How to measure battery capacity of various chemistry?

[drummerdimitri]

I have a few old Li-ion and lead acid batteries that I would like to test for actual capacity however I am not sure at what current draw their rated capacities where deduced. I have the measuring equipment that gives me a mAh/Ah reading.

If i am correct, Large lead acid batteries' capacity are based on a 20 hour constant current draw. I have the measuring equipment needed just not sure how to set the load.

Also, to what voltage should they be discharged? I am only interested in knowing the actual capacity today to know how they stand up and if they need replacing.

[drummerdimitri]

BUMP!

[KC0PPH]

Li-Ion batteries should be discharged at 1C for measuring capacity. I dont remember off of the top of my head what the Low Voltage Cutoff is as I only take mine down to around 3.7V per cell.

[Hiemal]

Low voltage cutoff for li-ions is usually around 3.0 volts i think.

[Rerouter]

If you want to compare to the rated capacity, then you measure at the load they specify for the rating

If you want to know the capacity for your specific use case, then you measure it at that load.

Lead acids you stop discharging at about 10.5V, li-ion you stop at about 3.2V

[drummerdimitri]

If you want to compare to the rated capacity, then you measure at the load they specify for the rating

If you want to know the capacity for your specific use case, then you measure it at that load.

Lead acids you stop discharging at about 10.5V, li-ion you stop at about 3.2V

Thanks for that!

So if my car battery is dying and I would like to test it's actual capacity vs its rated one (80 Ah, 800 CCA). I should discharge at a constant 80 amps?

If I am not mistaken, all lead acid batteries's capacity ratings are derived from a 20H rate.

[Rerouter]

for its actual capacity, test it at 4A discharge to a 10.5V cut off., (lead acid are rated at about 1/20C)

For cold cranking amps, measure the resistance to ground of your starter motor circuit to battery negative (can usually pull the starter relay and measure from there) this gives you your load, now the cranking amps are measured as the current it can draw without falling under 7.2V from a fully charged state, so roughly a battery ESR of 7.5 milli-ohm,

At a basic level if you see the battery voltage fall under 7.2V while cranking, its likely an issue, if not and you want to know the value, take note of the voltage at that starter contact to battery negative while starting, from there its easy enough to figure out how many amps it was pulling, then another start measuring just at the battery terminals and this will let you calculate the battery ESR pretty closely.