risky or what

[brin]

last night my mate came round to sort out some 10s batteries he had three flightpower 10s 4000s one cell on each one had puffed up so i removed one bad cell and replaced it with a good one from one of the others .didn't finish until 1 am. not bad as i usually get up at 6 don't u just love friends anyway he has had a few flights today and says it worked very well .one issue i had was the the joining strip i just could not get any solder to stick to it .so i crimped the plates together and epoxied them together, the other end was no problem i am not sure why it would not solder i used /lead free/lead/silver solder,i went from a electric solder iron to a gas one then onto a small blowtorch even used different fluxes but still no go any ideas all.

cheers
brin