The new iPad is basically a screen, a battery, and a strip of electronics. It turns out that the huge battery is needed, because the high-resolution 2048-by-1536 screen is a massive energy hog. And without a bigger power adapter than the previous model, the new battery takes lots of time to refill.
On tablets, we typically run two battery tests: straight video playback, and audio streaming through a 3G or 4G network connection. For the video playback test, we generally turn tablets up to maximum brightness.
The iPad showed a huge difference between its battery life on maximum and 50 percent brightness. At maximum brightness, with Wi-Fi and 4G radios on but quiet, the iPad lasted for 5 hours, 33 minutes of video playback. That's relatively short for a 10-inch tablet. The Asus Transformer Prime got 7 hours, 38 minutes in the same conditions; the iPad 2 scored 7 hours, 30 minutes.
Turning the brightness down to 50 percent, though, we got 10 hours, 54 minutes hours of playing the same video file, under the same conditions. Yep, you read that right - halving the brightness on the display doubled the battery life. That Retina screen is a massive power hog.
I've been disappointed in the battery life of 4G phones before, so I was prepared for the iPad to show only so-so results in the 4G streaming test. Instead, I got an amazing 25 hours of audio streaming, because the power-hogging screen was mostly off. Compared to the screen, the 4G radio sips power.
Streaming time doesn't have a linear relationship to battery size. The Motorola Droid Razr Maxx phone gets about 7 hours of 4G LTE streaming on a 3.3 watt-hour battery. If we were to follow a straight line from there, a 42 watt-hour battery would get an insane 89 hours. But 25 hours is more than enough.
However, you're likely to be using the screen at least some of the time, so Apple's average of 9-10 hours of use seems right, as long as you keep the screen at about 50 percent brightness.
Why Does It Take So Long To Charge?
The iPad's 42.5 watt-hour battery is unusually large for a tablet. It's more like a laptop battery. Both the 11-inch MacBook Air and Asus's Zenbook UX21 have 35 watt-hour batteries. The previous iPad and the Asus Transformer Prime, the two major competing tablets, both have 25 watt-hour batteries.But the new iPad comes with the same power adapter as the previous model, putting out 10 watts of power. That's much less than the standard MacBook Air power adapter, which puts out 45 watts to charge a smaller battery.
The result is that the new iPad appears to charge very slowly. It's charging just as fast as the previous iPad (it uses the same power adapter), but it's filling a bucket almost twice as big with the same trickle of water.
Charging time is roughly linear. So where the previous iPad typically took around four hours to charge, the new iPad is taking more like seven. That's definitely something of which to be aware. Leave your new iPad plugged in overnight, and you should see a full battery by the morning. If you don't, you probably aren't getting enough sleep.