I’ve had one of these Li-Ion battery jump starters for a few years and it works very well.
I’ve never had to jump my vehicle, but have used it on several others. It can also power a notebook PC or charge your cell phone. The only disadvantage is remembering to bring it in and recharge it once in a while.
There are jump starters available that have no battery, but use a super capacitor with a booster circuit. It can be hooked to a low battery, but not completely dead, and charge itself to a higher voltage and so be able to start the vehicle.
Here’s one: http://a.co/d/6B85H3J
The advantage would be that you don’t need to keep it charged, but on the other hand you can’t charge your phone etc.
Anyone have experience with one of these? Good or bad?