FYI, I just installed a transponder and wondered about the power
budget of the IPAQ 310. With a dead battery it pulls .465 amp @ 12.5
volts and fully charged it pulled .27. This is with Bluetooth off,
backlight 100% on all the time and just using the car adapter,
measured from the battery to the car adapter.

The batteries in these are easy to change, like a cellphone, and
extended batteries are available with a replacement cover that sticks
out and won't allow the original mount to be used. It might be nice to
have the extended battery in a different mount. Right now with a
transponder, encoder, etc. my total power requirement is .9 amps with
the IPAQ running off the airplane battery and .62 without it.
Currently I have a newer 12AH battery that should easily run for an 8
hr. flight. So it seems logical to pull the aircraft power off the
IPAQ and maybe even run the vario from it's backup battery after 5 or
6 hours.