An important concept, when talking about spot welding batteries, is to realize that the welding electrodes used are arranged close together and applied to the top of the tab.
This ensures that the welding current flow is very localized at the "head" or "foot" of the battery, only through the tab and the relevant battery terminal cap The welding current does NOT pass "through" the battery at all, from one battery electrode to the other, but ONLY along the very short, closely-spaced spot-weld/solder tab.
The net result on the battery electrodes is no effect, and no electrical current flows through the battery at all during welding, only through the millimetre-or-two separated spot-weld area of the two pieces-to-be-welded.
I have terrible nightmares of people reading thread like this for the first time, grabbing using 2 large crocodile clips and passing 300 Amperes though a tiny AAA cell from one end to the other. This would be extremely dangerous, so I think should be mentioned because I've seen it attempted before, with disastrous results.