I've been in a protracted discussion (now locked) regarding iPhone battery charging and whether it is harmful/helpful to use the iPad power adapter rather than the iPhone power adapter.
There are a lot of misconceptions related to iPhone charging and current ratings on the iPhone and iPad power adapters. As an electrical engineer, I'd like to clear up some misconceptions.
Apple supplies a power adapter, not a "battery charger," with the iPhone and iPad.
The iPhone power adapter supplies a constant 5V at up to 1A (rated–independent tests have shown that it can supply almost 1.8A).
The iPad power adapter supplies a constant 5V at up to 2 or 2.1A (rated).
The iPhone's battery charging voltage and current are regulated within the phone.
The iPhone's battery is rated at 3.7V. Charged with the raw 5V from the power adapter, it would be destroyed. Fortunately, the internal regulation in the iPhone prevents that.
The maximum current that an iPhone will draw from a 5V power adapter is right around 1 amp. It does not matter if the charger is capable of supplying more, that's the most that the iPhone will use.
Because of the internal charge regulation, the iPhone battery will not charge any faster on an iPad power adapter than on an iPhone power adapter.
Apple approves the use of the iPad power adapter with the iPhone. It is completely safe for that purpose and does not harm the phone or battery in any way.
A water analogy might make this much easier for people without electrical engineering background to understand.
- Water pressure (psi) is analogous to voltage.
- Water flow (gallons per minute) is equivalent to current flow (amps).
- A faucet valve is analogous to the iPhones battery charging regulation circuitry.
- A bucket's capacity (gallons) is analogous to a battery's capacity (amp-hours or AH).
Let's say you adjust the faucet to flow 1/10 of a gallon per hour. The five gallon bucket will take 50 minutes to fill up. Easy enough. But then...
You need a lot more water flow for a new sprinkler system.
You need a lot more current for your new iPad.
Since the water pressure is constant at 80psi, the only way to get more flow use a bigger supply line.
Since the voltage is constant at 5V, the only way to get more current is to use a higher current power adapter.
After installing the new higher capacity water supply line, you go out to your faucet and adjust it to 1/10 of a gallon per hour.
After plugging in the higher capacity iPad adapter, you plug in your iPhone and its charging regulation circuitry adjusts the current flow to about an amp.
The bucket still takes just as long to fill up.
The battery still takes just as long to charge.
The faucet is not under additional strain since the pressure is still 80psi.
The iPhone's battery charging circuitry is not under additional strain since the supply voltage is still 5V.
I hope that I've clarified this for those who want to better understand the subject.