270Mah is all you need to puff a cell

[dodgey]

Well, I repaired a 4s4p pack, but it seem I was not thorough enough in balancing all the cells individually before reassembling (I inserted some new cells). One cell out of one of the 4p banks puffed.

I'm now in the process of charging each cell (all 16!) individually before I start again, and looking at the charge figures - the bank of 4 that one cell puffed up in, were all 270 Mah in charge over all the rest (all the other 12) of the cells.

i.e. I am topping up each cell, and all of the are taking about 340 Mah, Apart from the "bad" bank which is taking 69 Mah till full. This leaves me with the conclusion that the "bad" (overcharged) bank reached maximum, and then the charger carried on putting another 270 Mah in the whole pack - power that most of the cells needed, but not the "bad" bank which was already full.

It seems 3 of the 4 cells in the bank coped with it, but one didn't - it puffed.

So perhaps, if my theory is correct - it can take as little as 270Mah over maximum to cause damage.

Of course the real figures are in volts, but Mah is all I have to go on.

[Rick Rotorhead]

Dodgey, I spoke to Mike Redmond at Arourra in Sheffield when I was reaserching the Cellpro balance charger, which they distribute. I can't remember the exact figure he gave me but 270 mah seems about right (about 10%). Lipos are more demanding in accuracy of use/charge cycles than even the info in these forum posts indicate. You really do have to be Anal about that overcharging and discharging. Mild abuse may not trash packs immediately, but will likely reduce their life span drastically. To give you an idea of how seriously it should be taken, Mike at Aurorra explained that if you charge indoors in the warm then take the packs outside and its cold it can really harm the packs. Why? because in the warm they take more charge in than they can hold when they get cold outside. So, indoors they get a correct charge, but when they get cold they become overcharged and die (or get ill). Now that can't be by much can it? So, see how demanding these batts are. I've got two packs with drastically reduced capacity because I would run them frequently until my Twister/Blade CP thing could not develop enough power to lift off - that wayyyyy too low discharge. Mike reckoned that packs treated exactly correctly (using their cellpro gear natch!) should certainly be good for 500 cycles. Badly treated packs and those cheapo packs that make great 'C' rating claims that they demonstate in practice are likey going to be on the way out after as little as 50 cycles. Thats another point, going under voltage in flight due to a pack not living upto its 'C' rating will also damage cells. Mike reckons a lot of cheapo packs claiming 20 C burts are just not upto it, but the makers monkey about with cell chemistry to give a better voltage reading than what it should really maintain under load.

[dodgey]

doesn't suprise me Rick - interesting reading!

My mistake was charging banks of 4P cells and treating them as 1 unit - assuming they were all in balance with each other.- after dicking around with soldering, mending tabs etc. SHould have separated every single cell and started from scratch, which is what I'm doing now. Fortunately, I have a few spare cells so I can do this, otherwise it's £220 down the pan!