Found this on hackaday.com, thought it may be useful for some DIYers.
The Poor Man's Battery Tab Welder
http://www.philpem.me.uk/welder/
(exerpt)
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.
In industry, resistance welding is used instead of soldering. Not only are the welded joints smaller than solder blobs, but they cause less damage to the cell. The only problem is the cost of resistance welding equipment. A low-end resistance welding machine can cost upwards of GB £2,000.
...
That's all well and good for industry, but when you only want to build up a few battery packs for an R/C car or re-cell your laptop battery, £2,000 is a lot of money. Thankfully there's a type of resistance welder that can be built for under £100 (well under £100 in my case). It's called a capacitance-discharge resistance welder.
(/exerpt)
Article: Poor Man's Battery Tab Welder
