For a long time I thought about a power bank with remote control output. I needed a 12V output in addition to the 5V, so I used a step-down voltage converter with current regulation in case it was necessary to limit the current for some device. 12V and 5V outputs can be switched on separately. 12V from the converter is also used to power the remote-controlled relay. However, only 5V outputs can be controlled remotely. The range of the remote control is stated by the manufacturer to be about 1 km in free space, but I soberly count on 100 to 200 m, which is quite sufficient for my purposes.

SAFETY NOTICE

When working with 18650 cells, pay close attention to polarity reversal and check individual connections several times with a multimeter. Use only well-insulated wires (even those cables from the battery basket are not exactly the first league of safety) and insulate the soldered joints well - with heat shrink tubing or several layers of electrical tape. Handling 18650 cells is not entirely without problems and can potentially be quite dangerous. A fuse on the ground wire or on the live wire is absolutely necessary (preferably use a 3A fast fuse). Choose cells with approximately the same capacity and charge them before use. In the era of sodium batteries, this danger will hopefully disappear.

Supplies

One single position switch
One two-position switch
Fuse and fuse holder
Holder for three 18650 batteries
Voltmeter (to check battery condition)
Radio switched relay
Three 18650 batteries
Power connector
DC/DC step-down/step-up convetrter
DC/DC step-down converter module 2xUSB 5V/3A
Lockable wooden box
Wood screws
Nut for better fastening of components
Electrician's chocolates or wagosclips

Step 1: Component Placement Planning

First, we think out how to place the individual parts in the box (I use a reinforced cigar box) and drill holes for the lever switches accordingly. One switch allows current to the 12V converter (which we need to adjust exactly to the 12V output to be able to power the radio controlled relay module) and the other switches the current in the first position of the switch to the normally open terminal of the relay, from which another wire leads from the com terminal to the usb converter to 5V (or it can be put the other way around, from the switch to the COM terminal and from the NC to the usb converter - that's how it is in the drawing), in the second position of the switch, the current from the batteries to the usb converter goes directly to 5V. Furthermore, it is necessary to place the battery holder appropriately and try to keep the phase conductor and the ground conductor as far apart as possible. I soldered a fuse holder on the ground wire to the cable, which I screwed together with the ground of the voltmeter and the ground of the 5V usb converter to the gnd terminal of the 12V converter.

I hope my clumsy drawing is understandable.

Step 2: First Test

We check several times whether everything is well connected, whether there is no polarity reversal and whether everything is firmly in place. Only when we are sure that everything is in order will we put the battery in the holder. If something starts to rust, we quickly remove the batteries and make sure they don't overheat. If one heats up, it is to be thrown away. In this case, we check the connections again until we find out where something could be leaking. In most cases, this is due to poor insulation of the cables, or at some point the polarity is reversed. To be sure, it is better to do the test outside.

Step 3: Conclusion

If everything works fine, you still need to be careful that the battery voltage does not drop below 10V. This means that it is necessary to charge them. Since only three cells are used, it is a good idea to use cells with the largest capacity possible. The capacity of one battery will most likely be consumed by the operation of the device itself.

It's just a prototype, but it works.