Measure voltage of 2 different batteries on Arduino

Question

Currently I'm using a resistor divider to measure the voltage of a 6v battery that is connected to an Arduino via a 5v power regulator. I also want to be able to measure the voltage of another battery (7.2v) on a separate circuit with the same Arduino.

The issue is that the batteries will be on separate circuits, and thus, have no common ground (batteries are not in series or parallel). Is this possible?

Answer 1

The simplest solution is to create a common ground. Since you want to measure two batteries, you have to connect their grounds to the Arduino ground. If you do that to both, the Arduino ground has become the common ground.