[idi banashapan]

there's a lot of people suffering the same thing with iPhone 4S and iOS7 (a quick google search will show this). since the update, the battery shows anywhere from 40% to 0% but suddenly dies, seemingly randomly. mine does it, as does a friends and his wife's, ONLY since the iOS7 upgrade. on iOS6, the phones were absolutely fine and never failed to give an accurate reading.

although your issue may well be a faulty battery, bear this bug in mind.