Absolutely right. A 25W iron will be nowhere NEAR hot enough to solder batteries, & in addition, you also need a fair amount of surface area in the tip so you can transfer that heat quickly & efficiently. That's one of the reasons why those 80W Wellers work, they have nice big & thick tips that allow for FAST heat transfer(the more area you can get the better, within reason, you do have to be sure you can control it, don't want the tip to touch anything else). And coupled with the other techniques mentioned(using some flux, scuffing up the surface of the cells, tinning everything, etc.), it should make the soldering go a LOT quicker, which is the best thing for the cells. That's why many have damaged their cells when connecting them, with less heat & less iron surface area, they end up holding the iron on too long, & that heats up the cells' interior, risking damage.....