ELECTRIC POWER SYSTEM THAT ALLOWS SWITCHING FROM A MAIN BATTERY TO A SPARE BATTERY, FOR SAFER NAVIGATION.

I previously demonstrated an electric motor power and control system that automatically switched on a backup battery when the main battery was low.
While the system worked, it didn't seem very useful if it didn't indicate when the first battery was low or if it didn't indicate when switching from the main to the secondary battery. The LED signaling system seemed ineffective during the day and from a distance.

I know that many radios provide charge status information, but this is a self-contained alternative system.

This setup (see diagram and real-world test, first and second attached images) includes the ability to switch power from one battery to the other.
Therefore, it will be possible to switch from the main battery to the backup battery as soon as you notice that the first is running low.
It will be possible to switch even if the main battery is completely flat or even damaged.

The system requires the use of four separate batteries but can be simplified to just two, as we'll see at the end of the explanation.
I made the diagram and practical test with four separate batteries for better understanding.
Don't worry about the size of the individual batteries; I used the ones I had available just to make the system work. You can use whatever technologies you like.

Explanation of Connections

As you can see in the diagram, battery 1 is the one normally used as the main battery (in this case, I used a 6-volt, 4.5 Ah AGM lead-acid battery, but any other technology will work).
Battery 2 is the one that will take over as a backup for our control (as above for the battery type).
The system is designed so that there is no possibility of them working together, so there will never be an accidental parallel between a flat battery and a charged one.
There is a dedicated battery for the receiver, the servos, the switches, and in short, all the loads connected to the receiver channels (as above for the battery type).
The ESC's BEC was not used, but a separate power supply was used for obvious reasons; otherwise, the system would not have worked properly.
Finally, there is a dedicated battery for energizing the relay coil. Since the relay (see photos 3 and 4) is 12 Volts 80 Amps, I couldn't use 6 Volt batteries to avoid overcomplicating the diagram and making it difficult to understand. (In this case, I used a 12 Volt, 2 Ah AGM lead-acid battery, but any other technology will do.)

A brief aside on the choice of relay.
It's a changeover relay with five contacts: two for the coil, one is the common contact, and the other two are the normally open and normally closed contacts. The relay switches between them.
I would have preferred a 6 Volt relay, but the ones I had didn't support high enough currents on the contacts. The ones I needed were too expensive. I paid less than two euros for these 12 Volt ones, including shipping.
They're the ones for cars, and 12 Volts is very common.

So, as already mentioned, a battery is connected to the receiver.
The negative terminal of battery 1 (main) is connected, together with the negative terminal of battery 2 (reserve), to the negative terminal of the ESC.
The positive terminal of battery 1 (main) is connected to the normally closed contact of the relay.
The positive terminal of battery 2 (reserve) is connected to the normally open contact of the relay.
The common contact of the relay is connected to the positive terminal of the ESC. (It is advisable to insert a fuse here.)
The ESC is connected to the motor with the two usual cables.
The ESC is connected to the receiver (with the positive isolated) with only the negative and signal wires (in this case, the active channel is channel 3, which corresponds to the left up and down lever on the radio control).
The relay coil contacts are connected to the two wires (out) of the ON-OFF RX Switch.
There is a diode between the relay coil contacts for protection only (it is not essential for operation).
The positive and negative input wires of the aforementioned ON-OFF RX Switch are connected to the 12-volt battery.
The three joined wires (signal-positive-negative) of the ON-OFF RX Switch are connected to the receiver (in this specific case, they are connected to channel 6, which corresponds to the knob on the top right of the remote control).
The ON-OFF RX Switch will draw power and commands from the receiver itself.

How the system works (see video)

Moving the left remote control lever (up and down) activates the motor, accelerating and decelerating in both forward and reverse.
In this condition (relay coil not energized), the ESC will draw current only from battery 1 (main) via the normally closed relay contact.
When we notice that the battery is discharging too much because the boat is slow (even with the lever fully up), or is completely flat because the boat is stationary, we will switch the power supply.
In the video, total battery discharge is simulated by disconnecting the negative terminal of the main battery (which in the real test is the right one) by removing the alligator clip.
At this point, you can see the electric motor stop even though the remote control lever is still fully forward.
Now simply turn the knob on the top right of the remote control. This way (on channel 6 of the receiver), the ON command will reach the ON-OFF RX Switch. This will pass current that will energize the relay coil. Energizing the coil will cause the contacts to switch; therefore, the circuit of battery 1 will open and that of battery 2 will close.
The boat can be brought back to shore, now using the full charge of the reserve battery.

If space is a concern and you want to minimize the number of batteries needed, here's what you can do.

Reduce the number of batteries to three (instead of four).
In this case, we could eliminate the battery used to power the receiver (and consequently all the loads connected to it via the channel slots), but this would not restore the positive of the BEC; absolutely not.
Battery 2 (the reserve) must also be connected to the positive and negative terminals of the receiver. All other connections should be left as they were.

There is a disadvantage, or at least an aspect to consider.
In this case, battery 2, a backup battery, will not be fully charged if needed because it will have to power the receiver, servos, any lights, etc., throughout the entire navigation.
The current draw will certainly not be as high as the motor, but it must be taken into account.
If necessary, I will make this second diagram.

Reduce the number of batteries to two (instead of four).
In this case, eliminate the two 6-volt batteries and use the 12-volt battery to power the coil, the receiver, and the ESC in case of a backup, taking care to lower the voltage to 6 volts for these two loads in parallel.
If necessary, I will make this third diagram.

I hope this is helpful.
For those less experienced, follow all the connections carefully, following the diagram to the letter.