When I bought two USB & Serial Cradles with my IPAQ 3970 I got only one power-adapter.

I were disappointed when I discovered that they were not recharging the batteries using USB power.
There were similiar products that charged batteries using USB power.

This is how I modified an normal USB cradle (IPAQ 38xx/39xx series) into a USB cradle with battery charger.

This is an picture of the original cradle PCB:

 

This is after the modification:

The red and black wires are removed, to remove any possibility to connect both power sources at once.

A thin wire is connected from one of R1's (unused) soldering pads to the "+" hole that were used for external power.