iPhone charger (left) and iPad charger (right).
A few days ago before I went to bed, I could not find my iPad charger for the life of me. Although I did not have an iPad charger handy, I found an extra iPhone charger on the floor of my room. Perfect, I thought, surely there is not that much of a difference between the two chargers. To my surprise, I was wrong. When I woke up the following morning I found that my iPad was only fifty percent charged. Why is it that my iPad can charge in about a night on the iPad charger but not on the iPhone charger? The simple answer comes down to basic current electricity.
| Source | Voltage (V=IR) | Current (I=V/R) | Power (P=IV) |
|---|---|---|---|
| USB Port | 5 volts | 0.5 amps | 2.5 watts |
| iPhone Charger | 5 volts | 1.0 amps | 5 watts |
| iPad Charger | 5 volts | 2.0 amps | 10 watts |
The table above shows the values of voltage, current, and power for three different types of chargers that can be used to charge an iPad. Although voltage may be the same, current differs, which is the main value we are concerned with when it comes to charging our precious iDevices. From a quick glance at the table, we see that the current of the iPad charger is twice the current of the iPhone charger. After this observation, the reason why my iPad took so long to charge with the iPhone charger becomes clear.
Let us assume that I can normally charge my iPad from 0%-100% in about five hours using the 2amp iPad charger. If we decrease the current and power by half by connecting the iPad to the iPhone charger, the iPad takes about twice the amount of time to fully charge (about 10 hours) than it would take on the iPad charger. Even worse, if I decided to charge my iPad through my computer’s USB port, it would take about four times the amount of time to charge (about 20 hours) than it would take on the iPad charger because USB current is only 1/4 that of the iPad charger.
If the iPad charger is the clear winner in terms of charging time, why not use the iPad charger for all of your iDevices? Theoretically, you could. Apple uses the same voltage on almost all of their iDevice chargers so that people can use them interchangeably. So, why not just buy all iPad chargers? In short, it is not necessary. Why waste money on electricity to charge your iPhone with a 10amp iPad charger instead of a 5watt iPhone charger when your iPhone would finish charging at relatively equal times with both? Supplying more current to the iPhone will not make it charge faster. Although the iPad charger has twice as much current as the iPhone charger, the iPhone battery circuit only takes the amount of current that it needs (around 1amp) so that it does not overheat. In many devices nowadays, oversupplying current is not a problem. Undersupplying current for rechargeable devices, however, can dramatically increase charging time.
Now that you have basic knowledge about iPad and iPhone chargers, you may want to think twice before using them interchangeably.
A Day in the Life of Physics 