One of the best things to buy on a Prime Day deal is a good portable jump-starter. Because, whether or not you know it, you very much need one. Your need for a jump-starter is a lesson best learned online—because out in the world, the lesson will be a hard one.

In my case, I left my headlights on in a rural stretch of Delaware, where my phone signal was about as good as a war criminal’s reception at the Hague. The only thing that got me off that Delaware hill was a passing park ranger, most of an hour later. And what he had in his truck cab was a portable jump-starter the approximate size and shape of a bag of coffee. Basically, jump-starters are little power banks with clamps—which release their charge in sufficient amperage to get even an old V6 Land Cruiser over the hump on a completely dead battery.

The two best jump-starters I’ve tested are both on sale for Prime Day, each for about the same price. The Wolfbox 4000A Jump-Starter is the one with the most capacity. The NOCO Boost X GBX45, meanwhile, is the most compact. Both are on sale for 20% off.

Be sure to check out our roundup of the Absolute Best Prime Day Deals and our live blog for this last day of the event. The Wolfbox 4000A Jump-Starter maintains a charge. And it safely releases a charge, without the sparking risk or reversed polarity issues that can plague Amazon models. During my initial tests, I was able to jump a V6 Land Cruiser from dead 15 times before dwindling to half capacity. But it charged 10 more times since then, before dropping to 40%. It's an absolute beast—with a hefty 89-watt-hour capacity.

The NOCO Boost X GBX45 is a compact, high-powered jump-starter: just 9 inches by 3 inches. It’s small enough to fit in a purse or under almost any seat. (Don’t stick it in a glove box. Glove boxes get too hot in the summer.) And yet, the GBX45 also packs a reliably big wallop. My engine turned over faster when jumping with this NOCO than when starting on a full battery.