I have an Empi Select
http://www.empi.com/empi_products/detail.aspx?id=146 0-60 milliamps with 2 AA batteries and haven't replaced the batteries yet. Because of the numbness, I've cranked it up to 50 for a couple hours at a time. I started to get a rash and thought the strength was too much so I only put it to 10. On my back I can stand 4.