I recently finished up my build of 2x 6s8P 18650 30Q packs. Well mostly finished… Having issues charging one of these packs in particular. I built these to replace my blown up Turnigy 6s 20000mah lipo bricks, hence the identical form factor. This is my first battery build and I will be running these two packs in series as 12S8P.
Here is a pic of one of them finished and the other under construction.
Here is a pic of one of them before kapton tape and heat shrink. I used 0.2x8mm pure nickel strip double thick across all series connections and triple thick across the 2 main +/- busses on each battery. Then paralleled the two 4p busses together to 8P using 8awg wire. Should be pretty solid from a current carrying standpoint.
No BMS on these and just balance charging. You can see the balance lead configuration in the pics.
The second battery (the naked one in the pics above) has a series cell that won’t charge. The very first time I put this pack on the charger I think I cell 3 got up to like 3.8-3.9V. The other 5 cells went up to 4.2 as they should. I decided to test these batteries out on my board in my pool (my efoil dyno) anyway and run one almost complete discharge cycle. They worked awesome (compared to my turnigy batteries). They barely got warm and had lots of run time. I ran the packs down to about 3.2V per cell (cell 3 might have been at 3.0). I up them back on the charger.
Battery #1 charged up just fine with every cell charging evenly and finishing off at 4.2V per cell.
Battery #2… not so much. Cell 3 never got much above 3.6-3.7v. Cell 6 stopped at like 4.1V. All other cells were at 4.2.
Here is a picture of the charge progress an hour and a half in:
I also tried charging this battery on ch2 of my dual battery charger to see if it was something wrong with the charger. Same results.
Cell 3 just really doesn’t seem to be taking a charge too well.
Disclaimer: i did have a couple sketchy spot welds that may have gotten fairly hot. Maybe these damaged the cell somehow?
Has anyone run into something like this before? What might cause a cell (or a set of 8 parallel cells in this case) to not get up above a certain voltage?
Thanks for the reply. I did check voltages on every cell before assembling. All were the exact same. like 3.1 volts I think. Perhaps I damaged one during the build process? Any clever ways of inspecting these cells after they have been assembled into the pack?
I missed this before. It is likely you damaged a cell in the building process. The main question is how much? Let it sit for a while and see if that one group is dropping voltage and self discharging significantly faster than the others. If so, then you are going to need to just pull it apart. If it is hold its voltage then try this:
Hook up some alligator clips and charge just that one bad group all the way to 4.2v. Then run a discharge test to 3v using your RC charger and see what capacity you get. They are going to measure a higher ah when using the small discharge amperage of your RC charger, but if it is reasonably within the calculated spec for 8 cells in parallel then you know that they aren’t so bad that you will start a fire.
I would then charge it individually to the same or a little higher voltage than what the rest of the groups are sitting at, then stop the charge. Let it sit and then try to do a balance charge from there and see what happens. I will say though, for a brand new cells you are starting off more out of balance than I would expect.
From the photo, it may be that your cell balancing circuitry for that group is active during charging process, therefor it would keep the voltage low from the the rest of the group.
Your Balancing circuitry may have been screw during your savaging the old bad battery pack…
