Few doubts about top balancing, compression, capacity test and chargers

filippomasoni

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Hello, I've ordered 8 280ah cells from Amy, and I should receive them in the next 2-3 weeks.

I have a few related doubts that I couldn't find answers to. I've heard and watched many videos about compression and top balancing but got confused on a few things.

1.
I'm planning on compressing the cells with the classic threaded rod and plywood which will serve as my battery enclosure as well, with handles for easier transportation. I want to top balance the cells first but I would like to avoid uncompressing them at 100% SOC to rearrange them in series because in theory losing the compression is not good and would render the compression useless. I could arrange in the final configuration and make some wires to parallel them, (don't need to be big wires in theory because I'll charge at a max of 20A and it's only 10cm). Otherwise, I have to discharge them at 50% after top balancing and I'm not sure how I should do that.

2.
How should I check for capacity? Should I buy one of those capacity testers for single cells and run that for each? (will take forever) or just trust the quality of Amy's cells and run a capacity test with the BMS at 24V? If I let the battery go to 0% to do a capacity test, do I run the risk of damaging it since it's advised to only go to about 20%? Or is that recommended only for regular daily use and if done only one time it's not an issue?

3.
Do I need one of those variable DC power supply or I can get by with one of these: https://shop.gwl.eu/Chargers-6V-to-36V/Charger-3-6V-20A-for-LiFePO4-cells-1-cell.html?cur=1
I'd like to have a small power supply, that works all over the world (AC 100-240V 50-60Hz) that I could keep in the camper in case of need and I don't like those bulky chargers. I'd also get this: https://shop.gwl.eu/Chargers-6V-to-...attery-charger-27-6V-21A-600W.html?cur=1#tab1 as my shore power charger, that also works worldwide.

4.
Finally, I'll probably need to store the battery for 4-5 months before I can start the build in my camper. I know I should keep them at 30-50% SOC, what do I need to discharge them to that desired percentage?

Little off-topic opinion:
In reality, I'm not too concerned with compressing because even without it they will probably last up to 7 years and I'm sure we'll have much more efficient technology by then and they will be obsolete. But I anyway need to make an enclosure since they will stay in a camper so might as well make a compression enclosure. If they last longer it will be better for the environment as well I guess.
 

Dzl

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2.
How should I check for capacity? Should I buy one of those capacity testers for single cells and run that for each? (will take forever) or just trust the quality of Amy's cells and run a capacity test with the BMS at 24V? If I let the battery go to 0% to do a capacity test, do I run the risk of damaging it since it's advised to only go to about 20%? Or is that recommended only for regular daily use and if done only one time it's not an issue?
LFP cells are designed to be cycled from 0-100%, but manufacturers and forum members generally recommend more conservative limits for daily cycling. But a few cycles down to zero-ish shouldn't be risky or meaningfully reduce cycle life. That is with new grade A cells, when you buy through resellers, its difficult to know if that is what you are receiving (in many/most cases it is not, if you are paying a lot less than the price for verifiability Grade A), so for grey market cells, there is a bit more of a question mark.

Since there is almost no energy stored at the lowest part of the voltage curve, you could choose to use a somewhat more conservative upper and lower limits for the capacity test, and just know that the cutoff might leave a few Ah uncounted. Or you could choose to test the full voltage range 3.65V-2.5V and make sure the cells meet spec.

You could use a single cell capacity tester but most people just connect a large load to the assembled pack (with BMS protection) and discharge until low voltage cutoff. If you care about knowing each cells capacity, or plan to do any matching, the single cell tester could make sense.

4.
Finally, I'll probably need to store the battery for 4-5 months before I can start the build in my camper. I know I should keep them at 30-50% SOC, what do I need to discharge them to that desired percentage?
Whatever you use to discharge them for the capacity test could be used to discharge them to partial SOC for storage. Or are you asking what voltage you need to discharge them to?
 

filippomasoni

Solar Enthusiast
Joined
Jul 26, 2021
Messages
102
LFP cells are designed to be cycled from 0-100%, but manufacturers and forum members generally recommend more conservative limits for daily cycling. But a few cycles down to zero-ish shouldn't be risky or meaningfully reduce cycle life. That is with new grade A cells, when you buy through resellers, its difficult to know if that is what you are receiving (in many/most cases it is not, if you are paying a lot less than the price for verifiability Grade A), so for grey market cells, there is a bit more of a question mark.
Great, thanks, so one cycle down to zero is not a problem. The cells I bought from Amy of Shenzhen Luyuan should be Grande A from what everyone tells, but they are obviously not binned, and even though they are "matched" and they sent me the sheet of capacity test after shipping, I'd still like to do a capacity test myself. I technically have time to do so, so might actually do one by one.

I can't find the capacity tester link anymore. Do you know of a decent one to buy? I remember those resistive types with a CPU fan, they were priced around 20-30 euro, so if I have time I guess it's worth testing each cell and also good to have to discharge them to my desired SOC as you said.

Since there is almost no energy stored at the lowest part of the voltage curve, you could choose to use a somewhat more conservative upper and lower limits for the capacity test, and just know that the cutoff might leave a few Ah uncounted. Or you could choose to test the full voltage range 3.65V-2.5V and make sure the cells meet spec.
What would be a more conservative upper and lower limit? 3.5 and 2.7?

Top balancing is usually done by parallel connecting all the cells. What happens if I capacity test with the cells in parallel? I should get an average of all the packs right? Is there a risk of having some cells go much lower than others?
Could this method be used to discharge them to my desired 30-50% SOC ?

Whatever you use to discharge them for the capacity test could be used to discharge them to partial SOC for storage. Or are you asking what voltage you need to discharge them to?
According to the Luyuan Tech Basic LiFePO4 guide 30-70% SOC is 3.25-3.30V. Are those little resistive capacity testers with a fan capable of discharging to a set voltage? So I can let them go down to 3.28V for example and I have the cells ready for storage correct?
 
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