1 idea for using batteries from old mobile phones
An old cell phone battery can still serve you well and be useful. The battery itself may no longer be charged due to the past years, but it has one useful thing that will more than last for many more years.
The fact is that the batteries of almost all mobile devices are equipped with a built-in charging controller. This is a small board located in close proximity to the battery, controlling not only charging, but also discharging, preventing the critical voltage from being underestimated. In addition, this small scarf protects the battery from short circuits. And some models also have a temperature sensor that monitors the operating temperature.
So, you can easily get this useful controller and use it for your 3.7 V lithium-ion batteries, say, the 18650 series - http://alii.pub/5becfz
Idea for using old cell phone batteries
We take the battery from an old mobile phone and carefully try to lift the upper plastic part.
Then we crawl up with wire cutters and bite off the flat lamellas coming from the battery.
Next, all that remains is to remove the board from the plastic protection.
Here is the charging controller itself.
On the sides there are areas for connecting the battery, marked with the symbols “B+” and “B-”. On the reverse side there are 3 pins for connecting to the phone. The two side ones are usually the way out.
We solder the wires to the controller going to the battery.
Then we solder a battery to the controller, consisting of 3 18650 batteries connected in parallel.
Solder the wires to the controller output.
We apply 5 V to the input and charge the battery.
As soon as the battery is fully charged, the controller opens the circuit.
Next, power can be taken from the controller output.