One of Nvidia’s key Tegra 3 selling points is "lowest power, highest performance." Basically, it claims that the added cores shouldn’t impact battery life. How does that hold up in real life? On The Verge Battery Test, which loops a series of 100 sites and downloads high resolution pictures with brightness set at 65 percent, the tablet’s 25Wh battery lasted five hours and nine minutes. That’s obviously not as long as the Galaxy Tab 10.1 or the iPad 2, but it translates to about six hours of continuous normal usage. I should note that I ran that test in "Normal" mode; lowering it down to "balanced" or "power saving" mode should increase battery life, and Asus has already promised a firmware update that should improve the already-solid endurance. (See update below.)
But, of course, the tablet has the keyboard for some great battery back-up. The keyboard houses an additional 22Wh battery, which when strapped to the tablet, let the entire package run for 10 hours and four minutes on that aforementioned battery test. You’re likely to get even more juice out of it when just watching a movie and if you use the tablet intermittently without the keyboard since the keyboard can actually recharge the tablet’s cell.
Oddly, the keyboard dock doesn’t come with its own charger, so if you want to charge them both at the same time you have to dock them together. It’s not convienent, and for $149 for a keyboard, I would expect Asus to include an extra AC adapter. It took close to four hours to charge back up both cells via the AC adapter.
| Battery Life | |
|---|---|
| Transformer Prime (Normal Mode, Verge Battery Test) | 5:09 |
| Transfomer Prime (Power Saving, Verge Battery Test) | 6:05 |
| Eee Pad Transformer (Verge Battery Test) | 7:27 |
| Transformer Prime (Power Saving Mode, 720p video rundown) | 9:19 |
| Eee Pad Transformer (720p video rundown) | 7:14 |
| *All tests run at 65 percent screen brightness |
UPDATE: A big update on the battery life: On our standard test with brightness set at 65 percent and power saving mode switched on, the tablet lasted six hours and five minutes — exactly an hour longer than when it was set to normal mode. Still, that's an hour shorter than the original Transformer, which lasted seven hours and 27 minutes on the same standard test (the same 65 percent screen brightness setting, etc). Asus maintains that the screen brightness is the culprit here, and I tend to agree with them — 65 percent brightness on the Prime is higher than on the original Transformer and most other tablets. I don't have a light meter, so I cannot measure the exact brightness equality amongst the screens.
However, there's another explanation for the shorter run time on our standard test: when I ran a video rundown using a 720p clip with brightness set at 65 percent and in power saving mode, the tablet lasted nearly two hours longer — yes, it ran for nine hours and 19 minutes. (By the way, video playback was extremely smooth at that setting.) According to Nvidia, Tegra 3 is optimized for video and thus doesn't use the added cores when playing HD video. Nvidia and Asus hold that the standard Verge Battery Test script keeps all cores active (even in balanced or power saving mode), and that those test results would be representative of somebody sitting down and playing a game like ShadowGun which draws on all the cores.
I am currently rerunning all these tests with the dock attached to give you an idea how much time you'd get with the extra cell, but I think it's pretty clear: the Prime is capable of running for over nine hours on a charge (even longer with the dock attached) with the screen being plenty bright, you just have to be mindful of what you are actually doing with the tablet and the power saving modes. Regardless, the battery life can be very impressive for the thickness and for the power inside.