Doesn't appear to be a way to monitor voltage in with the pi so I just ordered these for the lipo balance ports. They even have a low cell alarm:
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https://www.amazon.com/Readytosky-Battery-Monitor-Voltage-Indicator/dp/B01FHIMU0M/ref=mp_s_a_1_12?dchild=1&keywords=rc+battery+voltage+checker&qid=1590573960&sr=8-12
Bryan Waller