Kentli Lithium-Polymer rechargeable AAs

[Garth Wilson]

The battery itself doesn't drop off like that. There's a switching regulator in the end cap that does its darnedest to keep the voltage constant, but when the battery is too weak to do that, the regulator shuts down instantly and you don't get any more out of it. The switching regulator is partly why it's so expensive, and why it produces a lot of electrical noise as the article says.