and again, maybe third time lucky!
reposted, orig> I recently replaced one of the two dead Ryobi 14.4v batt packs which
I've never _re_built packs, but I have soldered them together.
The best way I know is to use a hot hammerhead iron - that's a soldering iron with a bit with two working ends, so you heat both cells at once.
The bit is about 1 inch long and 1/2 inch diameter, and looks like this:
| iron | || | ___||___ | _/ \_ | |_ bit _| | \________/ |
I have turned a few from copper, it's not hard - unfortunately I don't have any now, and I can't get back to the shop before mid-Jan (long holidays!).
You really need at least a 40 watt iron, although it can be done with a smaller one. Wait for it to get hot
People who make up a lot of packs tend to put the cells in the fridge before soldering, use 60/40 tin/lead solder rather than the leadfree type (naughty, but then the cells are full of cadmium ...), and make up a frame so the cells slide easily into the right position. You heat up the ends of two cells at once, make sure they are wet with solder, remove the iron and quickly slide one cell to meet the other.
Then cover with heatshrink for mechanical strength.
Never had a problem once I figured out how to do it - and you could of course practice with the hootered cells first.
It might be worth while googling "hammerhead soldering cell" , some good links there.
Below is a post I wrote in answer to a similar question, not an immediate answer but should have some relevant tips.
However, Tim's point about reversal during discharge sounds relevant. I've always been a good boy and never tried mixing cells tho'. Perhaps you could try to find the set of cells whose capacities match best.