Arduino Voltage Sensor decoupled

Using Arduino Sensors
1

Hi,

I have a RC motor ship model, and want to monitor and record current amd voltage. I have 2 separate power circles - one is the main 12 V one, served by a heavy duty Lead battery and experiencing current surges up to 20-30 A when spinning up to full trottle. Another is electronic circuit for monitoring, powered by separate battery. There is no electric connection between both circuits, all done connections are decoupled using optocouplers (direct in input, driving PowerMOSFETS in output circles), both for inputs like PWM signals, electric output for steam generator fan, etc. I still have a few in/outs of my Arduino Nano not used, and thought monitoring voltage and current and feeding them to the RC transmitter telemetry channel is a good idea. Current is easy with ACS712, but the voltage is more complex. All Arduino sensors are just the dividers, and require common GND. Is there a solution (in form of Arduino module, circuit plan, etc) how to do this? I can easily mill PCBs here, solder, etc, so not restricted to ready solutions.

2

Get an isolated voltage sensor.
A quick google on the subject turned up this sensor.

3

There are also through-hole linear opto isolators available which you use with an opamp circuit to communicate analog voltages in an isolated manner. For instance http://www.ece.umd.edu/~neil/micropower/power/solarcell/optocoupler300.pdf

4

Could also use a 'flying cap' circuit if you only want to measure the 'other' battery periodically.
A capacitor (say 100uF), connected to the two common contacts of a DPDT relay, is connected at idle to the battery with the two NC contacts, and periodically switched to the Arduino (the NC contacts).
Switching, measuring, returning, would only take ~25ms when a tiny DPDT relay is used.
Leo..