let them ever get too low.
We got our inverters and battery bank from a guy that has been in the RE business for close to 30 years. He showed us a chart of how many cycles the batteries can do vs how deep they get cycled before they have to be replaced. He went over all our loads, what we wanted to be able to run, and the fact that we wanted to improve our quality of life. Our setup is designed to get 10 years from the bank, according to all he said we needed.
We lived with basically junk for almost 10 years here, and there was way too many times I was up at 3:00 AM at 30 below zero trying to get a gas charger started because our bank was dead and the furnace blower quit. When we got the money to put in all this new stuff a little over a year ago, it was a big change for us. And we've been enjoying it. But taking care of those batteries so they last for their design lifetime is a top priority for me. They were a pretty sizable chunk of the total investment.
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Chris
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Chris, looking at your diagram, it looks to me like the batteries closest to the buss bars will take most of the load and most of the charge.
That's just an example diagram showing 8 batteries. I have 24 and there is a "tap" to the bus every two batteries on each parallel string.
It worked flawlessly yesterday with both good sun and wind. The bank was a bit thirsty after the previous day and we generated 44 kWh with solar and wind. The bank was floating by about 2:00 in the afternoon and it stayed perfectly dead even all day between the two strings during bulk, absorb and float.
This morning when we got up the bank was down to 25.0 volts and still dead even between the two strings.
I really think it's going to work fine. I'll know more when we get a few poor days in a row and pull it down to 60-65%, then get a good day when it bulk charges at 200+ amps for several hours.
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Chris
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Chris, looking at your diagram, it looks to me like the batteries closest to the buss bars will take most of the load and most of the charge.
That's just an example diagram showing 8 batteries. I have 24 and there is a "tap" to the bus every two batteries on each parallel string.
It worked flawlessly yesterday with both good sun and wind. The bank was a bit thirsty after the previous day and we generated 44 kWh with solar and wind. The bank was floating by about 2:00 in the afternoon and it stayed perfectly dead even all day between the two strings during bulk, absorb and float.
This morning when we got up the bank was down to 25.0 volts and still dead even between the two strings.
I really think it's going to work fine. I'll know more when we get a few poor days in a row and pull it down to 60-65%, then get a good day when it bulk charges at 200+ amps for several hours.
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Chris
Chris, what does your ammeter show going to or from each battery? Are they all equal?
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The ammeter only shows total bus input amps from the wind and solar. It doesn't show what's going to each battery. But batteries in parallel will only "draw" what they require at the particular charge voltage they're at. The voltage is the same across the entire string, even at very high charge amps. I have #4 interconnects on the parallel part, and each interconnect is only about 14" long (the width of each battery).
In a parallel string you can connect a whole bunch of different sized batteries and it still works fine. The smaller ones will take less current, and the bigger ones will take proportionately more current. For many people this is hard to understand or visualize, but it works fine.
Even if you have (in my case) 12 batteries in parallel in each string, there are slight differences in each one. As an example, let's say you have two different 12 volt batteries - one is fully charge, the other is discharged down to 50% SoC. Connect them parallel, then hook up a battery charger to charge them up. The dead one will come up to full charge and the fully charged one won't take any current.
If you have two different sized batteries in parallel - let's say one is 100 ah and the other is 200 ah - it also works fine. If they start out at full charge and you load them the bigger one provides more of the load current than the smaller one. Let's say you pull 100 ah out of this 300 ah parallel set of batteries, the bigger one will supply 66.6 ah of it and the smaller one will supply 33.3 ah. When you recharge them, the charging current is also in the same proportion.
With parallel batteries, as long as you have them matched so they require the same charging voltages for absorb/float, you can get away with just about anything. Been doing it for years.
Series is a whole 'nother ball game. Batteries in series have to be PERFECTLY matched or you will wreck them in short in order.
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Chris
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Chris, the whole parallel connection is equal thing is great if the actual voltage at each " battery " end of the conductor is equal.
I have no point to prove. I am only providing what I experienced and also understand that only goes so far. Because your batteries are all the same size, you should get an equal reading from a clamp on ammeter battery to battery, is all I'm saying, as otherwise some batteries will be exercised more than others. While my batteries were all the same size and in a configuration such as yours, I did not get an equal reading. I had problems cell to cell, battery to battery after only a few weeks. Maybe I need a picture of what you are saying vs. a drawing. Maybe that will put it into perspective.
http://www.anotherpower.com/board/index.php?topic=224.0
Some time ago, you posted your new way of battery arrangement which was the bees knees, what happened with that configuration? Were you looking for better by changing that?
