You'll want to get the best iron you can get with the widest tip you can get. An adjustable temp iron like the Hakko 936 that I use is nice but not a necessity. I used a Hakko 80 watt iron for a long time. It's cheap, comes with a big wide tip that's perfect for doing batteries. The key when doing batteries is to only have the tip on the cell for 1-2 seconds. You don't want to heat up the cells too much or you can damage them.