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...

There's a chance that the 2 packs were not in the same state of charge?

There's nothing wrong with combining packs like that regarless of voltage. I am curious how you formed them and what the capacity was. To be sure they should all be discharged to the same point (or charged to the same point) seperately.

That's the only thing I can think of.

I started by running a discharge sequence on the charger on both packs separately, then connected them together and ran a charge/discharge/charge cycle. Now, they did get unplugged from each other and sat separately for about 3 weeks before I connected them to test in the plane.

Would one pack 'dump' faster than the other causing a big difference in the state of charge?

Not sure what you're asking with the forming question...one pack is a 8x8 cell brick and the second is just the 4 next to eachother (lengthwise). The combined 'pack' is just the 8x8 brick with the 4 cell pack sitting on top and velcro'd into place. The capacity is 3300mah.

I was asking what you meant by "forming" the pack, but I wanted to know if you knew the capacity during the charge and discharge cycles?

Hmm 3 weeks may be a long time for the cells to set, but it still sounds as if they are at a different level of charge. It wouldn't matter if they were hooked together or not.

I would try to discharge each cell individually or to do a 300 ma charge on all of them for 16 hours, that should equalise any imbalance.

Hmmm...If I remember correctly, it was about 3100mah on the cycles.

Ok, will try to equalize and will let you know how it goes- MANY thanks for the help!

I'm guessing you think the batteries are still viable after their 'venting'?

He might, but I don't. Once they start goozing like that, they're not much good IMHO. The seals are breached, so the good stuff can get out and the bad stuff can get in...

My only theory as to why hovers around the assembly of the packs. Did you build both packs yourself? Are you fairly well-versed on building packs? If you bought the first pack assembled, then made the second one yourself, and this was your first time building packs...

I've built at last a dozen packs- all built from loose cells (as were these packs)...so I'm pretty versed in battery pack construction. So far this is my first 'leaking' pack...

Your cycle data seems ok for that many cells. If you had the equipment to do a discharge test and check all or each cell for voltage would be a good indicator if 1 cell was bad. You may also try temping each cell during charge and discharge. If all are equal and all maintain a good voltage then I think they are good to go.

That is a lot of cells to keep together and keep track of them all series together.