Capacity test setup - campervan battery bank

[krishkrash]

Hi all, I've got 3x Valence U27-12XP Lifepo4 batteries wired in parallel in a camper van. They were over-discharged over a year ago (red light is still flashing) but I was able to revive them and continue to use them afterwards. I'd like to do a rough capacity test by charging them fully, then running a load until I hit some low voltage cutoff level while measuring the total expended capacity with a battery monitor. A few questions:

1. Any issues doing the test on the full battery bank instead of on each battery individually?

2. What low voltage cutoff should I use to represent 0% useable state of charge? 10V?

3. I don't have an external BMS unfortunately so I will probably have to manually stop the test once I approach the low voltage cutoff. Any issues with this? Might have to monitor closely for several hours...

4. I know it's recommended to run the test at a certain discharge rate (0.2C) so if I expect to have maybe 350 Ah total capacity, that would be 70 A. I think I could get close to this by just running our camper induction stove at around 800 W setting (800W / 12V = 67 A). Does that sound reasonable?

Thanks!

 

[chrisski]

I'm a fan of only doing capacity tests if you will do something with the information you get. If you are going to use these batteries regardless of the results, than I'd recommend skipping the test.


Seems you already have experience waking it up.

1. Any issues doing the test on the full battery bank instead of on each battery individually?

The most useful data will come from an individual test. If you're goign to do this, than IMO do it all the way with three individual batteries.

2. What low voltage cutoff should I use to represent 0% useable state of charge? 10V?

That's what I test mine to. This is a battery not a cell test so if one cell reaches 2.5 VDC before the other, the BMS may shut down before all cells are discharged.

3. I don't have an external BMS unfortunately so I will probably have to manually stop the test once I approach the low voltage cutoff. Any issues with this? Might have to monitor closely for several hours...

I am not very good at watching stuff for hours at a time to stop it at a certain point. If you are then, fine.

4. I know it's recommended to run the test at a certain discharge rate (0.2C) so if I expect to have maybe 350 Ah total capacity, that would be 70 A. I think I could get close to this by just running our camper induction stove at around 800 W setting (800W / 12V = 67 A). Does that sound reasonable?

I'd monitor the DC from the battery if you do this. An inverter normally has a 15% loss, so your inverter may push 800 watts to the burner, but the conversion losses pulls ~940 watts from the battery. The inverter also won't have a steady efficiency loss, so this won't be the most scientific way, but it will work.

With discharging at .2C, the burner will likely be 800 watts at first, so starts at 67 amps at 14 volts, but will go up to 95 amps at 10 volts. it won't be a true ,2C discharge rate, but will give you an idea.

Here is the load tester I used for individual cells for batteries:

https://www.aliexpress.us/item/3256801272010454.html?gatewayAdapt=glo2usa4itemAdapt

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If you do this, a shunt will be a more accurate battery monitor.

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[krishkrash]

Thanks for the response and guidance! Everything you said for #2-4 makes sense to me.

I am planning to sell the van in the near future and would like to run this test 1) so I can set/tare the battery monitor 100% SoC capacity level (Ah) and 2) so I can have more confidence disclosing that the batteries are still functioning reasonably well to potential buyers.

Ultimately , the primary useful information I would like to find out is: how much useable capacity is left in the system of 3 batteries in parallel? Given that goal, I'm hoping it makes sense to do the following (even though it may introduce some error and not be a "clean/pure" capacity test):

• Run the test on the full bank instead of each battery individually, seems more representative of how the system will be used
• Run the test with a somewhat representative load (i.e. will roughly try and hit 0.2C but fine if it varies over the coarse of the test)
• I think your first comment is implying that running such a test is a risk in the first place so maybe I could mitigate that by running the test down to something higher than 10V cutoff (maybe 11V or 12V - per manufacturers charts, 12.5V = 10% SoC). Then the capacity value I have can be treated as conservatively low and I can set the battery monitor with that value knowing that the system may have slightly more.

Let me know if you have any additional input on that plan. Would be particularly interested to learn more about what kind of permanent damage or long-term degradation occurs from an over-discharge event. From my experience, the batteries were able to be charged again and seem to be functioning reasonably well but not sure if they may be just slowly degrading further over time.

The AiLi battery monitor I have uses a shunt so I think it should be sufficiently accurate to measure total discharged Ah and the instantaneous current out at any given point in time.