Spiderman’s Uncle Ben was known to say, “With great power comes great responsibility.” The same holds true for battery power. [Andreas] wanted to use protected 18650 cells, but didn’t want to buy them off the shelf. He found a forty cent solution. Not only can you see it in the video, below, but he also explains and demonstrates what the circuit is doing and why.
Protection is important with LiPo technology. Sure, LiPo cells have changed the way we use portable electronics, but they can be dangerous. If you overcharge them or allow them to go completely dead and then charge them, they can catch fire. Because they have a low source resistance — something that is usually desirable — short-circuiting them can also create a fire hazard. We’ve covered the chemistry in depth, but to prevent all the badness you’ll want a charger circuit.
The little circuit fits on top of a standard 18650 cell and uses two chips (one of which is just a dual MOSFET) and three discrete components. It does add about 3 mm to the cell. [Andreas] found that battery holders with a coiled spring would accommodate the extra length, but those with metal leaf springs would not.