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Dead AGM Batteries

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OTG:
So I took the "solar truck" out this morning to cut up a downed tree with the electric chainsaw (MK Triton single-cab with 3 x 36V 190W panels mounted in parallel atop the tray canopy, feeding a Mastervolt 20A PWM Charge Controller, 2 x 120AH 12V AGM Deep Cycle batteries in series for 24V, and a PowerJack 5kW Inverter - all about 3years old now).

The inverter powered up as usual, I went to run the chainsaw, and... nothing!  ??? I checked the Controller and it's LCD screen is indicating that the battery has "high resistance" and I now realise:
1. the only thing powering the inverter was the solar array (hence it conked out when the load came on), and more importantly...
2. somethin's goin' on very wrong with my batteries!  :P

So after a day out with the trusty old Stihl, I've just done a quick Voltage test and both batteries are reading OL (Overload).

I've never had a dead battery do this (usually it's just EOL/under-voltage) and I couldn't find anything on the Google machine about such a reading, so I figured I'd ask the electrickery whizzes here:

1. Whats likely wrong with these batteries?
2. What likely caused the fault/failure?
3. Any chance of resurrecting them?

Like I said they're about 3 years old and have had a reasonably easy life thus far (they only get used occasionally (always on sunny days) for DC Welding, Chainsawing, running a small cement mixer, etc...). They are cheaper no-name batteries, but still, I expected to get more out of them then I have.

Any thoughts/help/advice appreciated.

WooferHound:
You have 125 amphours in batteries
If the chainsaw uses 10 amps at 120 volts, then the inverter will pull 110 amps from the battery
This is not very healthy for the batteries
For long life you should keep the voltage above 11.8 volts

Pete:
My suggestion is that you have a high resistance joint on the battery connections or a loose connection somewhere. You said that the batteries are reading Overload. Not sure if you meant to say Overvoltage or not.
Anyway with a high resistance joint on the battery terminals , you would be reading overvoltage because you would be reading solar panel voltage not battery voltage. Also you would not be able to draw much current from the batteries because of the crook terminals.
So clean the battery terminals well.
Use some battery terminal spray to prevent corrosion
And get back to us with what you find
cheerio and good luck
Pete

MadScientist267:
I'm not even sure where to begin here.

1. Yes, for deep cycle, around the 1C rate (120A; I'll get back to this in a sec)... they put up with it for short periods... But this is just bad juju. My guess is that they're at the very least hybrid "marine/RV" deep cycle/cranking batteries...

2. Chainsaw is going to be a series wound motor, and working it's way up to if not at or even beyond whole horsepower... Which by definition (ignoring efficiency of the motor itself just for the moment, which just compounds the issue) is ~750W per HP. With starting surges being many many times this. At 1HP your running load amperage (again ignoring additional inefficiency of the inverter) at 24V is 31.25A. That's ~C/4 for the battery in question, still rather bad juju for anything with "deep cycle" in it's title. Then add the conversion losses that the inverter and motor bring in.

3. Woof, where's the "110A" come from? This is one of those "1 dimensional equations" you got going on there... O.o

4. "OL" is something meters with probes generally display and indicate one of 2 things... "Overload" or "Open Loop" (the latter applying to continuity). Either way, if you're seeing this on a standard multimeter with 24V worth of battery, "you're going it wrong" is the first thing that comes to mind. Either its not set to measure the right range of voltage, or not set to measure voltage at all.

5. Again ignoring some efficiency considerations, you're feeding the bank with a "label" of 570W. This means that you're theoretically capable of putting nearly *2C* charge rate into the battery at high noon on a clear day. This is insane. Just be glad you aren't using an MPPT controller... With plain PWM, because of how it's able to use the PV, you're probably a little under this at worst case, and then on this side of the coin, inefficiency is "working in your favor" by reducing that even more. [EDIT - Future readers: I really hosed this call up somehow lol - Addressed the correction in the reply below]

6. The other thing you mention is welding with these. "Please say it isn't so"... Yes it is *possible* to weld using batteries, but even ignoring everything else that needs to be taken into consideration with this (series inductance etc)... This isn't something I'd even remotely be considering with this size battery. You're begging for an "event".

Lead acid is forgiving, but it has its limits. I apologize if I'm assuming too much here but what I've read in both the OP and parts of replies so far, stood my hair up. What's described isn't a "large system" by any stretch, but you're still messing with just enough power to get yourself into a lot of trouble if something goes wrong.

OTG:
Thanks guys!

Ok... after more testing today, turns out the Voltmeter on my Digitech Clampmeter is on the fritz, hence the strange "OL" Voltage reading. I found if I waited about 10secs after taking the probes off the terminals you can see the V reading come back down into range, with readings starting from about 5000V as it scales down, per the pic)! So yeah, this POS meter was hardly helping my efforts figuring this out! >:(

Using a working Multimeter today, I found the battery voltages are reading fine (with the Panels/Controller isolated), each battery returning 13.06V/13.04V respectively. In series all looks good too, returning 26.10V.

However, the moment you try to feed/drain the batteries the Charge Controller/Inverter protest in their respective ways -
a. Charge Controller manual describes the displayed Error as follows - "Battery wires or battery fuse damaged, battery has high resistance - Check battery wires, fuses and battery". Even with this error, the Controller does actually continue to trickle in charge (about 1.6A) however.
b. The Inverter's display does function (per the pic), but whatever current is trickling through, it's not enough to produce any power.

So I've checked the wiring, breakers, cleaned up the terminals, checked all terminations, but the issue persists. The only thing I haven't done is try to pull a DC load directly from the batteries (like DC welding... don't freak out just yet MadScientist267!  :D ).

The problem as crudely as I can explain it is - the batteries have good voltage, but can't supply current - for whatever reason!? Any further thoughts much appreciated!

Now, MadScientist...

Thanks for the detailed reply - I'm interested in better understanding your "hair standing on end concerns"  :o oh ooohh  ;). A lot of your assessment was spot on!!! The Chainsaw is about 750W (31A DC @ 24V) and the welder (a DC Mig Readywelder) pulls about 200A DC. These batteries, if I recall correctly, have a max discharge rating of about 1000A (5s). So I'm within that spec, but to your point C ratings are another matter, and so I keep the welding to small "bush welding" jobs (seconds of welding) and chainsawing/jackhammering/etc... jobs to minutes (10/15/20max) - basically, if the Controller gives me the 75% DOD beep, I stop. So (from my perspective at least) the usage hasn't been too rough - or am I being overly optimistic?

And per point 5. can you elaborate on whats so insane about this theoretical charge rate? I'm not following and would like to better understand. Shouldn't the CC (be it PWM or MPPT) prevent any charging "insanity"? ::) (obviously depending on the units features (e.g. reverse polarity detection) and the array being within spec of the unit (i.e. nominal PV voltage, etc...)

And per point 6., the "event" you're referring to I assume could be summarised as batteries going "boom"? :P Per above, DC or "bush" welding is a pretty common thing, I'm within the max discharge rating of the batteries, try to go easy on them with each job, and I've done a bunch of it to date with nary an issue. So again if you could elaborate on what kind of event/s and what circumstances might lead to one, that would be greatly appreciated.

Thanks again guys, all your thoughts/insights/experiences are greatly appreciated.

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