I'm sure the phone can require more than 0.1 amp for some functions. For example, sending cellular data in a weak signal area can require up to 600 Mw for the transmitter. Under this condition the phone will get warm. So it's possible that the phone will charge slower with the 5 W charger than with a 10 W charger, because the 5W is being split between charging and powering the phone.
So the observations that a 10 W charger charges faster may be a result of this. However, it's not that the 10 W charger is charging faster; the 5 W charger is charging slower due to the additional drain of the phone.
That sounds right to me except the last part. Even on a 10 watt adapter the iPhone 4S didn't pull more than 1.03 amps. It didn't matter if it was the 5W or 10W, it rapid charges at .93 and pulls a max of 1.03.
I think some people make the mistake that rapid charging only happens below 80%, so maybe they are observing rapid charging on the 10w but noticing slower charging above 80% and not making ann apples to apple a comparison?
This is a very large but also instructive thread about a common misconception.
I think Mr. Finch is totally right. And I would like to add that his explanation also answers the question whether you can use a MacBook Pro Retina power unit (85W) to charge a MacBook Air (which comes with a 45W power unit). Turns out you can, without having to fear anything...
Apple officially states: (http://support.apple.com/kb/HT2346)
<< QUOTE BEGINS >>>
For instance If you have a MacBook (13-inch Late 2009) that normally uses a 60W adapter, you can also use an 85W adapter with that computer. You would not use a 45W adapter with that computer; it would not provide enough power for that MacBook.
Using an adapter of higher wattage than the adapter that came with the computer will not cause the computer to charge more quickly or otherwise operate any differently than using the adapter that came with the computer. The tables further down in this article show the style of connector that initially shipped with each model of MacBook, MacBook Pro, and MacBook Air.
<< QUOTE ENDS >>>
Note that Apple doesn't call the power unit a charger, because as Mr. Finch pointed out long ago, the charging circuit is in the iPhone or iPad or whatever... What we colloquially call a "charger" is in fact just a power unit.
I tried to read through the entire post but decided to jump to the end and ask a question that has probably been addressed before. If there is no software load on the battery, clearly the 10w and 5w adapters charge the iPhone the same and the temperature / charge time are also the same. But I would think that there is almost always a background process going on, e.g. if charging at night the iCloud sync/backup is using the wifi circuitry. Now if the 5W is not capable of providing the full current to the battery and the background processes, so that the battery charges at a slower rate than it is designed to, would it not be true that the heat generated is less than the 10W that supplies the battery and the wifi circuit with full current? In that possibly typical situation would not the 10W power supply result in more internal heat, not because of the battery charging process but because of allowing the background activity to run at max?
My total uneducated guess would be that this additional heat would only slightly affect the battery life, because the batteries are designed to be tolerant of extremes. Perhaps a slow steady charge is slightly better for the battery (not because of the wattage but because of the inability of the 5W to provide the full 5W to the battery), but is not a significant issue like with early lithium ion cells. If so, I would think that the users experiencing a problem should use the 5W adapter and/or see if background programs are causing a problem.
My educated guess is that any extra heat will not affect battery life in the slightest. Although I agree that IF the phone is sending gobs of data over Cellular while the phone is charging it will get warmer. The battery does not generate significant heat when charging, however. So if the phone was using gobs of data and sucking up lots of power from the external power source sufficient to make the phone warm the incremental difference in temperature will be small to unmeasurable. And Apple has considered this possibility; if the phone gets hot enough to affect operation (and the battery) the phone will detect this and shut down.
This high power consumption scenario will probably reduce battery life, but for a totally different reason. If apps on the phone use lots of power it will be necessary to charge the battery more often, and the life of the battery is measured in number of full charge cycles. And this reduction in battery life will be the same whether it charges fast or slow. I'm assuming, of course, that apps that use gobs of data will use it whether the phone is connected to its power source or not.
In response to JPSSnyder's quesiton 5W and 10W going to be irrelevant because the iPhone will treat both chargers as if they are 5W (or 1 amp at 5V.) That is, the iPhone will pull a max current of about 1.03 amps on a 5W and 10W USB chargind adapter alike. That is according to my tests of 12 different USB adapters that are 5W, 10W, and 15W; and 12 different cables on an iPhone 4S and iPhone 4.
When not at full load, rapid charging pulls .93 amps (.1 amps less) on both 5W and 10W adapters.
As for heat, I can't say with any authory or certainty but we know heat is not good. How warm I don't know. Moderate warmth may be not harmful but very warm or hot may be. Eneloop AA batteries get very very hot when charging and they can do over 2000 cycles. So we know rechargable batteries can handles a good amount of heat without significant damage. Obviously Apple has engineers that have taken this into consideration and probably one reason it doesn't charge faster on 10 watts.
So I suspect that anything othr than extreme heat is going to be "fine" for the iPhone. I wouldn't worry about it.
While rapid charging the battery already gets pretty warm on it's own. I don't think that some background tasks are going to raise the temperature a significant amount that it already is. Just using the phone with the screen on while rapid charging is far warmer than when in standby and charging, even if the phone is doing something.
Will a slower charger be more gentle? It could be? I'm paranoid. I have a 2.5W charger (.5 amp, which is the minimum USB spec) that I use for overnight charging specifically to prevent rapid charging. It's just not necessary overnight so why even risk it? I use normal 5W chargers during the day for mid day and in the car, though. I'm not recommending anyone go out and find a 2.5W charger and do this as well, but if you're worried about it's an option.
In case you're not aware, I'll repeat that the iPhone (and iPad) will rapid charge until the battery reaches about 80%. Above 80% it gradually reduces the charge rate up to 100%. This is to prevent harm to the battery. So it will cool off and slow down as it reaches full charge.
This testing (grahps below) is done with the Apple 10W USB adapter for iPad with genuine apple 30-pin cable. Screen was off, no background tasks running.
I don't have any technical knowledge in this field, as I'm a nework engineer by proffession.
I've an australian iPhone 5 since a year almost, and Apple supplied the same iPad charger(10w) with all iphone 5's in Austrailia . Unlike the US iphone5 charger.
And I never had any battery problems to this date, it doesn't even gets warm while charging.
I get a battery backup of around 10 hours (on screen time)
Message was edited by: ibnally
I am an Electrical Engineer at an aerospace firm. I design charging algorithms and software for batteries that cost more than the average house.
The incredibly patient Lawrence Finch has consistently been correct throughout this discussion.
- If you believe that your iPhone's battery life was shortened by the use of an iPad AC adapter, you are wrong.
- If you believe that your iPhone "feels warmer" when charging from an iPad charger, you are wrong.
- The iPhone charging is regulated within the phone. You could attach it to a power supply capable of 5V at 500 amps and it would not charge any faster or be damaged by the experiment.
- Your inability to resisting swilling down your entire 40oz. Super Big Gulp in one five minute sitting is not evidence that your iPhone is similarly unable to limit it's intake rate and quantity to what it needs.
I don't care if your sister's friend's brother claimed that his iPhone battery was destroyed by using an iPad charger. I don't care if ScooterBoy426 wrote that his iPhone melted because he used an iPad charger. Keep those stories to yourself and don't pollute this thread with vague, unscientific, anecdotal claims of shortened battery life or 'feeling warmer.'
I am neither as patient as Mr. Finch nor as nice (as you have no doubt noticed), so I will be direct: Unless you have expertise in electrical engineering, then limit yourself to asking questions in this thread and, if an electrical engineer answers your question, saying "thank you." That will go a long way towards increasing the signal-to-noise ratio in this thread.