I get a lot of use out of my portable mobile devices. Ever since I put a keyboard on my iPad, it has become my little workhorse throughout my day. Before work, I might be using it for a breakfast recipe and some light writing while I eat. During work it sits on my desk as a dedicated email device and I’ll bring it with me to meetings throughout the day. At night I watch TV shows on it and do all sorts of other things. All of this work takes a toll on the battery, and while it’s definitely up to the challenge, it does need to be charged regularly.
Not all of my mornings are “read a recipe online”-smooth though. Sometimes I’m rushing, and I often forget to put my iPad or my iPhone on the charger overnight. There might be an hour at most of available charge time from the minute I get up to when I walk out the door. I hear this question a lot: How can I charge my iOS devices faster?
There’s actually some interesting things going on here, and it all has to do with electricity and battery chemistry. Luckily, I have extensive experience with both and will try to explain all of it here. If you have an iPad and an iPhone, you’ll know that they each come with a different charger. The iPad (excluding the 12.9-inch one) comes with a 10-watt charger and the iPhone comes with a 5-watt charger. Each charger is outputting its power via USB which is always going to be 5 volts direct current. The iPad is a larger device than the iPhone, and it has a larger battery. Charging this battery up in a reasonable amount of time requires more power, hence Apple providing you with a beefier 10-watt charger. The iPhone’s battery is smaller and can be charged in a reasonable amount of time with only 5 watts.
Both chargers output 5VDC USB power. Couldn’t you plug your iPhone into the larger 10-watt charger to charge it faster? The short answer is, yes. To understand why, let’s look at exactly what we’re talking about electrically in this situation. Both chargers are outputting the same voltage, but they are each rated for different power so what’s changing? Well, power is voltage times current. This means that the current is the value that’s changing. Indeed, you may actually see this listed on certain chargers. A 10-watt USB charger will by definition be capable of delivering 2 amps of current. 5VDC x 2 amps = 10 watts. The 5-watt USB charger delivers only 1amp of current. 5VDC x 1 amp = 5 watts.
When it comes to charging batteries, current is both your best friend and your worst enemy. Current is what shoves the ions in the battery to their “potential energy” positions, ready to release and deliver power for you. So more current will mean more of that happening, and faster too. But that process isn’t free. You pay for it with heat build up and secondary electrochemical reactions that can degrade the life of your battery.
So battery charging is typically a compromise between fast and keeping the battery life long. Ok, but wait, wouldn’t that mean you’d be ruining your iPhone by charging it off of 10-watts instead of 5? Nope. You won’t damage it at all. This verdict comes straight from Apple itself. But why is this? Inside your iPhone, iPad, and even your MacBook is a lithium-ion battery. These batteries are amazing. Their chemistries have high voltage potentials relative to other chemistries, they’re very lightweight, and they can withstand thousands of charge/discharge cycles. However, they are very picky about how they are charged and if anything is wrong, they can be seriously dangerous. Just ask Samsung about that.
For this reason, all devices with lithium ion batteries have on-board charge controllers that regulate the charging. Many modular lithium batteries have protection circuits built into them by default. So no matter how much charge current is available, the controller will always have the last say about how much of it hits the battery. This is why you can charge your iPhone with a larger iPad charger. Just because the larger charger is rated for 10-watts (2 amps) doesn’t mean it’ll be delivering that all the time. It means it can deliver up to that if necessary. There is some portion of the iPhone’s charging profile where the extra power can be safely used, but it’s only some portion of the charge process, not all of it. This is why your iPhone will charge faster, but not twice as fast.
Can you go the other direction and charge your iPad with the iPhone charger to extend it’s battery life by charging it more slowly? No. This will not work because the iPad’s larger battery requires the extra power to charge it. The battery cannot be properly charged without the full 10 watts being available. This is why you’ll often see USB chargers say that they can or cannot be used to charge tablets. All tablets, not just Apple’s, typically require 2 amps. USB ports on most computers follow the USB bus protocol which means they can only output 0.5 amps or 0.9 in some cases. USB-C can output up to 3 amps. This is why your iPad may not charge when plugged into your laptop (though it can still transfer data over USB).
These charging rules basically apply across Apple’s entire line of products. You can always charge a smaller device/battery with a larger charger, but not a larger device with a smaller charger. Obviously this only works if the voltages are the same. You cannot charge your iPhone with the charger for your MacBook because the voltages are different (as well as the plugs).
I hope this was an interesting topic. We’re really getting into applied electrical engineering now!