The ammeter only shows total bus input amps from the wind and solar. It doesn't show what's going to each battery. But batteries in parallel will only "draw" what they require at the particular charge voltage they're at. The voltage is the same across the entire string, even at very high charge amps. I have #4 interconnects on the parallel part, and each interconnect is only about 14" long (the width of each battery).
In a parallel string you can connect a whole bunch of different sized batteries and it still works fine. The smaller ones will take less current, and the bigger ones will take proportionately more current. For many people this is hard to understand or visualize, but it works fine.
Even if you have (in my case) 12 batteries in parallel in each string, there are slight differences in each one. As an example, let's say you have two different 12 volt batteries - one is fully charge, the other is discharged down to 50% SoC. Connect them parallel, then hook up a battery charger to charge them up. The dead one will come up to full charge and the fully charged one won't take any current.
If you have two different sized batteries in parallel - let's say one is 100 ah and the other is 200 ah - it also works fine. If they start out at full charge and you load them the bigger one provides more of the load current than the smaller one. Let's say you pull 100 ah out of this 300 ah parallel set of batteries, the bigger one will supply 66.6 ah of it and the smaller one will supply 33.3 ah. When you recharge them, the charging current is also in the same proportion.
With parallel batteries, as long as you have them matched so they require the same charging voltages for absorb/float, you can get away with just about anything. Been doing it for years.
Series is a whole 'nother ball game. Batteries in series have to be PERFECTLY matched or you will wreck them in short in order.
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Chris
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Some time ago, you posted your new way of battery arrangement which was the bees knees, what happened with that configuration? Were you looking for better by changing that?
Yes, like I said, that arrangement made me nervous because I had no way to tell if a battery developed a problem. It worked fine and kept them balanced very good. But what if one suddenly developed a weak or dead cell? It required servicing and load testing once every three months to make sure every battery was good.
I want to get away from that 3 month service because it's no trivial task.
So I decided to try this new setup because it has less wire in the bank and I would like to get down to servicing the bank once a year. I can check water once a month without disconnecting or moving anything. I got a way to monitor the bank for proper performance without having to load test each individually every three months.
It's still an experiment and nothing is the bees knees. I'm trying it based on experience with these things showing me that more parallel and less series connections keeps things better balanced.
Frankly, I'm not even remotely worried about difference in current to batteries in the same parallel string. If there is any difference, it's very, very small to the point of being negligible. I know that because I already ran the bank at the full rated output of the inverters and no voltage drops problems at all. If all the batteries couldn't deliver their proper current I'd get a big voltage drop from the batteries to the inverter studs. That didn't happen. Charging is no different than full load testing - just that the amps flow in opposite directions.
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Chris
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Some time ago, you posted your new way of battery arrangement which was the bees knees, what happened with that configuration? Were you looking for better by changing that?
Yes, like I said, that arrangement made me nervous because I had no way to tell if a battery developed a problem. It worked fine and kept them balanced very good. But what if one suddenly developed a weak or dead cell? It required servicing and load testing once every three months to make sure every battery was good.
I want to get away from that 3 month service because it's no trivial task.
So I decided to try this new setup because it has less wire in the bank and I would like to get down to servicing the bank once a year. I can check water once a month without disconnecting or moving anything. I got a way to monitor the bank for proper performance without having to load test each individually every three months.
It's still an experiment and nothing is the bees knees. I'm trying it based on experience with these things showing me that more parallel and less series connections keeps things better balanced.
Frankly, I'm not even remotely worried about difference in current to batteries in the same parallel string. If there is any difference, it's very, very small to the point of being negligible. I know that because I already ran the bank at the full rated output of the inverters and no voltage drops problems at all. If all the batteries couldn't deliver their proper current I'd get a big voltage drop from the batteries to the inverter studs. That didn't happen. Charging is no different than full load testing - just that the amps flow in opposite directions.
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Chris
How will you be able to tell when you have a problem now vs. what you had arranged before? I mean, disconnect all but two ( in a string ) batteries in one bank and leave the other huge bank which is paralleled with the now two batteries. What will the gauge prove? I guess I'm not a believer ultimately in the meter approach. That is my fault and lack of experience I'm sure. If your " parallel " definition is true, then the meters will not matter. If one battery fails, will that not throw the whole arrangement out of balance for even a short time which would not be good? Truthfully, there is no way to configure a bank where in the event of a failure, there will be no adverse effect on the other batteries. It seems all these arrangements have been discussed and documented by someone else in the past and this arrangement is nothing new. Had you determined the other arrangement would not give a year worry free use? Going a year without a lot of service is a long time. Even a disconnected battery will show good until the SG starts to fall. I still keep my bank arranged as the previous thread depicts, No problems so far, but will have to go read dates for a timeline. If one fails, or I need that battery for something else, I take it out of the group and keep chugging away. I don't have to remove two or more to re-balance the system or shut it down completely. The difference being, your bank is system specific and mine I use to keep the entire fleet of lazy equipment batteries cycled.
