I have a new HiFei 300A-X ESC that has and needs an isolated ground between the Rx Throttle plug ground and the main Battery (-) terminal.
The EagleTree Seagull HV data logger current sensor is also the Volt sensor. It has only one red wire to go to the main battery, no ground to pair with it.

Question; How am I to receive a 10S volt signal (~40V) without connecting the grounds? HiFei says don't do it.

The ET instructions suggest using a separate Rx battery rather than a BEC to solve the problem, but I don't see how this would work.
The ET data logger gets it's ground from any of the servo sensor rail ground terminals.

Any ideas greatly appreciated.