Assuming the M12 CP1.5 battery pack, it’s probably three 18650s. Specifically, it’s probably three LG HB series 18650s, which handle high burst loads well, but hold only 1500 mAh. A single Sony VTC6 holds 2/3 the energy of one of those packs. Wait… why am I speculating? Youtubers tear down power tool battery packs on video all the time, and someone did that one. They’re Samsung 15Ms, which are a little worse than HBs.
Anyway, short runtimes are fine for most field repairs, which is the whole point of something entirely self-contained. Spare batteries can extend it indefinitely, but a battery soldering iron is probably not what I’d pick for extended soldering sessions.