High Voltage to Low Voltage Attenuating Isolation
An easy way to do it would be to PWM encode the HV battery voltage, transmit it across an opto-isolator, low-pass filter it and feed it into an ADC.




Use a 300:1 potential divider from your battery and feed the signal into an LTC6992 MOD input.
Drive your opto-isolator from the PWM output. You may need a transistor to drive it adequately.
Take the opto output, low-pass filter it and feed it into an ADC on your monitor circuit.


Things to watch out for:


You'll need a high-speed opto-coupler.
You may have to run a calibration on the device. Hook up a pot to the LTC6992 input and vary the voltage from 0 to 1 V while monitoring the signal received on the ADC input. Any offset / scaling can be done in your micro.


Thanks to Andy Aka for introducing me to this chip which pops up in many of his solutions.