Choose by the tip size as much (if not more) than the wattage. For example, the Weller Soldering Gun is 120w, but the tip is so small that as soon as it touches a cell all the heat is pulled away. The Weller suggested by Desolas has a lower wattage but a large tip.
The trick with cells is to have a lot of heat stored in the (large) tip so that when you put it on the cell, it heats the joint and doesn't just drain away. Also, using a paste flux (NOT a liquid, acid flux) ensures that the solder flows fast and all across the joint - another way to minimise the time the tip needs to spend on the cell.
Weller make some great irons for the job, of which this one
is the pick of the bunch. It is designed for stained glass leads, which suck out heat like a hungry tiger! HTH