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Testing your homestead battery capacity...

John Weiland

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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!.....

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David Baillie

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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

John Weiland

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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....

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D Nikolls

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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..

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Mart Hale

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I don't do capacity tests other than I know what loads I put on the battery and can refer to how much I can know from my past loads how the batteries respond. If they do the loads they need to do, for me that is good enough, I run lithium batteries, and I have created my own wifi battery device that I can record on an open source program domoticz the data.

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David Baillie

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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....

Mart totally wins for geek wizardry! His own monitoring! If I want to test a lithium it depends in the quality of the inverter and the batteries. Those drop in replacements that dont need comms to inverter you would have to charge until it does not take charge, apply a known load and discharge till it shuts down; i dont like that. If its a solark, lux or eg4 ( rebadged lux) and a battery that does two way comms you have a state of charge meter and an app so its way easier. I can apply a known charge to it add in the system running load and watch the percentage go down. If it matches spec you are golden as it ages I would expect it to drop more rapidly. The batteries im installing today are less than 2 years and so far no change over about 12 systems. Here is a screen shot of the lux app: the website version is better. Eg4 and solark have similar ones. The cheaper inverters not so great I dont install them as its too much call backs and confusing menus. There are other good ones of course.

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David Baillie

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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....

Mikael Korhonen

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John, you don't need shop-grade gear for a real-world test on LiFePO4. The cheap way: fully charge the battery, then run it down through your inverter with a steady resistive load like a space heater, and track the watt-hours with a shunt-based monitor or even a plug-in meter on the AC side. Note the total Wh when the BMS hits low-voltage cutoff, convert back to Ah at nominal voltage, and compare with the label. The flat discharge curve just means voltage alone won't tell you much — the Wh counter does the real work, which is exactly why the bluetooth BMS estimate plus a periodic full run-down is the practical routine for most folks.

Rad Anthony

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I think what you're doing is sufficient. You only really run it down when using an inverter or a heavy load so that common sense will tell you when you are really running your battery down. Unless you and neighbors or the local communities are investing on pulling and recycling the lead plates with fresh acid, may not be worth the time and effort. It's good knowledge and wisdom to have though. Do you have a way of recycling lipo if they stopped making them for good. Same with guns and ammo. Lead acid you can at least jerry rig if you have to lol. I'm not sure how often folks recycle the lipo when they get bad. At work we just log the install date and load test every now and then. If there's a storm and the generator runs for hours then you know thats heavy load. If you have a rain storm during the well now you have to find the charge to charge without the sun. Hope that makes sense.unlees you have a stream or wind for continuous power aka ac, working with batteries you sort of have to be mindful of your usage and takes more maintenance.

Chandrapal Mehula

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posted 5 days ago

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My position, for what it's worth: a capacity test on LiFePO4 is worth doing once when the battery is new (to confirm the rated Ah is real) and then only occasionally, because the BMS and a shunt already give you most of the answer.

A low-tech way to do it with gear you probably have:
1. Fully charge until the charger is in float and the tail current has dropped below roughly 0.02C (the BMS or shunt should read 100%). Let it rest for an hour.
2. Zero the shunt's amp-hour counter, disconnect all other sources, and run a steady known load through the inverter. Aim for roughly C/5 to C/2 so the result is repeatable. A resistive load like a space heater is ideal.
3. Stop at the BMS or inverter low-voltage cutoff, or at about 2.9 to 3.0 V per cell if you are watching individual cells. Don't push below that.
4. The amp-hours the shunt counted is your usable capacity. Compare it with the rating (and with your own earlier test) and log the temperature, since cold cells will read noticeably low.

Two cautions. A single-point capacity number can mislead if the shunt isn't calibrated, so check its reading against a clamp meter. Also, with LiFePO4 the voltage curve is so flat that you can't judge state of charge by voltage the way you can with lead-acid. The knee at the bottom comes suddenly, and in a parallel bank the weakest or lowest-resistance battery hits the cutoff first.

For ongoing health, a practical trigger is to watch for these:
- Cell imbalance at the top of charge (more than about 50 to 100 mV spread).
- Amp-hours consumed per day that no longer match your loads.
- Hitting the low-voltage cutoff earlier than before at the same load.

If you see any of them, do a full capacity test, and test parallel batteries individually if you suspect one is lagging. A fully charged pack that still reaches 80 to 90 percent of its rated Ah after a few years is generally considered healthy.

John Weiland

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Chandrapal Mehula wrote: ....3. Stop at the BMS or inverter low-voltage cutoff, or at about 2.9 to 3.0 V per cell if you are watching individual cells. Don't push below that..

Thanks for all of the excellent and helpful responses. One question about the statement clipped above: Won't the BMS automatically stop discharging at the low voltage cutoff? In other words, won't this prohibit me from pushing discharge below that voltage limit even if I wanted to? Thanks!....

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D Nikolls

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John Weiland wrote:

Chandrapal Mehula wrote: ....3. Stop at the BMS or inverter low-voltage cutoff, or at about 2.9 to 3.0 V per cell if you are watching individual cells. Don't push below that..

Thanks for all of the excellent and helpful responses. One question about the statement clipped above: Won't the BMS automatically stop discharging at the low voltage cutoff? In other words, won't this prohibit me from pushing discharge below that voltage limit even if I wanted to? Thanks!....

A correctly functioning BMS will, yes. Relying on the inverter alone might let you draw one weaker cell way too low, though.

David Baillie

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posted 4 days ago

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John Weiland wrote:

Chandrapal Mehula wrote: ....3. Stop at the BMS or inverter low-voltage cutoff, or at about 2.9 to 3.0 V per cell if you are watching individual cells. Don't push below that..

Thanks for all of the excellent and helpful responses. One question about the statement clipped above: Won't the BMS automatically stop discharging at the low voltage cutoff? In other words, won't this prohibit me from pushing discharge below that voltage limit even if I wanted to? Thanks!....

Yes it will but set your inverter low voltage cutoff a little higher Than that to avoid a battery level shutdown.

Arvin Dale

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One measurement distinction may help with the AC-meter suggestion above: a plug-in AC meter measures energy delivered after the inverter, not battery-terminal Ah capacity. Dividing those AC Wh by the battery's nominal voltage gives an estimate with conversion losses and voltage assumptions mixed in; it is not a like-for-like comparison with the Ah label.

For the DC measurement principle, Victron's SmartShunt manual, section 5.1 explains integrating battery current over time to count Ah. For comparisons over the years, keep the measurement point, load and manufacturer-specified test conditions consistent. AC Wh can still be a useful record of how much usable energy your household loads receive; just label it separately from battery Ah. There is no need to treat a protective BMS shutdown as the target for a routine check.

Disclosure: I am affiliated with Much Solar, and this reply was prepared with AI assistance and checked against the linked documentation, not offered as a personal test report. Our AC/DC battery measurement-boundary guide explains which inverter and auxiliary losses sit inside a reading. It covers efficiency comparisons, not a battery-capacity test procedure.

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