This tiny 3000-amp A3 Gooloo jumpstarter, small enough to fit in a glove box, is the lowest-cost jumpstarter whose reliability I can personally vouch for. The Gooloo A3 is nearly half off right now for Amazon Prime Big Deal Days, available at the eminently reasonable price of $70 ($60 off). This little A3 portable jumpstarter, which in many models also includes a tire inflator, is the model that's served me ever since the day I found myself stranded on an isolated stretch of Delaware, apparently impervious to cell phone signals. I was saved by a lone passing truck, whose driver happened to have a little portable Gooloo jumpstarter. I drove directly to an auto store, bought this little A3, and never looked back.