>>1211508>Even easier than an auxiliary battery and isolator is to just get a dual purpose starting battery.This is a really bad idea and will result in the battery being over-discharged, which will kill it and likely leave OP stranded and having to buy a new battery.
OP needs a few AGM (absorbent glass mat) batteries specifically for his power pack.
https://en.wikipedia.org/wiki/VRLA_battery