Miniature Jump Starter

$20 is a great price. I paid ~$60 on sale at Harbor Freight a few years ago. They do work if the engine isn't too cold or cranks too long. Definitely not the power of the lead-acid battery, but better than nothing. One oversight is that most turn off the "crank" mode after about 1 min, which confuses many people and also makes it hard to use as a general 12 VDC source to test things. Another problem is their capacity decays over time. I recall something like 50% left after 3 years or so. But, large jump-packs with lead-acid batteries aren't known for lasting much longer either, indeed I have had a lot of car batteries fail within 3 years lately, and a few from Autozone within just a few months, though the prices continue to rise.