I bought a cheap USB solar power bank off eBay about 18 months ago to use in emergencies when I need to charge a phone, torch or radio etc. I only paid about £10 and it worked pretty well.
It's been in its case in the shack unused for about 6 months and when I was having a clear out the other day I found that the lipo cells inside had swollen. They had pushed the solar panel off the top of the unit!
Luckily I caught it, these things burn well!