Do laptops charge faster when turned off?

Reader Jeff Boulter writes: Does a laptop charge faster if it's turned off? I've always wondered if I should shut it down if I need to charge it as fast as possible.

The conventional wisdom has long stated that you should shut down your laptop if you want it to recharge quickly. It makes sense, right? If the computer is not using power to operate, it can use more of that power to charge the battery.

On the other hand, there's a "common sense" argument the other way: The lights don't dim in your house the more you turn on. And your stove burners don't put out less heat if they're all turned on. Does a laptop charging system really have anything to do with whether it's running or not?

To get to the bottom of this, I did two things: Asked the computer makers and tested for myself.

First, the computer vendor responses, most of which come direct from their engineering staff. Most vendors, including Dell, Apple, and HP said laptops charge faster while turned off. Sony hedged a bit, saying "it depends... but generally it will charge a bit faster with the notebook off." Only Gateway was the standout here, saying the laptop would charge the same either way.

I then put three notebooks to the test, draining the battery, charging them while turned off and measuring the time it took, then draining again, turning the laptop on, and timing the full charge while doing serious CPU work, playing video nonstop with the Wi-Fi turned on. The results for two of the systems were in line with predictions: A Toshiba and a Dell laptop both took more than twice as long to charge while turned on than when they were turned off.

Then I tried the same experiment with a new Apple MacBook Air. The results were shocking: It took about 50% longer to charge while turned off than on. I asked Apple why this would be and the company said it had no idea, offering no explanation at all for the anomaly. It's not really a bad thing, just a curious result that I hope someone will be able to explain eventually.

In the end, Sony's answer indeed might be the right one: It may really all depend on how the circuitry of the laptop is designed, and how power is allocated while a notebook is running. That said, it does look like most notebooks will charge faster while turned off, and quite a bit faster at that.

That said, note that a far bigger factor in charging is how powerful your A/C adapter is. In my informal testing, I found that ultralights with tiny A/C adapters took far longer to charge than bigger computers with beefy power charging units. In the case of the Toshiba, that was a full nine hours while the machine was turned on. You'd think that a small battery would charge faster than a big one, but that simply isn't the case: It's the adapter that really matters.