About 15 years ago I was writing software for an embedded device made by another company, and they sent us a unit for testing. It had a small rectangular rechargeable lithium battery that was charged via a DC jack.

At one point I hadn’t kept it charged, the battery went completely flat, and after that it would no longer charge at all. When I called the company, they said the battery was now too deeply discharged and required an “intelligent” charger to revive it. They sent a charger with a slot for the bare battery; some LEDs blinked in various patterns for a while, and eventually normal charging resumed.

I’ve always wondered what that charger actually did, that the built-in charger was not capable of. Was it performing some kind of analysis to decide whether the battery was safe to recover (e.g. after deep discharge), or was it simply applying some initial charge ignoring the battery’s protection circuitry (and at what risk)?