Hi Genghis,
I use aircraft power for my gps, however recently I needed to get external power, as I would be flying an aircraft without a power outlet.
I got one of
these items. It changes my system from 1 hour battery life, to 5 hours. It's got a 3400mah battery, so should allow you to extend your battery life even further.
Basically it has an USB output. So long as you have a cable to charge your PDA from a USB outlet, you can recharge it with this.
Unfortunately it appears to be out of stock at present, but no doubt they'll have more in soon.
In relation to mounting, the obvious one is a windscreen suction mount. Have you discounted this because of sunlight? These are easily available for most systems, and aren't particularly expensive (more so given that you don't want it to power your pda).
dp