I realize this is an old thread, but I keep running across these old discussions and finding I have suggestions or better info to offer
I dont like them, potential fire hazard and they basically will jump maybe 5 cars and you are done in most cases. If the battery is totally stone dead it may not even jumpstart it. They are more handy to charge your phone or something imho.
Ive worked in the towing and repair industry a LONG time, everyone and I mean everyone uses jump n carry. Id say about 90% use the standard JNC660 and the others might use the next model up. In a given day I will jumpstart 10-15 vehicles on the road and not need to charge the pack (although you can charge it off a running vehicle). My own personal JNC660 https://10carbest.com/best-jump-starters
I left sit for over a year and then finally it was to weak to start a V8 and charging it off my truck for 5 mins it fired up the V8.
We are talking thousands of jumps on some of the ones at work, zero failures. BUT its a different concept. It can power stuff, can jumpstart everything short of a fully stone dead diesel (6.6 duramax is no issues, forget a 7.3, 6.0 or cummins however unless you have a pack on each battery).
For most its overkill and too large, Ill happily use it however.