Adding backup battery?

Not sure if this is the right place to post this.

I want to add a fuse that cuts off power in case current draw for some reason gets too high. But the problem then is that if all power cuts off. I will have no idea where the boat is if its in auto mode, and I also have no way to drive it back to land safely. What’s the best way to add a backup battery that can at least give me enough power to keep the system alive so I can have gps position and a chance to steer the boat to safe land?

I would never add a fuse. Like you say, that’s the end if everything. If you want protection, use some kind of current limiter. In order to keep your electronics alive, you will need a separate battery that comes into play when the voltage drops below a certain level, but it should feed the circuit that causes the problem. All in all quite complicated circuitry that needs electronic skills.

[Current Limiting and Voltage Scaling — Copter documentation](https://Current Limiting and Voltage Scaling)

The way i got around it was to fit a pair of 5v 18650 UPS modules. One for servos and pixhawk and the other for communications and pixhawk
I needed this as i am running solar so draining the main battery is a real possibility. A pixhawk has 3 separate 5v rails and im running power in to the servo and usb rails so as long as one of them has power the pixhawk will stay online.

I have a sail as backup propulsion but you could add a small air propeller and backup battery.

What I had in mind is a simple power bank that will be hooked up to the Pixhawk USB port. But I am not sure and have not tested if Pixhawk will use the power bank while the battery is still hooked up to it or if there will be some over voltage. This way at least you will always have the GPS position of your system even if the main battery dies

that will work fine, that is essentially what I am doing but im using the usb header on top of the pixhawk rather than the micro usb port.