Want to learn about batteries? Adafruit has a great guide for picking the right battery for your DIY projects, but if you prefer video, they have got you covered there as well.
The video covers a bunch of basics that are useful for all of us, like what milliamp hours means, what nominal cell voltage is, and what exactly rechargeability refers to. From there, it breaks down a number of battery types, like alkaline, coin and button cells, and more, then goes into which types of DIY projects they're best suited for. If you make electronics, it's a great video to learn the basics, but there's plenty here for pros to learn too.
Collin's Lab: Battery Basics [YouTube]