Telemetry Solutions base stations are built with external charging capability. To use it, you just need to connect a 12-5 volt converter to a 12 volt battery then connect the other side to the base station USB. 

In fact, even when using the external power, the base station is really operating on the internal battery.  The external power is just there to keep the internal battery topped up.  The beneficial side effect is that if you ever want to unplug from external power, you have a fully charged base station ready to go!