I built a 5S2P pack using 10 * 18650 cells (Callie's Custom, unprotected 3100mah /w tabs, basically Panasonic CGR18650CH) and have encountered a weird behaviour that I'm struggling to explain. Specially the 5S series pack reads ~18.5v for about 1 second but then will drop out (like continuity is lost). The problem is observed on both halves of the 5s2p pack, and indeed on 4 separate 5s packs I have built.
Now the obvious issue would appear to be a connection or pcm but I've been able to exclude these as causes. I don't use a PCM, but instead have wired a JST-XH balance board into the pack (using this I can see every cell reading perfectly). I've charged the pack with an Icharger in balance mode (works fine), but a similar turnigy balance charger doesn't like it (presumably something to do with the voltage cutting out, whereas the icharger might ignore this?). Using the icharger the cells charge and balance out perfectly (4.20v).
I've not been able to isolate it to a specific cell, as I can read 14.8v consistently if I take the reading from the top 4 or bottom 4 cells in the 5 series pack. I've concluded from this that I don't have a connections problem.
Cells were spot welded together. I then did a very quick/light solder onto the connecting tab to solder in the jst-xh balance wires to each cell junction.
Any help would be much appreciated, as I'm not super experienced building battery packs. Actually I'd rather buy the pack but can't find anywhere to buy this configuration. The 5S2P configuration fits perfectly in our circular underwater housings.