How can I measure the supply voltage of batteris without a multimeter?
If the first XBee has an analog input then use it to measure the battery voltage.







simulate this circuit &ndash; Schematic created using CircuitLab

Figure 1. Battery voltage sense.

Create a potential divider to bring the maximum battery voltage below the maximum input voltage for the ADC.

You would then need to figure out the scaling factor and set the remote to control your gauge. This would be easy to achieve with a potentiometer on the analog in during setup.