Testing your homestead battery capacity...

For those that use battery storage for home power generation or even powered small electric vehicles or appliances, do you check the capacity of your batteries annually or every few years?  I have a battery load tester to examine general battery health, but for deep cycle batteries that are rated in amp-hours, how to you test for how far off of the original "100%" that your battery capacity has declined?  General internet searches have revealed some rather sophisticated devices for which load, discharge rate, etc. can be programmed, but I was wondering if I was missing something that would be more low tech, but still give a duration measurement on when the battery was depleted.  Thoughts?  Thanks!.....

John Weiland wrote:For those that use battery storage for home power generation or even powered small electric vehicles or appliances, do you check the capacity of your batteries annually or every few years?  I have a battery load tester to examine general battery health, but for deep cycle batteries that are rated in amp-hours, how to you test for how far off of the original "100%" that your battery capacity has declined?  General internet searches have revealed some rather sophisticated devices for which load, discharge rate, etc. can be programmed, but I was wondering if I was missing something that would be more low tech, but still give a duration measurement on when the battery was depleted.  Thoughts?  Thanks!.....

John, for larger lead acid deep cycle banks i used to apply a c5 to c10 load on the inverter and hold for 15-30 minutes with all inputs and other house loads off. I usually used space heaters. The speed at which the voltage dropped would give me an approximation of ability. You start fully charged with a good absorb cycle and repeat after say a year and see where they are in comparison. Its pretty time consuming and to have someone do it expensive so its kind of gone the way of the dodo bird. Add to that the switch to lithium and its a dying art. Ill still perform a test if someone asks me to evaluate a house they are buying or figuring out if they are going to make the winter.
Cheers, David

Thanks, David!   Can you tell me why this is not done routinely for LiFePO4 batteries?  Granted, a bluetooth version would be able to give you an estimate based on the battery management system display, but wouldn't one want to be able to 'real-world' test that capacity.  And even if the discharge curve for Li is flatter than that for legacy lead-acid tech, if one was detecting a 'fully discharged' status well before the amp-hour rating on the battery, this would indicate where power deficiencies may be located.  Finally, it seems from some videos out there that comparing LiFePO4 battery specs often involves validating the advertised Ah rating and so this often is done, albeit with shop electrical control units that appear more expensive and sophisticated than I wanted to deal with. Thanks for insights here....

John Weiland wrote:Thanks, David!   Can you tell me why this is not done routinely for LiFePO4 batteries?  Granted, a bluetooth version would be able to give you an estimate based on the battery management system display, but wouldn't one want to be able to 'real-world' test that capacity.  And even if the discharge curve for Li is flatter than that for legacy lead-acid tech, if one was detecting a 'fully discharged' status well before the amp-hour rating on the battery, this would indicate where power deficiencies may be located.  Finally, it seems from some videos out there that comparing LiFePO4 battery specs often involves validating the advertised Ah rating and so this often is done, albeit with shop electrical control units that appear more expensive and sophisticated than I wanted to deal with. Thanks for insights here....

I expect it is skipped because it requires some gear and some work..

For me, I am relying on my shunt and Victron Cerbo to do a reasonable job of monitoring energy consumption; if I find that voltage is nearing the low voltage cutoff and the amp-hours consumed since last full charge are not ballpark of what I am expecting, I will investigate individual batteries in the parallel bank.

I do load test and roughly capacity test the budget LiFePO4 batteries I use before putting them in service, but this is a bit of pain as I don't have a spare inverter or a high power DC load available at this time, so it requires installing the new battery as the sole power source, running it down to the inverter cutoff point with space heaters, and then seeing how many amp-hours (from the Victron charge controllers) it takes to reach full charge. Then if all good reconnect the other batteries..


To be honest I rarely look at any of this in the summer half of the year.. it tends to just work, tons of wasted power. Fall is time to check on it before the dark returns..