Many AA or AAA battery chargers charge batteries in pairs, but plenty of devices use 1 or 3 batteries, meaning that some of your batteries get overcharged and some get undercharged. NiCd and NiMH batteries also benefit from an occasional full discharge, which most normal battery chargers won’t do.
If you’re anything like me you’ll end up with a lot of rechargeable batteries, none of which end up being charged properly, and some of which turn out to be completely unusable. It’d be perfect if you had a low-power battery charger that you could leave on all the time, that would charge your batteries individually, automatically discharge them, and give you an idea of their real capacity. That’s what you’ll make in this tutorial!
Note: To make this nice and easy, the charger uses Espruino’s GPIO pins to directly charge and discharge the batteries. This means it can’t charge batteries very quickly (it can take days to charge and discharge them!). NiMH batteries benefit from an occasional fast charge/discharge, which this charger can’t provide without some extra components – so it’s not perfect, but it is easy to build and will make sure your batteries are always topped up.
For more detail: Smart Battery Charger (no soldering!)