I bought one of these (DBPOWER 600A 18000mAh Portable Car Jump Starter). I've used it a couple times to jump other people's cars. Also used it to power a laptop and a hotspot for a few hours during power outage.

Works well. It goes on sale for ~$40 every now and then.

I have one of these (DBPOWER 600A Peak 18000mAh Portable Car Jump Starter) charged up and in my "trunk". Actually the little alcove on the left of the trunk. Doubles as a massive portable battery for tablets and phones as well.

Edit: got it on sale for $59 around xmas time.

this.

i charged it up for a few hours and i have jumped three cars with it within a year. it would make a great gift for someone that owns a car and knows how to open the hood.

I got this one about 2 years ago. I absolutely love it. Charge it every 6 months or so.

https://www.amazon.com/gp/aw/d/B01D42TYFC/ref=mp_s_a_1_1?ie=UTF8&qid=1515620888&sr=8-1-spons&pi=AC_SX236_SY340_FMwebp_QL65&keywords=dbpower+600a+peak+18000mah+portable+car+jump+starter&psc=1

Personal testimony, I hadn't charged mine in about a year, when a coworker asked me if I could jump her car. Turned it on and the display said "49 %" charge and it jumped her V6 TrailBlazer just fine. It was dead after that though. Recharged it when I got home and it was good to go