There is another possibility I did see mentioned, (I could have missed it). I don't know how much amperage/voltage your original charger was supposed to provide to the phone but the wall chargers are supposed to adjust for both the voltage and amperage depending on what is plugged in to them. I have one charger that outputs 15v @ 2 Amps. The standard voltage is 5v DC. But I can plug a 5v device into it and the circuit in the charger understands. If the charger was bad or wasn't correctly sensing the circuit in the phone it could be destroying the batteries. In short, if my 15v charger malfunctioned I could see it being a serious problem with my 5v devices. Or it could be, as stated so well previously, a crappy battery.