I mostly use mine in the truck, so I leave the PDA plugged into 12 volt power at all times. If I unplug the power, the screen dims to save battery life and the screen goes to sleep after a few minutes. The GPS receiver has a 35 hour run-time, so I rarely need to plug it in.
There are larger batteries available and you can buy those adapters that allow you to use AA batteries in place of the lithium ions if you need extra capacity and do not have the ability to charge via 110V or 12V power.