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If you scratch something often enough some good will come of it even if it is the scab on a sore and you should have left it alone.
Chris you and I use a similar amount of power each day, albeit we live pols apart, me in the tropics you in insane cold at times. I have lived with 96X2 volt cells connected series parallel to form a 24 Volt system, then a 48 Volt bank of 2 Volt cells, and now again a bank of 2 volt cells forming 48 volts, and I know which I prefer.
We all need to be careful about what we say,
Even if you have (in my case) 12 batteries in parallel in each string, there are slight differences in each one. As an example, let's say you have two different 12 volt batteries - one is fully charge, the other is discharged down to 50% SoC. Connect them parallel, then hook up a battery charger to charge them up. The dead one will come up to full charge and the fully charged one won't take any current.
You are correct but to suggest that we can connect two batteries that are at considerably different potential is misleading, wet cell batteries have low internal resistance which means if we parallel two that are at different states of charge and therefore terminal voltage, massive amps will flow albeit for a brief period of time.
I am being picky I know but let us all strive to be as accurate as possible in our posts, I claim no moral high ground, the opposite, it's because I am down here looking up that I can see what I see and trust me it is not always pleasant, the Scottish men have a lot to answer for.
I will now crawl back into my trench so most of the flack will go over my head.
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You are correct but to suggest that we can connect two batteries that are at considerably different potential is misleading, wet cell batteries have low internal resistance which means if we parallel two that are at different states of charge and therefore terminal voltage, massive amps will flow albeit for a brief period of time.
Allan, yes that was a bad thing to say now that I look at it. You, of course, are correct. Amps will flow from the fully charged one to the one that's at a lower state of charge and they will equal out. I guess that was my point - you can do it and charge or discharge them and the two will equal out with no adverse effects on the batteries (as long as they are both healthy).
Truthfully, there is no way to configure a bank where in the event of a failure, there will be no adverse effect on the other batteries. It seems all these arrangements have been discussed and documented by someone else in the past and this arrangement is nothing new
This is correct, Watt. I think the key is early detection of that failure so it can be dealt with before it causes failure of other batteries in the bank. I made a video of what it does when I hooked a little 2 amp 12V battery charger to one string in the bank and posted that video in an earlier post. Within seconds it caused enough imbalance to see it on the meters.
My theory here is that if the amp-hours of both strings are identical, there should never be an imbalance in voltage. If I lose even one cell in one battery at some point, that changes the amp-hour capacity of one string. And it will show up as imbalance on the meters during load (lower voltage on the string with the failed cell) or charging (higher voltage on the string with the failed cell).
I read your post earlier and I tried something else this afternoon. We went walleye fishing last night (didn't catch any fish) and the batteries in the boat were down on charge so they read 12.4 volts at-rest. Those batteries are 95 ah each - Group 27 marine deep cycle batteries. I got an idea from your post - 95 plus 95 is 190 ah. The batteries in my bank are 200 ah. This is all "nameplate" stuff because one T12 weighs double what those two Group 27's weigh combined. But that's what it says on the tags.
Our bank was floating most of the day and I float those T12's at 28.0 volts (temperature compensated was 27.7 volts today). I disconnected one battery in Bank 2 and replaced that battery with the partially discharged Group 27's by hooking them into the bank with jumper cables to see what happens. The voltage on Bank 2 instantly dropped to 13.7 when I hooked them up. The voltage on Bank 1 went to 14.0.
Now, with these two boat batteries we have 10 less amp hours on Bank 2 than we have on Bank 1. I left them hooked up all afternoon (for about 3 hours now), and they're still hooked on to Bank 2 as I write this. The balance meters evened out within a couple hours, but now after about three hours the voltage on Bank 2 is showing higher than on Bank 1.
