Originally Posted by bertrik
For the rockbox project, I've looked extensively at the hardware of various sansa players and never saw any evidence for a battery gauge component. The battery does have a wire additional to the plus and minus wires, but that appears to be a battery temperature sense wire.
I think in this context the "gauge" would be software based. I think some more advanced devices (e.g. phones) continually record time/voltage measurements, and if you fully discharge the battery they can then use the values to update the battery calibration. I don't know if simple devices like the Sansas do it though.