My iPhone 4S has had it. At 11pm last night, the battery was on 86%. The alarm on the phone went off at 6:30am this morning and went flat about 5 seconds into the alarm to Is it definitely the battery or could something else have caused this? Is there a battery monitoring app? I've got an old iPhone 4, but the battery is slightly lower power at 1420mha (4s is 1432mha). Should that matter? Can any of the cheap eBay one's be trusted? Or are they likely to be much lower-power than advertised?