>>> Anyone with a smartphone knows the problem: After 18 months or so, it holds a charge for less and less time as the battery begins to degrade.

Really? That's still a thing? These aren't nicads. I've found that my phone doesn't report full charge as often, but it still lasts for a similar amount of time. My 5+yo netbook's battery is still reporting 80% of its design capacity.

Imho, such apparently dramatic falls in capacity often have more to do with running apps rather than physical degradation of the battery. Talk to me after a reset to factory settings.