A lot of electronics projects require portable power and so do some of my ideas. One of the most popular rechargeable batteries to achieve this is the 18650 Li-Ion battery. They can be found in a lot of devices like laptops, electric scooters and bikes, and toothbrushes.
It's pretty easy and cheap to get your hands on some of these battery packs that are old or defective. Most of the time though, it's only one or several batteries in a pack that are broken or dead, the rest of the batteries can still be salvaged. A while ago, I got an entire box of broken laptop batteries from an IT guy. I wanted to disassemble them to test the individual cells and use them in my projects.
To reuse the batteries I made a charger for 4 18650 cells and a capacity tester to see how much energy they can still store.