I am completely satisfied that if I lose even one cell in one battery I will be able to detect it with those meters and this bank wiring configuration. The reason is because the bank always runs at a certain system voltage - let's say it's 26.0 volts for easy example. The balance meters should show 13.0 volts each. But if you have even a slight imbalance in one string from a failed cell it will cause either a drop in voltage on that string (under load) or a rise in voltage (under charging). At the same time it causes a corresponding rise or fall in voltage from the 13.0 on the other string, which makes it very easy to detect on the analog meters.
In the above example, if the system voltage is 26.0 and Bank 1 is at 12.95 because of a bad cell, Bank 2 is at 13.05 - a difference of .1 volts which is easily visible on those meters.
I've done enough experimenting now to know it works and will detect even a slight imbalance. My batteries are due for service again in June under my previous regimen. I'm going to leave it hooked up this way and skip that service interval. Servicing involves starting the generator and switching all our loads to genset, shutting down the inverters, solar and wind. Then remove each battery from the bank and record at-rest voltage, load test it and record loaded voltage, then leave it set and record at-rest voltage after it recovers from the load test. It's a long drawn out affair that takes 4-5 hours, and I hate doing it because my back is about shot after handling those freaking heavy batteries.
I'm going to leave it this way until the September service interval and just check water once a month. I'll service them as normal in September by load testing, etc.. If they all show identical like they did for the last service interval I will then consider this wiring method to be "proven" and it works.
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Chris
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Chris each to their own, but you have over engineered caring for batteries. a device called a hydrometer is a simple and easy way of checking each cell, just use a quality hydrometer. No need to run a generator, or have the batteries out of service for hours, just take a reading of each cell at around the same time, and they will never be all perfect.
A battery bank volt meter and observing it generally tells one everything necessary for safe and efficient operation of a battery bank.
I try to water mine once a month, take SG readings once a month, take cell (2V) readings once a month and I can see what my system is doing from the office by looking at my laptop. Apart from that I get on with life.
If I remember correctly you yourself once posted that flooded lead acid batteries live longer if kept bubbling rather than under charged and I fully agree, apart from water use and with due diligence to battery temperature they love it.
As example I can tell you my batteries are at 58.8 volts, 12.2 amps going in, solar panels are supplying 1.67 Kw (being limited by the MPPT's) system load is 0.9 Kw, today my batteries have received 9.1 kWh in and supplied 7 kWh they have not yet attained float although very near at 99.8% charged, my total AC load so far today has been 6.7kWh. And it is 1115 hours, I have fridge and freezer connected, 4 ceiling fans and one refrigerated air conditioner and two computers running, all of which are always in service oh and the tellie talking to it's self.
And worst of all there is no grog in the house, so I must schedule a trip to town sometime this week or it will be a sad weekend.
Allan
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Allan, I made a mistake when I built my battery case.
Our batteries are in an unheated utility room that is built on to the house. The rest of our power equipment (except the generators) are also in there. The power room has a concrete floor and it's insulated so it never gets below freezing in there in the winter time. But the battery case is also insulated and there's a small fan that draws air from our kitchen and blows it into the battery case to keep the batteries at around 65° F in the winter and no more than 80° F degrees in the winter. It seems the batteries are always about 5° F colder or warmer than the house air.
At any rate, the case has two racks. 12 batteries on the bottom rack and 12 on the top. It is made of 2 x 4's and plywood with foam insulation around the plywood shell. When it came time for battery service the design flaw in it became apparent. I can't get to the cells on the batteries in the lower rack without either:
- pulling all the top ones out to get to the caps on the lower ones thru the slots between the 2 x 4's that they set on
- pulling all the bottom ones out to be able access the caps.
I don't know what I was thinking when I built that confounded thing. Floor space concerns in the power room, I guess.
I don't remember the exact dimensions on the batteries, but they're roughly 2 feet long x a little over a foot tall, by about a foot wide. Each one weighs about 155 lbs with electrolyte in it. When I discovered the design flaw in the battery case I made a little stand that sets on the floor, and that's at the same height as the lower rack. I can reach in there with a 13mm wrench that's wrapped with electrical tape except for the box end on it and get the nuts and cables off a battery. Then grap the rope handle and slide it out on to the stand without blowing my nuts out so they're danglin' down around my knees.
It's one of those things where if you had it to do over, you'd do it different if you know what I mean. >:(
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Chris
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Chris, Sounds like you are convinced. I am going to have to go with your experience and also wish you the best. I will try and remember to check back in Sept. as to your success with this system. I am always looking for better myself. I can't count or remember all the exact schemes I've used which failed and know that our current scheme keeps the SG for each type of battery right on spot. The forklift/traction batteries stay just under 1.285 and the GC2 batteries are 1.300 and the 4d and 8d are just about 1.270 at these temps ( around 75F ). I noticed Allan mentioned this " for the battery SG " detail.
