You went to a fun event that was out in the middle of nowhere and, as chance should have it, you’re the last person to leave. You go to start your car and find that the battery has died. It’s getting dark, you have no phone reception, and it’s supposed to be well below freezing tonight. Thankfully, you have a portable jump starter. These devices take up very little trunk space, and they could literally save your life. Here are a few features to look for when choosing one.