You could use a LiPo battery and a Power Module connected through "Power" labeled connector. This method allows to get readings of the battery voltage.

You could use a standard 5v @2A microUSB charger to do it. Use the microUSB port as the image below:

The servo rail input can be used as a Power method

Notice that the power connector has attached two ADC channels in order to read battery level. If you don't use this connector, you could use the two middle pins to have analog voltage lectures for a sensor/device.

** Power connector **
 | .  .  .  .  .  . |
   |  |  |  |  |  |
       ch0 ch1

Warning: The range of input of the ADCs are 0~5v. Do not connect anything that exceeds this voltage.