As you have mentioned with name tag ratings, I have a bit of a mess with mine. One group is 1000ah at 24v. Another group is a traction battery 935ah at 24v. The other a mixture of, yet all strings of this group are of equal name tag ratings, GC2 208ah, 4d and 8d batteries. Sometimes in that very mixed group, I have 4 to 6 group 31 stud mount batteries particularly in the winter. All of these groups are in series to form a 72v nominal bank.
I noticed you said something about never mixing batteries in series. I have done so with the only exception being allowing absorb to run as long as if I had the full battery string as the largest group. IE... at times as much as 1200ah at 72v. Right now, I figure and base my system for 72v at 935ah for load at 40% dod or just on 27Kwh over night. No troubles so far, but who knows after 5 years. It's been only around 6 months abusing these at 72v. I abused them even worse when ran the system at 48v.
Good luck and I do understand doing what shouldn't be done or wouldn't be done otherwise.
You are correct but to suggest that we can connect two batteries that are at considerably different potential is misleading, wet cell batteries have low internal resistance which means if we parallel two that are at different states of charge and therefore terminal voltage, massive amps will flow albeit for a brief period of time.
Allan, yes that was a bad thing to say now that I look at it. You, of course, are correct. Amps will flow from the fully charged one to the one that's at a lower state of charge and they will equal out. I guess that was my point - you can do it and charge or discharge them and the two will equal out with no adverse effects on the batteries (as long as they are both healthy).
Truthfully, there is no way to configure a bank where in the event of a failure, there will be no adverse effect on the other batteries. It seems all these arrangements have been discussed and documented by someone else in the past and this arrangement is nothing new
This is correct, Watt. I think the key is early detection of that failure so it can be dealt with before it causes failure of other batteries in the bank. I made a video of what it does when I hooked a little 2 amp 12V battery charger to one string in the bank and posted that video in an earlier post. Within seconds it caused enough imbalance to see it on the meters.
My theory here is that if the amp-hours of both strings are identical, there should never be an imbalance in voltage. If I lose even one cell in one battery at some point, that changes the amp-hour capacity of one string. And it will show up as imbalance on the meters during load (lower voltage on the string with the failed cell) or charging (higher voltage on the string with the failed cell).
I read your post earlier and I tried something else this afternoon. We went walleye fishing last night (didn't catch any fish) and the batteries in the boat were down on charge so they read 12.4 volts at-rest. Those batteries are 95 ah each - Group 27 marine deep cycle batteries. I got an idea from your post - 95 plus 95 is 190 ah. The batteries in my bank are 200 ah. This is all "nameplate" stuff because one T12 weighs double what those two Group 27's weigh combined. But that's what it says on the tags.
Our bank was floating most of the day and I float those T12's at 28.0 volts (temperature compensated was 27.7 volts today). I disconnected one battery in Bank 2 and replaced that battery with the partially discharged Group 27's by hooking them into the bank with jumper cables to see what happens. The voltage on Bank 2 instantly dropped to 13.7 when I hooked them up. The voltage on Bank 1 went to 14.0.
Now, with these two boat batteries we have 10 less amp hours on Bank 2 than we have on Bank 1. I left them hooked up all afternoon (for about 3 hours now), and they're still hooked on to Bank 2 as I write this. The balance meters evened out within a couple hours, but now after about three hours the voltage on Bank 2 is showing higher than on Bank 1.
I am completely satisfied that if I lose even one cell in one battery I will be able to detect it with those meters and this bank wiring configuration. The reason is because the bank always runs at a certain system voltage - let's say it's 26.0 volts for easy example. The balance meters should show 13.0 volts each. But if you have even a slight imbalance in one string from a failed cell it will cause either a drop in voltage on that string (under load) or a rise in voltage (under charging). At the same time it causes a corresponding rise or fall in voltage from the 13.0 on the other string, which makes it very easy to detect on the analog meters.
In the above example, if the system voltage is 26.0 and Bank 1 is at 12.95 because of a bad cell, Bank 2 is at 13.05 - a difference of .1 volts which is easily visible on those meters.
