OP here, thanks everyone for the help and advice!

We got one of these when our battery was dying. It was hard to keep charged -- you can really only start a car once with it, and then you've got to charge it again. But if you keep it in your car in the summer or winter, it won't hold a charge. It should be safe to use, but may or may not actually do what you want it to.

It'll work. Just charge it up.

It should be fine, it will just need a long charge before you use.

I have one I bought off Amazon. I used it a couple weeks ago to jump a dead battery in my junky car (it has a bad alternator that needs to be replaced...just haven't had time to do it yet). It worked fine.

I will say that with the lithium ion jump start, you only get a couple chances to start the car before it dies. Pulling power from the battery into the car rapidly depletes the lithium ion battery. For example, I used it twice on my completely emptied car battery and it went from 100% charge to about 60%.

So if you're having bad issues with the car battery, don't expect to get more than a couple tries before the lithium ion jumpstart battery is depleted.

I bought an Xtreme Boost Lithium Ion JumpStarter from costco 3-4 years ago. I forgot about it until I needed a boost today (I didn't use it).

How do I know if it's still good or safe to use? It has sat untouched in the box since purchase. I'm charging it right now. Anything else I need to know? I'm unsure what to trust from google so am asking here. TIA!