hey, i dont know if this will help, but in my inverter AND inside my dashboard are low-voltage cutoff switches, so my chargers runs even when my devices are charged, (including for my jumpbox powerpack)
however i recommend this since you commented "idiot moments:" and during these times the dashboard will ensure to always have enough power avail for the starter, so the jumppack stops charging at a low battery then resumes after engine start,
my inverter is belkin300watt, idk price thou..
idk if this is chaper or simpler, definately less eductional,