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First time caller, long time listener: I've seen this question asked on a few other sites but with no "definitive" answers.

The Apple website on iPhone batteries is ambiguous, at best, and the closest it comes to answering the question is saying that a monthly deep cycle is a good idea. (reference)

So, what do you think -- is there an adverse impact from repeated, overnight charging of your iPhone 4? Does this negatively affect battery performance over the long term? Or is there some chip/ software that acts to shut off the charging once the battery is topped off, thereby protecting your hermetically sealed little power source?

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Even if the answer was "no," would you really change your life style around it? Keeping up with some phone charging ritual will be more undesirable than a more quickly ruined battery. –  XAleXOwnZX Mar 23 at 22:02
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3 Answers

up vote 23 down vote accepted

All this hype about how to charge your batteries is blown out of proportion.

Although some batteries (I forget which types exactly) can have longer lives if charged at "optimal" rates and temperatures, the reality seems to be that it does not extend the life by much. One of the research papers I read concluded something like if you charge your device at around 17 degrees from 0% until 75% full you can extend the lifespan by about 1/5 of the total life span.

I have charged my devices for days at a time, and my iPod Nano has been plugged in for months, and holds a charge the same as my other nano (I have two) which almost never gets used and only charged ocasionally.

In conclusion, just do whatever is convenient for you.

EDIT: Here are some links to the apple pages related to their iPhone batteries:

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First of all, iPhone uses lithium-ion batteries, so you should follow the same advice as for the laptop batteries. Look at this and this questions at SuperUser.

The most two damaging things to Li-Ion batteries are deep discharge and heat. Deep discharge is when you use the device until it shuts down, then wait and attempt to turn it on. That's bad practice - you should stop using the battery and think of charging it as soon as the device turns off for the first time. Deep discharge can severely damage the battery. Similar batteries are used in profesional Bosch power tools like drills and drivers. Obviously battery is a heart of such tool. When you continuosly use a tool its battery can get discarged suddenly and then the tool will shut off - you will be holding the switch pressed but the tool will just stop rotating. The manual says clearly that you shouldn't try to release the switch and press it again to try to restart the tool - according to the manual that can damage the battery.

Heat lowers battery lifetime. Heat comes from leaving the phone under direct sunlight, carrying it in a pocket close to your bode in hot weather, putting it next to a heater and so on.

Obviously you shouldn't do stupid things like piercing, burning, short-curcuiting and dropping the iPhone. All other factors like how often you charge it will have very limited impact on the battery lifetime. Even Bosch power tools manuals don't ever mention anything about how good or bad is charging the batteries often.

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Apple recommends to fully discharge the battery from 100% to 0% once a month. I guess this is different than a deep discharge since they don't mention anything about attempting to turn on the device at 0%. –  Senseful Nov 23 '10 at 6:44
    
@Senseful: Yes, that's not deep discharge, that's just normal usage. –  sharptooth Nov 24 '10 at 9:11
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Each charge 'cycle' is registered within the phone's software (same on a MacBook). If you think your battery is having issue (and is still under warranty), Apple will look at the number of charge cycles on it and make a decision against that.

Here's a pretty interesting, in depth article from 2007 on Apple's take on 'charge cycles': http://www.macworld.com/article/58916/2007/07/iphonebattery.html

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