# Evaluate battery capacity

Hi everyone,

My lead battery (12 OPzS) is getting old (9 years). I am figuring out how to evaluate its actual capacity. The steps would be:

1. Diminish the reporting frequence to VRM to limit error on times
2. Equalize, measure the electrolite density
3. In the evening, take note of time (h1) and SOC (S1) of the end of solar charging or at the very beginning of decreasing SOC;
4. Cut off the feed from panels
5. Next day follow SOC until 50% (s2)
6. Note time (t2) and put back on the feed from panels
7. From VRM note total load L between t1 and t2
8. Cx=100*L/(s2-s1)
9. x=(t2-t1)/(s2-s1)

I will not try to evaluate the voltage: not accurate. The electrolyte density should be enough

Any other method?

Thanks

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Hi, to measure the capacity of a battery, don’t look at SOC. Not to see if its full, not to see if its empty, and not to see what its capacity is either

I would:

1) fully charge the battery. Fully charge means continue charging until it doesn’t accept any significant current anymore while voltage is at absorption level.

2) write down value of kWh discharged meter in BMV.

3) discharge it with approx C2 current (= 1/5th of rated capacity) until empty. Ie voltage at 10V orso.

4) compare new value of kWh discharged meter with old value.

Ps. to measure it in amps, look at what amperage you draw and multiply with amount of hours it took to discharge it.

Ps. once empty, charge it: never leave a lead battery in discharged state; as doing that will reduce its life time due to sulfation

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