Old agm batteries have different amp hours

deegore

Hi, I have a battery bank made of 5 155ah agm 12 volt batteries connected in parallel. I haven't had any problems with them. Just for fun, while I had them disconnected for some remodeling, I decided to test the capacity they have left.
I charged them all to 100 percent. Then I put them on a 50watt air pump to see how long each one would run until the inverter started to beep. 
I took the hours each battery ran, and multiplied by 4.14 amps that the pump draws to get the amp hours.
The batteries were very different. 150Ah, 104Ah, 76Ah, and 2 at 50Ah.
I'm wondering what my best bet is to try to get another year or two from the batteries. Should I hook then back up and go until I start running out of power?, Discard a couple of the 50Ah?. I read some mention of putting a diode in between each battery to stop the weaker batteries from draining the stronger ones, is that a good solution for me? Thanks for any ideas

BB.

Don't bother with the Diodes--They will only make a bad problem worse.

There are some brands of batteries that offer a "recovery" method to try and bring back some life (Condcore/Lifeline brand)--But not recommended for other brands and AGM batteries.

Recharge all the batteries to 100% State of Charge (typically recharge to 14.4 volts for 24 hours or so). Then disconnect and let them "rest" for a day. See if they stay full charged (>~12.7 volts) after one day or not. Any batteries that are below 12.7 volts are probably "junk" at this point and could self discharge the rest of your batteries.

If the batteries are all >~12.8 volts after one day--I guess just reconnect and use them... The 50 AH batteries won't add much energy to your bank--But they did not cost you any more money at this point.

One thing to check--Batteries can be sensitive to how they are wired in parallel strings. Need to match wire lengths/resistance between each string. If wired "wrong", some batteries will be cycled (discharged/charged) more than the others:

http://www.smartgauge.co.uk/batt_con.html

How long have you had the bank? 5-7 years is, typically, a pretty good life for quality AGM batteries.

Are any of the batteries showing "positive post extrusion"? Where (typically) the positive post is being pushed up out of the battery case. This is usually a sign of corrosion inside the battery and that it is not long for this world.

Another sign is do some of the batterieds look "inflated"--A bit like a balloon. This can be normal (AGM batteries are sealed (have an over pressure relief) and can "inflate a bit" in normal charging (you don't want to hear the valve "hiss"--That is typically over charging/over voltage during charging)--Not a good thing. But just resting/being used looking "inflated", that can be from the plates sulfating--Again a permanent loss of capacity and battery ready to be recycled.

-Bill "not a battery engineer" B.

deegore

Thanks, the batteries are 6 year old marathon brand. No post extrusion, but maybe look slightly inflated. I've never heard them hiss. 
I will charge them all up and check the voltage after they sit for a day. It will take a while because my charger is only 15 amps.
Thanks again for all the info.