It works!! So happy right now
Built a circuit board to fit with a small battery in the battery tube and convert 310V DC into a regulated low voltage for the BlueROV2. The circuit has a battery current feedback loop to limit battery charge current to 5A, but will provide up to 20A total to the ROV (ROV+battery charge).
Power is connected to the autopilot enclosure with a relay, and the battery is connected to the board by a different relay. Relays are activated when topside power is connected. If topside power is lost, the battery will continue to provide power. Relay 1 command from the pixhawk turns off the relays.