Battery Bank Configuration

[ChrisOlson]

I service my batteries (24 Rolls-Surrette T12-250's) every three months.  The service procedure involves fully charging the bank as the first step.  Then, with the generator running and my RE system shut down, I flip my big Ronk 200 amp manual transfer switch.  The transfer switch disconnects gen power to the inverters while simultaneously disconnecting the inverter output to the loads, and switches all the loads over to gen power.  It totally removes the inverters and battery bank from the loads so the inverters go idle.

I takes about 4 hours to service the bank.  Those batteries are heavy and hard to handle, but I pull them off the bank in pairs to check them out.  I stick a 2" wide piece of painter's tape to the top of the battery and record the at-rest voltage of it on the tape with a marker.  I check the electrolyte level in the cells, and check the SG of each cell with a hydrometer and record it on the tape.  I then apply a 50 amp load to the battery with my Sun AVR for one minute and record the loaded voltage that it shows after one minute on the tape.  If the battery checks out it goes back online.

When I serviced my bank in September 2011 I had the batteries hooked up this way:



What I found was batteries all over the place on SOC.  Some of the batteries varied from others by up to .28 volts at-rest and the ones with low at-rest voltage fell flat on their face during the load test.

After servicing all the batteries I sat on my upside down five gallon pail staring at the bank while drinking a beer and decided this is not good.  I got 12 series connections in the bank and 24 cables going from the bank to the bus.  No two batteries are ever identical.  But the challenge is, when you connect them series, just how do you determine which ones are matched so they stay even during charging and discharge?

So I changed all the bank wiring around to this:



I eliminated 12 cables from the bank to the bus.  I eliminated 6 series connections in the bank.  I figured, what the heck - with pairs in parallel like that, and connecting two pairs in series, maybe it will stay balanced better.  It's worth a try.

I got a little behind on my bank servicing this last time with the holidays and stuff.  I just got it done yesterday, for the first time since I had changed the wiring around.  It worked!  The furthest off any two batteries were in the entire bank was .04 volts this time.  They all tested 1.270-1.275 SG, and all load tested so close to identical that it's not even worth mentioning or recording the difference between any two.
--
Chris

[WooferHound]

Are you sure about that second battery diagram ?
It looks like some of the batteries are shorted out . . .

[Watt]

Are you sure about that second battery diagram ?
It looks like some of the batteries are shorted out . . .

They are not shorted, they are in parallel series.   Equal to 

  l-- +                    24v  buss bars             -       -----  i
  l                                                                               l
  l                                                                               l
  l ---  +  12v group  -  - l                                            l
       l                             l                                            l
       l                             l                                            l
       l                             l                                            l
       l- +12v group -   -  l -- l                                       l
                                          l                                       l
                                          l                                       l
                                          l - l -- + 12 v group - --i    l                   
                                              l                               l    l
                                              l                               l    l
                                              l                               l    l
                                              l                               l    l
                                              l -- + 12 v group - -- l --l   




that. IF it worked.

 

[ChrisOlson]

Are you sure about that second battery diagram ?
It looks like some of the batteries are shorted out . . .

Yep.  It works.  You take two batteries and make one big one with parallel connections.  Then hook the parallel groups in series.

My theory on why it works better is because batteries in parallel tend to support one another even if they're a little bit different.  Series connections between batteries are always bad because if, for instance, the charging voltage is 30 volts and one battery is at 14 volts, the other will be at 16.  The one that's "lazy" gets deficit charged and the other one gets the snot boiled out of it.

By grouping the batteries in pairs with the parallel connections it must've eliminated the problems with slight mismatch between batteries that tends to show up more with series connections.

I thought about doing groups of three in parallel.  But then I would only have 8 cables from the bank to the bus instead of 12 cables.  It's 9 feet from the bank to the bus and my inverters draw 720 amps @ 24 volt nominal at full load.  With 12 cables I can get by with 1/0 from the bank to the bus, as each cable only carries 120 amps.  If I used a triple parallel configuration, then each cable would have to carry 180 amps and I'd have to use 2/0, and I'd still get more voltage drop to the bus than I do with the 1/0 and 12 cables.

