There was an old payload I flashed to it at one point, it would always boot whatever payload.bin was on the root of the sd card (I had to boot directly into atmosphere as hekate would mess with my extra ram oc at the time).
It was from a cancelled project called dragonboot or something (it was supposed to be a rcm loader and jig that looked like a gamecart and would sit in the cartridge slot when not in use). The payload could be flashed to any rcm loader though so I used it on this one and I never needed to reflash it again, not sure if someone still has the files, or something similar, but that was the best because I could just put the new payload.bin on the switch sd card whenever an update was needed, instead of needing to reflash each time.
The original battery was 150mah, which are really cheap to get hold of, you just need to get one that is physically no bigger than the size i mentioned earlier.