user18878: I'm building a CANBUS network based on an arduino to go into an offroad racing truck. The vehicle has two separate battery systems, and I would like to monitor the voltage while still electrically isolating them from the arduino itself. For the voltage monitoring itself, I assume I can use a simple voltage divider into an Analog pin. To electrically isolate the arduino, would an Opto Isolator work? I found this one: https://www.sparkfun.com/products/9118 But it appears that it's designed for low voltage to control high voltage, as opposed to the other way around. 

user13594: 1) using current sensors based on Hall effect: https://www.sparkfun.com/products/8882. Their sensing circuit/leads and analog parts are electrically isolated.
The drawbacks are: to calculate the voltage, need to wire some load resistor with known value to batteries, but you can switch on the load circuit by relay during measurement; susceptible to magnetic field, some shielding might be needed.
2) to use isolation amplifiers: please see http://www.analog.com/en/products/amplifiers/isolation-amplifiers.html

user6628: An optoisolator is good for digital signals, but won't transfer an analog signal accurately.  
Instead, you could put ADCs on the different circuits and optoisolate their communication with a master Arduino.

18878: <p>I'm building a CANBUS network based on an arduino to go into an offroad racing truck. The vehicle has two separate battery systems, and I would like to monitor the voltage while still electrically isolating them from the arduino itself. For the voltage monitoring itself, I assume I can use a simple voltage divider into an Analog pin. To electrically isolate the arduino, would an Opto Isolator work? I found this one: <a href="https://www.sparkfun.com/products/9118" rel="nofollow">https://www.sparkfun.com/products/9118</a> But it appears that it's designed for low voltage to control high voltage, as opposed to the other way around. </p>
 13594: <p>1) using current sensors based on Hall effect: <a href="https://www.sparkfun.com/products/8882" rel="nofollow noreferrer">https://www.sparkfun.com/products/8882</a>. Their sensing circuit/leads and analog parts are electrically isolated.</p>

<p>The drawbacks are: to calculate the voltage, need to wire some load resistor with known value to batteries, but you can switch on the load circuit by relay during measurement; susceptible to magnetic field, some shielding might be needed.</p>

<p>2) to use <a href="https://electronics.stackexchange.com/questions/51116/measuring-voltage-while-maintaining-isolation">isolation amplifiers</a>: please see <a href="http://www.analog.com/en/products/amplifiers/isolation-amplifiers.html" rel="nofollow noreferrer">http://www.analog.com/en/products/amplifiers/isolation-amplifiers.html</a></p>
 6628: <p>An optoisolator is good for digital signals, but won't transfer an analog signal accurately.  </p>

<p>Instead, you could put ADCs on the different circuits and optoisolate their communication with a master Arduino.</p>
