Home Forums Peripherals & Multimedia iTunes, iPhone, iPod, iPad, Apple TV Recharging iOS Devices

Is it faster to recharge via the AC adapter or via the computer? Or does it even make a difference?

I take it you haven't just tried both methods and compared results?

FWIW, my iPhone 6 charges much more quickly via the charger than via the USB port in my keyboard. I've never plugged it directly into the computer.

I have yet to try/compare results as it seems I seldom have identical conditions. I suppose I could force such, but was hoping an electrical engineer or such would have objective/scientific data (rather than my anecdotal observation & conclusion).

That all said, it seems reasonable (to me) that the AC route would be faster. Alas, all too often, what I think is reasonable, just ain't so. <Sigh!>.

the voltage should be constant in either event. Power is current x voltage, and power is what charges, so the current is the variable.

Normally, current is supplied "on-demand". Voltage = current x resistance. The device to be charged applies a load (resistance) to the supply, and with the voltage fixed, a predictable amount of current (and thus power) is drawn.

There are a number of standards regarding USB power, and no provision for any real "communication" about the power state on either end - how much power is available, or how much power is desired or needed. The data pins might be used to communicate this information, but at the time it wasn't practical to put a microcomputer inside everything that might want USB power, to try to talk wiht the supply and negotiate a deal. So peripherals would just draw what they wanted to, and if the host determined too much power was being drawn, all it could do is pull the plug.

They solved the problem by applying simple "drop resistors" from the power supply to the tx and rx pins. This dc power didn't affect communications, but set up predictable fixed DC voltage on the tx and rx pins. A standard of currents was set up for differnet voltages. Now the peripheral could "know" how much current was available before applying a load. It could vary its draw, or pop up a message on the device to indicate there wasn't enough power available. Rather than just falling back to the previous standard when drop resistors are not available, some devices (*cough*iphone*cough) will display a message saying the host is incompatible, rather than just drawing the minimum they know shoud be available.

So now that you have some background... wall chargers etc (dedicated usb power sources of any type) now come with drop resistors, to inform the peripheral how much current they can draw. Most are either 1, 1.5, or 2 amps. Two different sources that both advertise 2 amps should both supply the same amount of power. Those are limited by the size and capacity of the parts used in their power supplies, and can vary considerably. Cheap ones from china tend to not produce "clean" (noise-free) power when you approach their limit, or they may simply shut down when you get near their limit. (or HACF)

Computers are a little smarter. They almost always have some circuitry monitoring the current draw, and if the peripheral tries to draw more than allowed, they get the plug pulled on them, and won't auto-reset until you unplug the peripheral. (this also halts usb communication to that port) Computers typically provide the same 1, 1.5, or 2a limits. Laptops on battery power may reduce the available amount to conserve battery power. In theory they should be able to vary the amount advertised, but the standard doesn't provide a way to change the amount on-the-fly.

So.... in theory, both should be able to charge a peripheral at the same speed, if they're both rated to provide 2 amps of current. (2a x 5v = 10 watts) I thought I read something about 5amps being a new available value also, but not sure. Both power supplies and computers could be more limited though. So you really can't tell by looking at them what amount of power they can provide, how fast they will charge. Although the information is available, I have yet to see a peripheral tell me what amount of power the charging source is providing. AASPs can get a test cable from Apple that lets them charge a device, but hook an ampmeter inline to monitor charging power, to make sure the ipad/iphone is drawing proper current, the average user has no easy way to tell.

Many thanks V1 for sharing your insight. That is exactly the info I was seeking. Not only was that a wonderful reply, I may have even learned a thing or two.

Charging Apple iOS devices can be confusing. The iPhone will run and charge from a normal USB port such as the one on your computer, on the other hand while the iPad can run from the USB port on your Mac it will not charge from it. (I have an external hub that has two ports specifically designed to charge an iPad and four standard USB ports that will not, an iPhone on the other hand will charge from any of the six ports.) The higher power charger that comes with the iPad will charge an iPhone more rapidly than the charger that comes with the iPhone, but there have been concerns expressed, at least at my local Genius Bar, that consistently using the iPad charger on an iPhone may reduce the iPhone batteries longevity due to overheating. I have never kept an iPhone long enough for overall battery longevity to become an issue, so I can't verify that statement, but it definitely takes less time to charge the iPhone battery. When an iPad is plugged into a port that has insufficient power to charge it the iPad will throw an appropriate notification on the screen.

It may be even more confusing than that. When my wife’s iPad Mini 3 is hooked up to a USB port of a Mac or attached USB hub, it always states next to the battery icon that it’s not charging. That turned out to be incorrect: it does in fact charge, but without some indications of the charging process, including the display of the partial/entirely green battery image. That shows up when she uses the charger.

While on the subject of battery charging and charging speeds I ran across this CNET article and this web site and ended up with a whole new viewpoint on the "simple" lightning connecter cable. It turns out there is no small amount of intelligence built into the lightning cable connectors and there is a wide range of difference in how the cable itself is constructed. The cable construction alone can make a significant impact on how much of the power coming out of the USB port actually reaches the iOS device and a corresponding difference in how long it will take to charge the battery in the iOS device. The cable construction made sense when I though about it although I did not realize how much effect it might have. The intelligence in the lightning connector itself was eye opening.

I have noticed the same thing on my wife's iPad, but although we both have the same iPad model mine does not charge the battery to any detectable degree.

My iPAD charges with the USB to the MBPro . . . but noticeably charges more quickly with the A/C charger to the wall outlet.

P.S. I haven't actually monitored the charging times of either method to the minute, but the A/C is a faster method.

Maybe when you a moment...

Will your wife's iPad charge normally when it is connected directly to your machine?

Just wondering if it's a machine or iPad related issue, or even the cable.

Curious and curiouser...