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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.

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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.

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    Instead of an ADC, I'd use another microcontroller with an integrated ADC (even a snall Attiny85, just 8 pins and three analog pins) powered by the battery it has to sense. Every x seconds (or milliseconds, but going really fast is useless I think) it wakes up, makes a conversion, sends it on the serial interface and sleeps again. This way you will just need one optoisolator (on the serial wire going from the attiny to the receiving circuit). using an external ADC will require three optoisolators for SPI or a strange configuration for I2C (because its wires are bidirectional)
    – frarugi87
    Mar 10 '16 at 10:20
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    The voltage sensing side was going to have it's own microcontroller anyway, so I'll probably do that! Mar 10 '16 at 17:27
  • If you are using separate microcontrollers on separate battery systems, maybe a wireless solution would be good.
    – Dave X
    Mar 12 '16 at 16:10
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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

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