I have a 16 cell pack of GP3300s (actually Epic 'monster metals') that I've used for about a half-dozen flights or so with no problems what so ever. I just completed an application where I need to use a 20 cell pack, so I built a 4 cell pack out of the same type of batteries to connect in series with the 16 cell to 'create' a 20 cell pack. I cycled both packs connected together to form the big pack a couple of times on the charger, then put them in the airplane. Went to test in the shop, and after about a 10 second (if that) motor test, it sounded like some of the batts in the 4 cell pack were 'boiling', and I noticed some leaking fluid around the positive end of 2 of the cells.

Did I kill the little pack? What did I do wrong? I've used 'combined' packs before, but never with such a large voltage disparity...I'm guessing that's bad.

Any hindsight insight would be great...