What's going on with my six year old AGM bank?

TimC

WI/UP Border
Joined
Apr 17, 2020
Messages
99
I know this battery bank is on its last legs. I just want to know if this is normal end of life stuff or am I seeing an anomaly. I'm not really looking for what to replace the bank with if it is toast but if you insist I will listen.

I have what started out as 144 aHrs in my bank. It worked well for five years. And continues to work during the day when my 200w of solar keeps everything going. The last two camping trips I have had my refrigerator shut down due to low voltage each night. It was on High cut off when that happened. So I changed it to Low cutoff to see if it would keep running. Yes, I know this will drain my battery. It did continue to run below 9v.

My real question is about my battery bank, not the fridge operation. I don't want to make you read the meters but if it helps I've included a series of shots of my ammeters in a test early this morning when my solar was not producing much.

1- 12.64v - First shot shows ammeters with most everything off. Load on top, production on bottom.
2- 11.48v -Second photo shows just seconds after turning the fridge on.
3- 10.26v -Less than one minute later. Last image showing fridge volt meter at 9.8 volts
4- 9.81v -two minutes later, fridge shut off after photo taken
5- 12.59v -Two minutes later, most loads off
6- 12.65v -13 minutes later, most loads still off

Production is less than load as it was reset late afternoon with hazy skies and metering overnight use. I had to turn off the fridge during the night as it was set on High cut-off and would cycle on and off constantly. During the late afternoon (previous day) the ammeters were registering 13.2v while actively charging.

Thanks,
Tim
 

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RCinFLA

Photon Sorcerer
Joined
Jun 21, 2020
Messages
1,026
Six years is not unreasonable. Lifespan really depends on how used.

Looks like you are suffering from high cell impedance. For lead acid too much float charging can build up too much lead oxide on positive plate increasing cell impedance. This extra lead oxide can be burned off with some discharging.

AGM's usually have pure lead plates with no grid support structure to corrode which is normal reason for high impedance in flooded lead-acid.

AGM's are electrolyte starved meaning they don't have extra to spare. If overcharged they will vent hydrogen and oxygen and eventually drive all the water out of electrolyte. This is a common problem causing high impedance in AGM's.

My guess is electrolyte is dried out.
 

TimC

WI/UP Border
Joined
Apr 17, 2020
Messages
99
...
AGM's are electrolyte starved meaning they don't have extra to spare. If overcharged they will vent hydrogen and oxygen and eventually drive all the water out of electrolyte. This is a common problem causing high impedance in AGM's.

My guess is electrolyte is dried out.
So, am I overcharging if I have a Good quality MPPT connected and have it charging each day? The system has been sitting at full charge for many months without a load on the batteries. If I'm understanding this right I should regularly put a load on the batteries and draw them down to 12.5v or so?

I can accept that they are at the end of their usefulness. I was hoping to get a couple more years out of them but I also hope a million dollar check appears in my mailbox someday...

What is the responsible way to recycle/dispose of eight 18 aHr AGMs?
 
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