Mandatory warning that those are absolutely life-threatening power levels, and you’ll need appropriate engineering expertise and training to work with them safely.
It’s likely you’re already aware of that, but I need to make it clear to anyone reading this thread that this isn’t in the realm of “things to play around with”.
I’d recommend starting by measuring (or at least estimating) the efficiencies of your components, ideally including measurements at different current levels and temperatures to determine whether they have a significant effect. There’s a simple breakdown of the values for an ROV powered by an Outland Technology Power Supply in this comment.
Once you have an understanding of the losses in your system, you can decide whether it’s worth monitoring the tether voltage independently, or just estimating the expected voltage drop given the measured power draw of the ROV. This thread covers some higher current power sense modules that can be used to measure the low voltage power.
If you do decide you want to measure the tether voltage then as mentioned that should be possible by configuring an additional power sensor in the autopilot parameters.
In terms of the wired connections for that the simplest approach would be to use a voltage divider[1] to drop the 700V signal down to a level the flight controller board’s ADC is capable of measuring (on the Navigator that’s 3.3 or 6.6 V), and just set an appropriate scaling factor in the parameters to scale the measured value back up to the correct voltage. If you go that route I’d recommend having the voltage divider (and your power switching and smoothing circuitry) inside its own enclosure, like the Outland system does, so that only the 24V power wires and the low voltage signal wires need to go into the main enclosure with the ROV electronics.
Alternatively, you could set up some custom electronics to measure and monitor the power level, and report it to the vehicle’s telemetry stream using BATTERY_STATUS MAVlink messages, which would have the same effect but bypasses the autopilot (so you don’t get access to autopilot failsafes or warnings, but you may be able to more easily perform some more advanced power shutoff or reporting logic or something.
using a high total resistance, to minimise the leakage current to ground ↩︎