How do I weed out/group cells to put a pack together?

[BarkMau]

I have a bunch of A123 20 amp pouches, a Hyperion 1420, and 2 headway BMSs. Also, Agniusm kits on their way. I haven't been able to find information on how to capacity test the cells or other specific procedures prior to battery building. My cells arrived from China holding 3.27v each. So far, all I've done is charge each cell solo 95% and I'm observing if/how they hold the charge.

 

[dnmun]

if they are from the same lot, they are essentially identical.

 

[BarkMau]

Actually, I received what seemed to be 2 different batches. The differences I noticed: The one pictured on the right says "Made in USA" and "a2" on the center. The one on the left has a sticker "900mAh3.7V" and "a1" toward the center of the pouch. Other than that, they all read 3.27V.

 

[dogman dan]

Theoretically they'd be identical. But maybe the frockhead in the factory snuck a cigarette while he made one of em.

Ideally, you'd have a wattmeter to test capacity. I'm not expert enough to know how to measure internal resistance. don't the better RC charges do that? I don't think my cheapo ones do.


With nothing but cowboy tools, you could discharge them with a set discharge rate. For example, a light bulb or whatever that will always be the same for all of em. Then time the discharge to a particular voltage that is fully discharged. The ones that take longer have more capacity.

Any that get hot rather than warm like the others would obviously have really bad IR. Crude, but if they get noticeably hotter than the others they would be really really bad ones. It will take a higher discharge rate of course to even get close to getting em warm.

Break in the cells with some easy low dod cycles before you test em of course. Testing the capacity before they develop thier full potential would tell you nothing except which cells needed the break in most.

 

[ohzee]

when I built my batteries I first stacked the cells together in parallel and ran a few charge/discharge cycles so that
they were all in sync before building my pack. I think that's one of the first things you should do before you start to
build your battery. I used big binder clips to do this and it worked pretty good.

let me know if you need more details on what I am saying.

 

[GMUseless]

LiFe might be different, but with the sketchy quality of LiPo from Hobby King, I test every pack on a cell level before building into a parallel group.

I do a few things specifically. I first do a light cycle on the pack, maybe charge up to 70% (easily set on the Hyperion 1420) and discharge to 50%. I'm looking for gross errors with cell voltages.

If that looks good I do a 95% charge, resulting in something like 4.15 cell voltages, followed by a disharge to 30%. Here, I'm looking at cell spread. Does one cell drop faster than the others? Does one hit the cliff before 20%?

Next I'll discharge to the cliff. Here I'm closely watching the cell voltages and noting if one drops off way early. I.e. is one dropping to 3.5 while the others are at 3.7. This would have a significant impact on usable pack capacity.

Then I'll recharge back to 95%. This time I'm watching if one cell charges much faster than the others. If a cell that dropped in voltage early is now charging faster than the others, its a good indication of a weak cell.

Once I get them charged back up, I let the Hyperion balance the pack. Then the pack gets labeled, all cell voltages get noted, and the pack gets set aside for a couple of days. After that I'll check the voltages again to look for 'leakers.'

Any packs with leakers or signicantly weak cells will get set aside (then most likely mailed to Amberwolf:he loves separating the good cells from the weak ones! )

Then, I parallel up the mains for the packs going into parallel groups. But I don't parallel the balance leads at this point. I'll let this group go through several use cycles, each time keeping a close eye on each cell after discharge and charge. I still charge each pack solo.

Finally after 5 - 10 cycles, once the cells are proven to be good, I charge and balance each pack. Then I parallel the group at the mains and the balance taps and consider it a solid group.

 

[dnmun]

I have a bunch of A123 20 amp pouches, a Hyperion 1420, and 2 headway BMSs. Also, Agniusm kits on their way. I haven't been able to find information on how to capacity test the cells or other specific procedures prior to battery building. My cells arrived from China holding 3.27v each. So far, all I've done is charge each cell solo 95% and I'm observing if/how they hold the charge.

like i said they are all identical. you start the build by shorting all the cells together to bring them to the same voltage. since they are identical that will be pretty close to begin with.

you can charge them up while tied together with a single cell charger but that is not essential. your bulk charger is more capable than the single cell charger.

build the pack and add the sense wires. do not attach the sense wires to the BMS until you use your voltmeter to verify that each wire is one cell voltage higher than the previous one. in other words, make sure the sense wire plug is correctly wired before plugging into the BMS. this is critical.

attach the sense wire plug to the BMS, then begin charging. if it does not charge immediately, measure the gate voltage on the charging mosfet. if it is not 10v or so then something is wrong and has to be examined.

after charging, discharge into a load, like a space heater to make certain the BMS functions in discharge. if it does not discharge, measure the gate voltage on the output mosfets and determine why it is low.

do a few short charge discharge cycles to break in using the dummy load. after these break in cycles, you can then put the wattmeter on the pack and evaluate full capacity.

the headway BMS will stop at 2V so keep track of the cell voltages as you approach the 2V level for the lowest cells, and keep track of your Ah reading at this point too because it will be lost when the pack trips off at LVC.

immediately begin charging from the LVC. keep track of the cells which reached the LVC first. you wanna know in the future if they change when you do the capacity test again, so save the numbers.