I got 4/0 cables from the bus to each inverter, but those are only 6 feet long.
--
Chris

[Volvo farmer]

My theory on why it works better is because batteries in parallel tend to support one another even if they're a little bit different.  Series connections between batteries are always bad because if, for instance, the charging voltage is 30 volts and one battery is at 14 volts, the other will be at 16.  The one that's "lazy" gets deficit charged and the other one gets the snot boiled out of it.

I have the feeling we have been over this before, but everything I have ever read by solar installers and people who have been working with batteries for over 20 years, recommends the opposite. I have heard that it is parallel strings tend to stray in their SOG and that this tendency is reduced if one has a single string of big old honkin 2V cells.

Isn't a 12V battery just a series string of six 2V cells? Why are 6 2V 100ah batteries in series bad but a single 12V 10 ah battery in parallel with nine others good? Do all ten of those parallel strings really support each other?

[ChrisOlson]

I have the feeling we have been over this before, but everything I have ever read by solar installers and people who have been working with batteries for over 20 years, recommends the opposite

That's because they like to sell batteries   

I don't care what they say.  I'm going to go with what works and keeps my bank balanced.

Why are 6 2V 100ah batteries in series bad but a single 12V 10 ah battery in parallel with nine others good?

Probably because you don't have to deal with voltage drops and resistance across cables when all the cells are in one unit.  And being that all the cells in a multiple cell battery are the identical same age, and they were manufactured under the same "batch" or "lot number", they are closer matched to one another.

You only have to look in the automotive world to see that series connections are not good.  The Toyota Land Cruiser with its 24 volt system - nothing but constant problems with trashed batteries in those.  Google it on the internet and you can read about it on all the forums where guys are always coming up with new schemes to prevent smoked batteries in their Land Cruisers.

On the other hand, look at Class 8 semi rigs that have 4-6 12 volt batteries in parallel  - you rarely see battery problems in heavy trucks.  And when one does go south, you only have to replace the bad one, where with a 24 volt system you have to replace them both if one goes bad or it will very shortly smoke the new one because it runs at higher voltage than the old one.

So like I say, I go with what I see works.  I don't like series strings of any sort of batteries because all it takes is one failed battery in the string, and you are in for a constant headache and battle keeping that string balanced from that point forward unless you replace the entire string.  With parallel configurations you can put a 50 ah battery in parallel with a 225 ah, and it works fine with not a single problem.
--
Chris

[Watt]

I have the feeling we have been over this before, but everything I have ever read by solar installers and people who have been working with batteries for over 20 years, recommends the opposite

That's because they like to sell batteries   

I don't care what they say.  I'm going to go with what works and keeps my bank balanced.

Why are 6 2V 100ah batteries in series bad but a single 12V 10 ah battery in parallel with nine others good?

Probably because you don't have to deal with voltage drops and resistance across cables when all the cells are in one unit.  And being that all the cells in a multiple cell battery are the identical same age, and they were manufactured under the same "batch" or "lot number", they are closer matched to one another.

You only have to look in the automotive world to see that series connections are not good.  The Toyota Land Cruiser with its 24 volt system - nothing but constant problems with trashed batteries in those.  Google it on the internet and you can read about it on all the forums where guys are always coming up with new schemes to prevent smoked batteries in their Land Cruisers.

On the other hand, look at Class 8 semi rigs that have 4-6 12 volt batteries in parallel  - you rarely see battery problems in heavy trucks.  And when one does go south, you only have to replace the bad one, where with a 24 volt system you have to replace them both if one goes bad or it will very shortly smoke the new one because it runs at higher voltage than the old one.

So like I say, I go with what I see works.  I don't like series strings of any sort of batteries because all it takes is one failed battery in the string, and you are in for a constant headache and battle keeping that string balanced from that power forward unless you replace the entire string.  With parallel configurations you can put a 50 ah battery in parallel with a 225 ah, and it works fine with not a single problem.
--
Chris

Have a question for you Chris regarding the 50ah battery paralleled with the 225ah battery.

If, in equalize mode, that smaller battery is being cooked because the voltage and time required to equalize the larger battery. 

Got any thoughts?

[Cornelius]

Maybe you're on to something, Chris?

If you take LiIon batteries, you won't find Series/Parallel strings; only Parallel/Series (Parallel cells to get the amp up, then Series to get the voltage). (Ok; LiIon are different beast, but...)

Maybe 'A Truth' are about to be rewritten?

[Watt]

I guess this is Parallel/Series as well and I've had better luck with this arrangement.  But, I didn't document why I changed from the previous drawn arrangement. 

