I bought one of these "car jump start battery packs" for my wife's Yukon last January. I charged it when it came and stuck it in with the jumper cables and stuff.
This weekend we went skiiing, and I left the keys in the ignition. By the time we got back to the car the battery was drained. I could have flagged someone down for a jump, but I remembered we had the battery pack. I was skeptical that a) it would be charged after sitting literally for 12 months and b) that it would really have the amps needed to turn over the 6.0L LS.
I attached it to the battery, waited maybe 30 seconds, and the damn thing started on the first try...
The status lights indicated it was probably half charged, so I charged it up the rest of the way on the ride home with the cigarette lighter adapter. Very impressed with this thing!
https://www.amazon.com/Starter-GOOLOO-18000mAh-Portable-Battery/dp/B0748D8KT6/