I actually just bought this 9000mah hand warmer off amazon and used it once last week to see how it performs. I'm actually impressed. I ran it on low for almost 3 hours and that was plenty of heat. Then i charged my phone from 40% to 100% and when done, the unit showed 1/3 battery life left. I think that's pretty good.
https://www.amazon.ca/9000mah-Electr...xpY2s9dHJ1ZQ==