Punx0r said:
It will be should be OK if the copper rests on the positive terminal of the cell - it's a hollow cap, so heat conduction to the cell interior will be poor. It's soldering to the bottom of cells that's the real problem (AFAIK). For soldering tabbed cells here, I put a card sticker (actually the centre pieces from the insulating rings normally put on cell tops) on the cell bottom where the tab was welded, laid the copper bus on the sticker, folded the tab over the top and soldered. That way the card insulates the cell from the soldering heat.
Excellent idea... Put the cardboard "puck" in the center of the top, lay the copper bussbar on top, fold the tabs legs over and solder.
The positive hollow cap seems it's no problem to solder, in truth.
I did a test yesterday to appreciate that the positive is no problem (not the best test know, but just to give a gross idea)...
I charged the soldered cell @1.0A and discharged it completely @ 0.5A to measure capacity with my Foxnovo 4S digital charger (4.20 to 2.75 volts). The soldered cells was one of the very DEAD cells in the batch that i scavenged from Makita packs (sitting at 0.03volt when scavenged...). I wasn't even planning to use it, apart from testing the spotwelded tab's robustness but pulling apart the tabs with plyers (I took the negative tab off)... Well I had another cell that was dead, sitting at 0.04 V (both dead cells were originally both sitting dead, in the very same Makita pack, at the same voltage and were both in parrallel with each other (2P). So I figured they should be of a comparable damage/loss of capacity from sitting at the low voltage).
Well upon discharge I got similar discharge capacity for both cell (soldered and unsoldered ) :
Dead cell (#114) with soldered bussbar on positive terminal : 1923 mAh
Other dead cell (#116) but no solder, pulled from the same Makita pack (#116 and #114 were in parallel) : 1928 mAh
Seem comparable to me. But when I tested the other 128 good cells with the exact same charger/parameters, they were, on average 2196 mAh (± 91 mAh, 95% confidence interval ; n = 128).
(Rated 2000 mAh ; typical 2100 mAh @ 0.2C).
I want to try some testing for the negative terminal in the future, as soon as I can find the time to do it.
Was thinking of taking a dead cells, open it with a copperpipe cutter, gut out the internal of the cells.
Then put a thermocouple/temp probe from the inside of cells, right at the negative teminal, put some thermal conductive paste to fasten there, right on the negative post.
Measure temp profiles from the inside while soldering at the same time... I it doesn't go over 50°C, it's golden.... I think the tip of my 100W welder is at least 250°C... so gotta work fast.