DrMichael I have no idea! My best guess would be some sort of LIPO pouch but I don't really want to tear down my unit just to answer this question. If you really need to know this, I'd suggest asking Customer service or the manufacturer, rather than asking in a user forum like this. In practice, I keep the unit plugged in (micro-usb) while using it for PCB inspection, etc.so I can capture recordings on my desktop and just push the power button to use the internal battery when I'm up trying to support my aging eyesight when reading component markings on ICs etc. Why do semi-conductor manufacturers think it's smart to have barely legible markings? One day these designers will get older (hopefully so, when you consider the alternative!) and Karma will kick in. Agism has already taken over the graphics design sector. Let's keep it out of electronics!