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 on the internet—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 percent 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.

Best Capacity: Wolfbox 4000A Jump Starter

This Wolfbox jump starter does two things well. It 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 percent.

It's an absolute beast—with a hefty 89 watt-hour capacity. But it's also able to maintain that charge as effectively as any other jump starter I've tested. If you press the Boost button, overriding the low-voltage safety cutoff, this Wolfbox can juice a starter battery that's as near to dead as a healthy battery can get.

I wouldn't use it as a battery maintainer, and it's not as useful in sub-zero temps. But for most of the year, this Wolfbox is all I need to ensure I never find myself on a roadside waiting for a jump. The Prime Day deal is modest, 20 percent off. But the starting price is also modest. At $100, it'll buy you 10 times that value in peace of mind.

A Compact, High-Powered Jump Starter: NOCO Boost X GBX45

This little 1250A Boost X jump starter, from well-regarded brand NOCO, is the most compact jump starter I recommend, 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.

This said, the capacity is lower than the Wolfbox—so you'll want to charge it back up between uses. Note also that the cord length and clamp size are a bit short. This did not cause a problem for me, but it may be an issue on some car models. It's a great jump starter for its size—on sale for about the same price as the Wolfbox.