# Derating Ah rating of batteries as they age

Solar Expert Posts: 199 ✭✭✭
How can I determine the current Ah rating of my bank?  The bank is about 18 months old, and I suspect that they don't hold as much as they did when new.

The bank is 2 parallel strings of 8 GC2s in a 48V nominal bank.  Originally, they were 416 Ah...  How can I update my Classic with the right number so that my SOC readings are reasonably accurate?

I know there's no way to be absolutely certain what the Ah rating is, but is there a way to calculate or measure some property of the batteries that will get me close?

• Solar Expert Posts: 65 ✭✭
2twisty said:
Originally, they were 416 Ah...  How can I update my Classic with the right number so that my SOC readings are reasonably accurate?
It may just have to be via trial and error by slowly decreasing the set battery Ah in the Classic until you see the Classic enter float charging mode at the same time as the battery SoC reads 100%?
Have you been measuring/logging specific gravity of each cell? As the maximum SG of a cell drops, that is a reflection of reduced capacity.

The absolute method is to discharge bank until it no longer can supply enough voltage/current to support your loads. However, Lead Acid batteries are stressed by deep discharge *taking the battery to 20% or less state of charge).

In any cases--Equalize the battery bank while monitoring your SG every ~30-60 minutes (when all cells stop rising, that is the new "max SG" for each cell).

Once you have the new SG--Now monitor the SG. You typically want to get >90% state of charge once or twice a week (do not try for 100% SOC every day or even several times a week--That is pretty hard on a battery bank--essentially "equalizing" every day). And once a month equalize.

If you find the bank is not charging to 90% (or is charging to 100% often), then adjust the charging voltage set points/timers/end charge as needed. The actual state of battery charge is what matters--Not the estimated capacity.

And, once the battery bank no longer supports your needs (say 2 days of "no sun" at nominal loads)--Then the battery bank is no longer fit for your needs and should be replaced.

Estimating your battery bank AH capacity to +/- 20% is probably the best you can do (measuring SG of cells, voltage of discharging batteries in the string). And the "typical" worn out battery is when it has lost 20% of capacity (Concord AGM "warranty" specification)--I am not sure I would go to much effort to estimate/measure "exact" bank capacity.

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Solar Expert Posts: 5,287 ✭✭✭✭✭
The easy way to guess along with Bills comments is to load test. A 1500 watt heater load should not drop the resting (solar off) voltage more than 3 volts on a 48V nominal. If it is after 5 minutes of load, it is time for new batteries if the SG is good.
"we go where power lines don't" Sierra Mountains near Mariposa/Yosemite CA
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