Lead acid discharge limit at c/10 rate

[stevec]

I’ve just ordered up an electronic load tester to check the capacity of the various batteries I have around the place (including the hab batteries). I’m trying to find the cutoff voltage at which to terminate the test for a 12v lead acid battery of around 100Ah capacity. The tester goes up to 150w so am looking at discharging at the C/10 rate. I’ve not found a definitive answer. Any suggestions?

 

[autorouter]

I seem to remember that the cutoff voltage is about 10.5V. At that point the battery is practically empty. I also think manufacturers do this test on a sample of their battery, and regard it as a destructive test. I don't think there would be much value left in a battery after a test like that.

There is an alternative if you can find a chart that shows the resting voltage versus state of charge for the variety of battery you are testing. Then you could fully charge the battery, and discharge for 10 hours at the C20 rate. A new battery would have dropped to the 50% level. If your battery has dropped to the 40% level, then a bit of a calculation will tell you your battery capacity at its 100% level, compared to a new battery.

I think in general manufacturers quote the C20 rate rather than the C10 rate, so better to do a C20 test for comparison purposes.

 

[Hoovie]

I seem to remember that the cutoff voltage is about 10.5V. At that point the battery is practically empty. I also think manufacturers do this test on a sample of their battery, and regard it as a destructive test. I don't think there would be much value left in a battery after a test like that.

There is an alternative if you can find a chart that shows the resting voltage versus state of charge for the variety of battery you are testing. Then you could fully charge the battery, and discharge for 10 hours at the C20 rate. A new battery would have dropped to the 50% level. If your battery has dropped to the 40% level, then a bit of a calculation will tell you your battery capacity at its 100% level, compared to a new battery.

I think in general manufacturers quote the C20 rate rather than the C10 rate, so better to do a C20 test for comparison purposes.

This is the method I use to check a battery capacity. Remember to let the battery rest for at least an hour minimum after the load test before checking the voltage.

Depending on the battery, you might be able to find an actual spec sheet with the expected voltage at different levels of discharge so could be a straight cross-check, but for most batteries people end up with, a generic voltage vs SOC can be the best available.