https://www.amazon.com/Sunbeam-Heat-Rechargeable-Cordless-Back/dp/B004VWX8HU

I purchased this to help me at work. It uses rechargeable batteries so I basically bought two additional sets of rechargeable batteries so I can rotate them out. On the highest heat setting the batteries last for a few hours.

It's basically a velcro belt and I can use it for my terrible low back pain or cramping, depending on the day. It gives me the ability to walk around and not have to be plugged into the wall.

Hope you find something that works for you!