Let's say I want to have a spare car battery in my car.....

[sota]

the jump pack I have has a car cig lighter adapter to charge it.
I only remember to plug mine into the wall once every couple of months, and it's never even had a 1 (of 4) bar degradation.

[GTI19403]

VOXX POWV35 is a nice option for what you're looking to do. I have one in the car we use to leave at the airport.

[JaySmith]

I had one of those small lithium packs in the glove box for a year, then my father calls me over and asks for a jump for his motorhome. I took my truck with dual batteries, but for giggles, i took the battery pack. The motorhome's v10 ford engine didn't turn over, only clicks. I hooked the battery pack up and sure enough, it fired right up.

[HanzMcFeely]

Dual batteries under the hood with an isolator is the easiest way to go. Pretty much just need a length of any old wire to bridge with. The isolator/charge controller sets em up in parallel and keeps the second battery full. Cuts in/out at 13.7v. I’ll snag a couple pics of the setup on the suburban for you. I forget what i paid for the isolator, amazon deal recommended by streaker

[RockIsland]

Dual batteries under the hood with an isolator is the easiest way to go. Pretty much just need a length of any old wire to bridge with. The isolator/charge controller sets em up in parallel and keeps the second battery full. Cuts in/out at 13.7v. I*ll snag a couple pics of the setup on the suburban for you. I forget what i paid for the isolator, amazon deal recommended by streaker

How is that easier than throwing a Li-ion jump pack in the glovebox?

Also, 99% of passenger vehicles do not have any accommodation to put an extra battery under the hood. Heck, many don't have enough room for one.

[JenniferG]

[Carson]

As others have said, get a NOCO. I have the Boost X GBX75. Also bought the NOCO 10 charger.

You get what you pay for. The NOCO is expensive. I had a couple much cheaper jump packs that didn*t last one use.

And also as been said, an unsecured, charging battery in the passenger compartment is dangerous.

[HanzMcFeely]

How is that easier than throwing a Li-ion jump pack in the glovebox?

Also, 99% of passenger vehicles do not have any accommodation to put an extra battery under the hood. Heck, many don't have enough room for one.

Easier because its there, more convenient because never goes dead.

With some simple backyard fab work, i bet one could make 99% of vehicles accomodate a second battery

[JDshoots]

There is another type of starter out there they call “Super Capacitor Jump Starter”.
Amazon has a bunch but none from companies I’ve ever heard of. The advantage of a starter based on capacitors is you can charge them up with any voltage. So even you dead battery without enough snot to start the car can slowly fill the capacitors enough for them to be dumped quickly and with enough current to do the job. I have been meaning to get on just haven’t looked into it further.

[Emptymag]

Mrs EM bought something a year or so ago. It's sitting in the seat pocket on the back of the driver's seat.
Battery is nearing it's end of life, so maybe I'll get to try it out.