Try it on for 20 minutes on the highest setting and did not feel like it's getting warm.

I bought this for my wife who drives school bus. Opened the box read the instructions and plugged the battery pack into my computer to fully charge it per the instructions. Computer immediately crashed, total power down. I have never seen any USB accessory do that. Connected battery to vest and turned it on high, after about 2 minutes, could feel some heat. Went online looking for information about the battery pack crashing the PC and found nothing. She'll try it tomorrow and decide if she's going to keep it, or return it