AGM battery health check

CALLD
CALLD Solar Expert Posts: 230 ✭✭
in Off Grid Solar & Battery Systems #1

My 2 12v/260Ah AGM batteries are now where I'd estimate to be mid-way through their lives. The manufacturer promised 800 cycles to 50% DOD under optimal conditions (temperature, charge rate, discharge rate etc.).

I decided to do a moderate discharge test, load test and internal resistance test:

1st Test from 100% to 85% SOC:

Load 36 Amps for 90mins.
Start voltage under load = 25.4v
End voltage under load = 25.1v
3 sec rebound voltage with load removed = 25.4v
60 sec rebound voltage with load removed = 25.7v
Internal resistance based on 3 sec rebound = 0.0045 Ohms per battery (manufacturer says it should be 0.0035 at 100% SOC)

2nd Test from 85% to 70% SOC (The slightly abusive test):

Load 120 Amps for 15 mins.
Start voltage under load = 24.3v
End voltage under load = 23.6v
3 sec rebound = 24.8v
60 sec rebound = 25.2v
Internal resistance based on 3 sec rebound = 0.0050 Ohms per battery (manufacturer says it should be 0.0035 at 100% SOC)

The results seem fairly satisfactory to me, however I do perhaps feel that the only way to know for sure if they are healthy is to run them down to the end voltage (21.6v) at the 10hr rate and see if I actually get 260Ah, and my gut feel is I wont - may probably only see 200Ah or so before the cut-off voltage.

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Comments

  • BB.
    BB. Super Moderators, Administrators Posts: 33,613 admin
    #2
    I don't like to take lead acid batteries near dead--I just don't think that older batteries (especially) will survive very well (may fail immediate or up to several weeks after taking them to zero state of charge).

    Realistically--My suggestion is the batteries are "good" as long as they run the loads you need. If you expect the batteries to support to 60% state of charge--Then test them to 50% state of charge--If they work, then all is good. If the batteries colapse prior to reaching your expected end point (60% SOC), then the batteries are "bad" for your needs. If they fail by 50% SOC, then they are on their last legs and should be replaced soon (and taking the batteries to zero useful energy storage--i.e., fail prior to 50% SOC, the test itself may kill the batteries).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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