Testing: The New Nexus 7's Battery Life

By Norman Chan

The new Nexus 7 has a much higher resolution screen than last year's model, and yet it has a smaller battery. Here's why that's not a cause for concern.

There's so much to immediately like about Google's new Nexus 7 tablet. It's noticeably thinner and lighter than last year's model and its 1920x1200 resolution IPS display has the same effect on last year's 1280x800 screen as the iPhone 4's screen had on pre-retina displays. These advantages are tangible and instantly recognizable, giving the Nexus 7 an all-important "wow" factor. The intangibles, then, lie in two other critical areas: day-to-day performance and battery life. I'm going to save the performance discussion for next week after more testing, but wanted to talk about by experience and thoughts on battery life for this tablet (and this class of tablets in general).

On face, the physical improvements made to this year's Nexus 7 sound troubling for battery life. The thinner and lighter form was achieved partially by reducing the battery capacity from last year's model, down from 16Whr (4,325mAh) to 15Whr (3,950mAh). iFixit's teardowns of the two generations of Nexus 7 tablets don't indicate who the manufacturers are for these parts, so I assume that the lithium-ion polymer battery technology in each of these tablets is comparable. The 1.5GHz Krait SoC in this year's Nexus 7 is built on a 28nm manufacturing process, while the 1.2GHz Tegra 3 in last year's model is built on a less efficient 40nm process. According to Google, the efficiencies offered by the Qualcomm chip and features in Android 4.3 make battery life implications a wash--the new Nexus 7 is supposed to run for 9 hours of use--the same as (if not better than) last year's battery life.

Then there's the power considerations of the new screen. Conventional wisdom tells us that as LCD displays get bigger or more pixel-dense, they require brighter and more powerful backlights to maintain the same image quality. That's why that the iPad 3 with Retina Display had a bigger battery than the iPad 2, and was also thicker and heavier. It's likely why Apple's Haswell MacBook Airs didn't get higher-resolution screens, instead opting for long battery life as their killer feature. And it's also why many people don't think that Apple will be able to release an iPad Mini with a 2048x1536 resolution panel this year without compromising its size and weight. As John Gruber wrote recently in his thoughts on the topic, something has to give.

But I'm here to tell you that that's not the case. In my testing of the new Nexus 7 (which doesn't account for long-term/multi-month use and potential battery degradation), the battery life was excellent. I also think that the difference in usage scenarios for 7-inch tablets compared to laptops, phones, and "full-size" tablets requires that we valuate battery life differently. I'll get to that argument in a bit, but first let's look at the numbers.

Testing was split into two types of tasks: web browsing and video streaming. For both of these tasks, I was interested in two metrics: the total number of hours I could use the Nexus 7, and the amount of battery drain put on the battery per hour (as gauged using the HD Widgets battery percentage indicator). The latter metric is the more interesting one, and I based it on the battery drain rate starting at around the 60% mark. Mobile device users will recognize how battery percentage markers tend to move real slow when the battery is full or above 95%, and then suddenly seem to drop quickly at mid battery charge. On my HTC One and iPhone 5, the time it takes for the battery to drop from 99% to 97% seems to last several times as long as the drop from 59% to 57%. That's a consequence of the way lithium polymer batteries measure and report their charge, and correlated with the reason why topping off the last 20% of those batteries takes so long. With that in mind, the reported battery drain rate at mid charge is likely the best metric for power consumption. (Other conditions: Wi-Fi on, brightness at 50%, Bluetooth and NFC off.)

With sustained web browsing--scrolling up and down a page and loading new pages every 15 minutes--the 2013 Nexus 7 lasted just under 8 hours before received a low-battery warning. At mid-charge, the battery was being drained at an average rate of 13% per hour.

Video streaming over YouTube (this HD video on repeat) actually consumed less power, and the new Nexus 7 held up for 9.5 hours of sustained video streaming--beating Google's estimates. Battery drain for this task was measured at an average rate of 10% per hour at mid charge.

No compromising so far.

But I don't think the majority of people use their tablets that way--in multi-hourlong stretches at a time doing only one type of task. The daily use case for a small tablet is far more varied, and my bet is for shorter durations. In anecdotal testing, my daily use case for the Nexus 7 averages checking and replying email for half an hour every morning, browsing the internet for two hours throughout the day, and watching less than 30 minutes of streaming video at night. In that scenario--and with little to no thought put to battery conservation--I only had to charge the Nexus 7 every three days when the battery dipped below 15%. (Wired's recent review came to the same conclusion.)

And therein lies what I think is a significant difference between how battery life should be managed and valued between the Nexus 7 (along with other small tablets) and other mobile devices. For the majority of Nexus 7/iPad Mini owners or potential buyers, my guess is that these small tablets are secondary devices that complement other computers. Unlike a full-size tablet like the iPad or Nexus 10, I don't think the Nexus 7 is appropriate as most people's sole computer. As a consequence, secondary devices are by definition not as critical in day-to-day use as a laptop or smartphone. My work laptop is something I need to have fully charged because I could be using it all day, and the same goes with my phone. That's not the case with either my iPad or the Nexus 7. If I wake up in the morning and see that my phone only has a 25% charge, that's cause to freak out. Seeing the same thing on a Nexus 7 doesn't cause me to bat an eye--that's still enough of a charge for the few hours I might use it that day.

This is why I disagree with people like John Gruber about there needing to be a compromise when it comes to high-resolution 7-inch tablets. The new Nexus 7 proves that not only can a thin and light tablet support a screen for a full day of continuous use, and actually living with the Nexus 7 shows how meaningful that battery life is in the context of a multi-device household. That's my bid for users and reviewers to stop solely relying on the full-to-drained battery metric for evaluating battery life on tablets.

Two final things to note, in regards to battery life. One place where the Nexus 7 (and Android 4.3) potentially falters is in idle battery consumption. On each night over the span of a week, the Nexus 7 consistently used up 5% battery over the 8 hours when I slept. At that rate, it's a device that you can't just have idling for several weeks without charging. This was with Wi-Fi left on, so the Nexus 7 was persistently checking email, pushing some notifications, and updating apps. Idle battery life can likely be extended by turning Wi-Fi off, but I think it's unreasonable to manually switch Wi-Fi off and on every time you go to bed.

Also, the 2013 Nexus 7 support wireless charging through the Qi standard, just like the Nexus 4 phone. It's compatible with the wireless charging dock that Google sold along with the Nexus 4, but charges at a slower rate than the 10watt wall wart. During the quick-charge portion of the battery (eg. below 80% capacity), Qi wireless charging moved battery percentage at a rate of 5% every 15 minutes. I recommend the accessory if you have the new Nexus 7--it's much more convenient than plugging in that tiny micro USB cable.