Q: I am a new iPhone owner and have a question about the iPhone’s battery: I understand that the battery can recharge fully approximately 400 times before it’s life begins to shorten. I have it on an FM transmitter in the car which charges it and I plug it into a wall USB charger while at my desk and at home. I am also considering purchasing one of those battery pack cases for the phone. My question is whether the battery is bypassed while plugged in or whether it continues to drain while being continually recharged? Am I reducing my battery life by leaving it plugged in while driving, at work and at home?
A: Like most modern electronic devices, the iPhone uses a lithium-ion battery, which although offering a limited number of charge cycles, does not have the issues with minor top-up charges that older rechargeable battery technology had. This means that you can plug in your iPhone into a charger regularly and this will not have any impact on the overall battery life.
Just to be clear, a “charge cycle” refers to a complete recharge of the battery, so if you are regularly topping up your iPhone battery, you are only using fractional charge cycles.
When the iPhone is running from external power, such as in your car or at your desk, the iPhone operates from the external power and charges the battery simultaneously. In this mode it does not consume any battery power, and therefore is not constantly recharging. Once the battery is fully charged, the charging circuit cuts off completely and the iPhone simply runs from external power. This is indicated by the plug icon appearing over the battery indicator.
Note, however, that since the battery is not being used at all when running from external power, it may lose a bit of power naturally and the iPhone will therefore occasionally apply a top-up charge. This would be similar to the way in which the iPhone would lose a bit of battery power even if it were sitting on a shelf turned completely off—batteries drain naturally, albeit it slowly. For the same reason, Apple recommends that you do not leave your iPod or iPhone constantly powered from an external power source, since that would effectively be the same as leaving it powered off on the shelf; the battery won’t get used at all and can suffer from disuse as a result.
Realistically, however, none of this is going to be an issue unless you are leaving your iPhone plugged in to external power for several days at a time, which most people are unlikely to do for a device such as the iPhone.
For more information, we have a more detailed explanation on iPod Battery Life and Charging in our May 9, 2008 Ask iLounge Column.