If Tesla powers a car with 7,104 of those batteries, why shouldn’t I use four of them?

Type 18650 batteries can be found in many applications. The huge battery packs used in electric vehicles often consist of thousands of those units. Hook up a few of the cells in series and parallel, et voilà you have your notebook battery.

Wanting to utilize these lithium-ion batteries as a portable power source for Marta, I was planning on increasing the capacity by wiring up four of them in parallel using a battery holder.

As one cell only provides 3.7V, but most of the electronic components need an operating voltage of 5V (i.e. mainly the LED setup), we need a DC-to-DC step-up converter. Additionally, the battery pack needs to be charged (ideally from an ordinary USB power supply). A charger/discharger boost converter board serves both purposes. To prevent the batteries from catching fire and overloading, I tried out a cheap battery protection circuit, which worked very well.

Unfortunately, the multi purpose charger/discharger PCB was not as good as I hoped. There was a constant high frequency noise when current was flowing. After some time, the whole unit got hot as well. Using all of the components would have meant more soldering. The maximum output current of 1.2A wouldn’t even have worked out well with the later abandoned super-capacitor UPS circuit, so the whole idea was discarded after a short time and instead a few different power banks were checked out.