BatPhil writes – “Over the past few years, I’ve built up a few battery packs for myself and for other people. Most of them worked fine – in fact, one of the first packs I built over five years ago is still in service, working fine in a torch in the bottom of my cupboard. The big problem with soldering to batteries is that you tend to damage the plastic separator, and the cell seals. This – as you might guess – is not a Good Thing. In some cases, solder can splatter over the cell’s pressure relief vent. There’s a reason the datasheets make a big fuss about the vent – in an overpressure situation, the vent is used to release the excess pressure in the cell. Needless to say, blocking the vent with solder is never a good plan, unless you’re trying to get a Darwin Award, or you happen to enjoy watching your battery pack undergoing rapid, uncontrolled self-disassembly.” Link.