  ( + )                 24v buss bar                         ( - )
     l                                                                    l
     l                                                                    l
     l   l -- (+) 12v group - ---- + 12v group - --l   l
     l   l                                                             l   l
     l _l                                                             l_ l
         l                                                             l
         l                                                             l
         l -- (+) 12v group - ---- +  12v group - --l

I keep the cable lengths the same for the y connections and the interconnects. 

[ChrisOlson]

If, in equalize mode, that smaller battery is being cooked because the voltage and time required to equalize the larger battery.

It doesn't appear to.  Last fall when I put the boat away I took both batteries out of it and put them in series on my bank and hooked them up to the bus.  I figured it was better to use them and maintain them that way than leaving them in the boat.

They're just little Group 27 deep cycles - 95 ah.

I have monitored those quite regular because they're not in a battery case, and they seem to be doing fine alongside those big suckers.  I've equalized the bank with them hooked up and they boil good, but they have never taken any water yet after being on there for close to three months.
--
Chris

[Watt]

Thanks Chris, I've been a bit too afraid to give that a real go cuz I'd read where it may be disastrous. 

[Wolvenar]

I also place my boat deep cycles inline with my battery bank on the inverters charger. I have never had a problem even with a controlled equalization

[ChrisOlson]

I know.  I've read the same thing.  But in parallel it seems that the internal resistance of the battery determines how many amps it's going to draw during charging at a set voltage.  The smaller battery must increase its internal resistance as SOC comes up so the larger one draws a bigger proportion of the amps going to them.  At least that's what it appears to do.
--
Chris

[ChrisOlson]

I also place my boat deep cycles inline with my battery bank on the inverters charger. I have never had a problem even with a controlled equalization

I should've mentioned that the batteries in my boat are 2005, so they're 7 years old this coming spring.  I've put them on the bank every single year instead of leaving them in the boat.  They're still like new.  I got an older Minnkota (I think it's a 565) trolling motor on the boat and I can troll at the lowest setting for an afternoon and evening of walleye fishing for 8 hours steady and the trolling battery never drops below 12 volts.  That would be about 50 amp-hours out of the battery, and I consider that pretty good for (what was last summer) six year old batteries.

Early last June my wife and I were fishing on Cedar Lake and found a school of walleyes off the edge of a sunken island and we had so much fun we didn't quit until about 1:00 AM.  So I fired up the old Merc 40 four cylinder and headed into the landing.  It was like a five mile run up the lake to get to the landing.  Got 'er up on plane and we were enjoying the night run up the lake when the motor quit about 2 miles from the landing.  The frickin' reed valve block that goes around the crankshaft between cylinders #2 & 3 busted.  So I raised the outboard out of the water, dropped the trolling motor in, threw 'er in high and started out for the landing on electric power.

By golly, that six year old battery got us there, but it was not fast.

What was really funny was that my wife figured since we're "trolling" might as well throw a bait in the water and drag it.  About 200 yards from the landing she caught a rock bass and that little sucker fought so hard she figured she had the Monster Walleye on the end of the line   
--
Chris

[ghurd]

Have a question for you Chris regarding the 50ah battery paralleled with the 225ah battery.

If, in equalize mode, that smaller battery is being cooked because the voltage and time required to equalize the larger battery. 

I'm not Chris, but we do it all the time.
85, 120, a few pairs of golf cart 6V 220, maybe an 8D sometimes.  No problems.

I should've mentioned that the batteries in my boat are 2005, so they're 7 years old this coming spring.  I've put them on the bank every single year instead of leaving them in the boat.  They're still like new.

I am kind of confused about that.
Are these in the 24V system the new batteries with the gawd awful high voltage and long duration equalization routine?

dropped the trolling motor in, threw 'er in high and started out for the landing on electric power.
By golly, that six year old battery got us there, but it was not fast.

Yikes.
Slower is more efficient in that situation.
With six year old batteries at 1AM, I'd have been running at about half the peak MPH.
Still slower, but a better chance of getting back with dry oars.
I HATE rowing.

Had to row up wind with a broken oar and a net.
Actually had to do it twice in the same week.
(different boat, different broken oar, different Chinees Merc, different lake, same net)
The only thing that would have been worse was doing it at 1AM.
Yup, I HATE rowing.
G-