Summary

Three 18650 batteries and few more things can provide an unregulated 12V power supply for emergences. As you see from the tension curve of the 18650 battey, the battery pack tension will be up to 4.5x3=13.5V when batteries are at 100% and 3.75*3=11.25V up to 20% of batteries charge. A fake battery is printed and in order to shortcircuit it's terminals a metal forniture joint is used (see image, you can buy it at less than 1$/1€ https://www.amazon.it/Giunto-Mobili-Connettore-Armadi-lavoro/dp/B084DQX666).
The battery pack can power many different kind of devices: motors/attuators and arduino boards.
If you need a regulated power supply you should consider to user four batteries and a 7812 chip to get very stable tension.