Delete comment from: Ken Shirriff's blog
@Matt Claudius:
IMO just about all of these USB "chargers" are constant voltage power supplies. (IIRC the USB spec calls for 5 V +/- 5% or so.) Basically a mini version of the average desktop PC power supply.
All the rest is up to the device connected, which also contains the actual charging circuitry (along with a bunch of switch-mode converters and regulators to generate the various voltages needed - look at, say, the full datasheets of MP3 player ICs from e.g. Sigmatel or AustriaMicrosystems, they're surprisingly complex beasts). You can bet that battery monitoring is reasonably sophisticated when LiIon/LiPo cells are involved (you neither want them to go bang when overcharging nor die when deep discharged).
Therefore the choice of "charger" shouldn't normally influence charging, assuming voltage remains well-regulated in the face of whatever current is demanded.
I can only speculate as to the cause of your battery misfortune. I do know that lithium-based cells are not generally fond of heat (plus charging does warm them up), and keeping them very full further speeds up aging. Such a rapid loss of capacity would point to an already damaged cell, possibly a production issue.
Dec 13, 2012, 3:24:02 PM
Posted to Apple iPhone charger teardown: quality in a tiny expensive package