I've done enough experimenting now to know it works and will detect even a slight imbalance. My batteries are due for service again in June under my previous regimen. I'm going to leave it hooked up this way and skip that service interval. Servicing involves starting the generator and switching all our loads to genset, shutting down the inverters, solar and wind. Then remove each battery from the bank and record at-rest voltage, load test it and record loaded voltage, then leave it set and record at-rest voltage after it recovers from the load test. It's a long drawn out affair that takes 4-5 hours, and I hate doing it because my back is about shot after handling those freaking heavy batteries.
I'm going to leave it this way until the September service interval and just check water once a month. I'll service them as normal in September by load testing, etc.. If they all show identical like they did for the last service interval I will then consider this wiring method to be "proven" and it works.
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Chris
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Chris, Sounds like you are convinced. I am going to have to go with your experience and also wish you the best. I will try and remember to check back in Sept. as to your success with this system.
I'll see if I can remember to report back in Sept when I pull them all out and see what I got. After the last two days I don't expect any surprises, but one never knows I guess. I'm satisfied at this point that I'm not hurting them in any way, and that the gauges I put on each half of the bank will indicate a problem in the event it would develop, so the problem could be dealt with.
One of the downsides to multi-cell batteries is that you have a lot more cells. My 24 batteries have 144 cells. If I had 24 2V cells I'd have only 24 cells. So I got 6x more chances of having a cell go bad at some point than a 24 cell series/parallel bank. The batteries are under warranty for almost 6 years yet. But still, with this type of bank I think it pays to have a monitoring system on it that works so in the event one does go tits up it can be caught right away and fixed before it causes further damage.
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Chris
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Well, my batteries would normally be due for service again - including load testing them all. But my new bank balance meters show no problems. Our system is running at 25.0 volts tonight with 9 amps load @ 240 volts on the inverters and both meters show 12.5 volts (photo with my cell phone - not too good of quality):
[attachimg=1]
So I skipped pulling them all out and doing the load test. I checked water and the whole bank took about two quarts. Saved myself 4--5 hours that it normally takes to go thru all 24 batteries.
Just to make sure it is working I hooked one of my 95 ah boat batteries (which was fully charged) up to one string with a set of jumper cables. With 9 amps load on the inverters the amp draw on the bank is about 100 amps. It only took 10-15 minutes and I could see the side of the bank that didn't have the extra battery hooked up was already a needle's width different than the other side. So it's working.
September is my next normal service interval for our bank. I think if the balance meters show no problems, I'll skip that one too.
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Chris
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Nce to see positive results.
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Just a report back on this, as I promised I'd relay the results......
I ended up pulling the batteries out in July and checking them all because they got up to around 110 degrees in the heat. The controllers had the absorb voltage down to usually 28.2 volts and the batteries got lots of solar power charging because of bright, hot sunny days - several days at 98-100 degrees.
On July 26 I noticed on my analog monitoring meters that the needle on Bank 2 was slightly above 14 and the needle for Bank 1 was slightly below 14. I'd never seen that before so I decided to check them and pulled them all out. I found one battery on Bank 2 that had boiled itself down to where the plates were exposed to the air, and that battery had an at-rest voltage of 12.97 volts when I pulled it off the bank. The hot weather must've caused them to use more water. It took about 5.5 gallons to top them all off.
So I didn't make it to September as originally planned. But I'm glad I caught that really low battery right away. The battery seems to be fine after I watered it. I checked it again today (and that's what made me think of this thread) and its at-rest voltage, fully charged, was 12.74 - much closer to normal.
The bank wasn't due for a water check for two weeks when I pulled them all out. I made a mental note that when we get 100 degree weather that the electrolyte level in the cells has to be checked more often.
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Chris
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Thanks for the update Chris, looks like the meters are paying off.
A question, didn't you change the wiring a bit also?
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Nice update Chris, and good catch.
(Glad to see you're still about, thought you must have gone fishing or something... been awfully quiet of late!)
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Yes thanks for the update.
It seems your setup is advantageous.
This next spring we should be in the market for a new battery bank, if the final decision of battery lends itself to this approach I will likely duplicate the double meter setup, maybe with some extra ideas I had about this recently.
Don't be such a stranger ;D
Hope to see more posts from you.
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Good point on the water usage versus heat Chris. Thanks. Glad you caught it before it did serious damage.
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Yes, almost back at the start I removed the series connections on the bank and moved them to the bus. Those series connections at the bank caused some irregularities in charging voltage that seemed to go away with the series connection made at the bus. I don't think that was a huge deal because it always evened out.
The only reason I can come up that may have caused the irregularities is that the series connection was shorter than the leads to the bus. With the series connection at the bus, now all the cables carrying the heavy amps are all the same length.
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Chris