To achieve high-voltage (>1000 volts) isolation, infrared transmission was used between the on-battery monitoring electronics/MCU and the hierarchical comms system.  I updated the schematic for this 4th iteration and then routed this 10mm x 30mm PCB, which is then double-overmoulded with a soft epoxy (first at low presure so as not to damage components on the PCB) producing a fully isolated design that’s easy and cheap to manufacture and test.