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I have notice lately (in last two months or so) after disconnecting my iPhone 4s (with iOS 6) from the charger in the morning after an entire night of charger it immediately drops from 100% to 98%. Is this a sign of a problem or normal battery aging.

I do perform the monthly battery drain til power down as recommended by Apple

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2 Answers 2

up vote 7 down vote accepted

It’s neither a problem nor a sign of ageing.

iOS devices, since the first iPhone, have used a method of charging their Li-ion batteries, which is different from normal charging as you know it.

Suppose your device is at 10% charge and connected to the charger. The device will charge up to 80% in 2 hours. This is called fast charge. Now the device will charge up to 100% in another 2 hours. This is called slow charge.

When the device reaches 95% actual charge, it displays “100%” as the current charge in the status bar. The device keeps charging till it reaches 100% actual charge, at which point charging is stopped and the battery discharges naturally. Note that still the displayed charge is “100%.” Now, when the battery reaches 95% while discharging, charging starts again and continues till you reach 100%.

This cycle is repeated until you disconnect the charger, at which point the displayed charge changes from “100%” to the actual charge, which may be anywhere between 95%-99%.

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+1 : Cristal clear explanation. Thanks. –  Rabskatran Oct 19 '12 at 15:04
    
You’re welcome. –  duci9y Oct 19 '12 at 15:05
    
Wouldn't this cause power degradation on the that top 5% of the battery - causing it to age faster? –  Anthony Oct 19 '12 at 15:39
    
Nope. That is too obvious a situation to have been not thought of. –  duci9y Oct 19 '12 at 15:47

Modern devices with a battery (iPhones included) are intelligent with charging and stop charging when the battery is full. Since the power supply of the adapter will be switched off at that point, your battery will start to slowly discharge just like when it would be unplugged. To avoid wearing down the battery by continuously "topping off" the battery after such a discharge while still being connected, it will not resume charging automatically until a certain threshold for discharging has been met.

The behavior you experience is this small discharge which happens during the time your battery was fully charged, and the moment you unplug the device. The confusion comes from how Apple decided to display the battery level, but the reasoning is that users will be more inclined to believing there is a problem with the battery when it appears it never charges to 100%.

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