Hi. I bought batterypack from last group buy. It was wery low quality and i took it apart. 2 series were drifting all the time and now i did measure cell voltages. I wonder if this is caused by bad welding or crappy cells? 8 of the 4 sets (8s4p) were all 3.82V per cell and these 2 were drifting like crazy.
When i connect them in parrallel it should balance itself all 4 cells?
don’t just connect drifted cells together in parallel. They will balance themselves but your have no control over the current flowing from the “charged” to the less charged cell. 0.1V or so doesn’t matter but I see you have 4.07 and 3.69V cells there - that is too much. Either connect only similar level cells together or discharge/charge them one by one.
That’s what thougt. Just need to find something how i can charge/discharge them individually.
What would happen if you just connect them together? You might have a very small undetectable micro spark but I’m guessing it won’t damage the cell at all.
As I said - you don’t have control over the current and the current might go above the charge current the cell is rated for.
A 25R is rated to 4A charge current. It can discharge a lot more so in a worst case scenario there could be more than the 4A flowing from the charged to the less charged cell. How much current there actually is I don’t know. But I don’t want to be the one finding out 
a simple trick would be a really thin cable. Just hook your cells up in parallel with it and you have kind of a fuse to avoid higher currents
Very good idea!
Or use a light bulb to discharge the pack to a similar level as the lowest cell
This is with a BMS too? Sorry to see you haven’t had much luck with this pack. Without a BMS I have only seen drift of about 0.12v between parallel groups. I only charge to 40 volts anyway so no issue for me. Plan to check the pack once a week from now on.
No bms have no effect on this. Problem is that 2 of 4-packs are actually only 3p because of bad weldings. So those two charge and disharge faster than the others. I haven’t had any drift (0.0V) expect those two which were always ±0.2-0.4V
Might be easiest to discharge to the point where they are all even and then repair the pack and start cycling from there?
Look that pic above. I balanced those 2 drifted packs and both balanced to 3.83 and 8 of them are 3.82 so i’m going to charge them with bms and then go without and check them every week.
Highly recommend picking up an imax b6 charger for like $20 just so you can slowly charge and discharge the cells. Doing this at 1A will reveal any broken cells. Nothing should happen if it is a good cell. It’s also possible to measure the internal resistance I believe.
Edit: PXSS recommends matching the cells by internal resistance so they don’t drift unevenly. i.e. they will be very similar in parallel.
I do own one but it’s broken. How does it reveal when battery is gone? I can measure resistance with my multimeter.
Internal resistance is a bit different. It’s a physical limit of the battery itself, so it effects the max discharge, how fast it discharges etc. Ultimately the IR is an important factor of how you should group your batteries. If they do drift, they will more likely drift as a pair and keep the same/similar voltage. But you should pick up another imax b6 or something like that.
But how does imax reveal broken cells? It just doesn’t charge or can’t balance or what?
Oh!!! Ok yeah, basically high IR = bad low IR = good match them alike. But also charging and discharging at 1A will reveal any bad cells because it is such a safe procedure (less than 1C) if any cells get hot or go el combusto on you are probably bad.
And if you want to go the cheap route you can also use a voltmeter and a resistor to calculate it.
Usually, a bad cell will have much higher IR than normal. I got a couple bad Lipo packs from HK once. Each had a bad cell that where below 3.0v and with very high IR
