I've been using a "Portable Power Station" that I picked up at Costco, for a few years. The charger is built in, and I keep it plugged into the wall until I need it. Up until last week it would start anything that was dead, from my lawn tractor to my 429 Cobra. (Last week it failed to start the Cobra (dead battery), so I think it has reached it's end.) It has 2 12-vole outlets, and a tire pump, but I've never used either.
At the same time, I keep my Cobra battery on a Deltran Battery Tender whenever it is parked. It usually kept the battery ready to start the car, even though I may go months between runs. The Cobra bettery finally died after 5+ years, which is pretty good for rarely being used.
I'm thinking of getting a waterproof Battery Tender for my Mustang, since it sits outside for weeks without starting.
Gregg
2001 Proton Yellow #1379