Absolute Power


Secrets of the iPad 3

This blog originally posted on the Low Power Engineering Community 4/5/12.

I had pretty much decided not to purchase the new iPad (aka “iPad 3”) because I don’t use my first two iPads as much as I could. They are just something extra to carry around, and they don’t quite replace a laptop when I need one.

Of the rumored new features on the new iPad (Retina display, A5X processor and quad-core GPU, 5MP iSight camera, and 4G LTE), the only one that seemed significant for my use was the 4G communications, and that wasn’t quite enough to push me into those opening day lines. However, I’ve changed my mind, and am now playing with my new iPad. And the feature that pushed me over the edge only showed up at the last minute on the new feature rumor list—a significantly bigger battery!

Battery capacity and charging mysteries explained
Battery capacity should really be the headline for the new iPad. Battery capacity jumps 70% to a whopping 42.5wH! That’s higher capacity than the 11” MacBook Air. The overall device is only marginally thicker and heavier than the iPad 2, but a 70% increase in battery capacity? The rumors were that Apple had come up with some magical new battery chemistry. Could it be that the rest of the world had fallen that far behind? Well, a closer analysis shows again that there is no magic. The new battery capacity of the iPad is very much in line with the size (and weight) increase of the new battery pack.

The weight of the battery pack has increased 54%, and the total volume of the pack has gone up by 77%, so the 70% increase in capacity is in the right ballpark. The amazing thing is how the overall product is put together with the same length and width dimensions and only a 0.6mm increase in depth, plus about 50 grams in net weight. With a net product-level increase of around 7% in size and weight, battery capacity has increased by 70%. The “magicians” in Cupertino have once again shocked the world with the same technology that everyone else has access to, but they designed it better.

The other chatter on the Internet concerns battery charging. There are many complaints about slow charging of the new iPad. This is no surprise, because while the battery capacity has increased by 70%, the charge rate hasn’t changed. Charging a 42.5Wh battery with a 10W charger will take a theoretical 4.25 hours, assuming 100% efficient charging. In reality, expect five- to six-hour hour charge times. Using a 5W (iPhone) charger or a computer USB port will take additional time, which is inversely proportional to the current supplied by the port. And all of this assumes the iPad is off (or at least not in use) during the charging. The answer to your charging problems: Use the included 10W adapter, and charge overnight.

Retina display—good, bad, but not ugly
It’s good that the battery capacity has increased dramatically, because while the retina display looks great it’s a definite power hog. I ran my baseline “Star Trek” movie test on the new iPad, and basic power efficiency looked pretty good. Playing the entire movie in the “Max Battery” mode (airplane mode on, display and sound at minimum) required just 2.13Wh of energy. That’s even better than the iPad 2 at 2.25Wh. The basic hardware is getting more powerful, but energy efficiency is increasing even faster. Isn’t technology wonderful?

Once the brightness was turned up the display looked fabulous, especially displaying high-resolution pictures and computer-generated graphics. My movie test didn’t look much better on the new iPad vs. my iPad2 though. It’s clear you’ll need higher definition (at least 1080p) video to see much improvement. And the news gets worse.

Given the baseline energy measurement, we can get a pretty good estimate on how much that beautiful display is going to cost in energy. Turning up the brightness to maximum on “Star Trek” increased the energy consumption to 11.48Wh! That means the additional energy needed just to run the display between minimum brightness and maximum brightness is 9.35Wh for the Star Trek movie (2:06:46), and power consumption just for the display is around 4.45 watts. Imagine using a 5W charger to try to charge this device. Turning on the display at max brightness immediately makes it impossible to charge the battery. All of the energy is used up just to keep that display nice and bright. The charging time issues will continue to swirl in the user community for some time, and have a real impact on how the device can be used.

I believe this is a potential design flaw in the new iPad. What’s the point of a big, beautiful Retina display, if practicality forces you to turn down the brightness to the minimum usable level? And we’re just starting to see the other impact of viewing high-resolution video—gobs of data. Getting the most out of the Retina display will put many additional strains on the product, from device storage limitations to the cost of data plans.

From my standpoint, the Retina display, while beautiful, isn’t worth the high energy (and data) cost. I can’t see the individual pixels on the new iPad, but to tell you the truth, I can’t see them iPad 2 either, without high-powered reading glasses. Is it possible that Apple has lost sight of the fundamental design principle that “form follows function”? Knowing what I know now, and had there been the option, I would have ordered a custom new iPad with the old display, and been very happy with the 70% larger battery.

The killer app
While these issues will keep the blogosphere busy, there’s another application of my new iPad that is a clear winner. I got the Verizon 4G LTE iPad, and the communications speed is very impressive. I’m seeing network speeds up to 13 Mbps on LTE, compared to 700 Kbps in 3G. These bracket the network speeds of up to 4 Mbps I get on my iPhone 4S on the AT&T network (you know, the 4G network formerly known as 3G). With much better coverage in my area, Verizon 4GLTE is looking like a winner, and I really can’t wait for the iPhone 5!

On top of that, the Verizon personal hotspot feature is included with the service plan on the iPad, so for the same $20 I used to spend just to enable the personal hotspot on my iPhone, I get expanded coverage across both AT&T (iPhone) and Verizon (iPad) networks—plus a faster network connection to share. And with the iPad’s new larger battery, the killer app for me is using my new iPad as a WiFi hotspot. I was traveling last week, and two of us used the iPad as a WiFi hotspot for more than 12 hours. That’s more than a full day’s work, transferring a total of 297 MB of data in email and Web browsing. At the end of the 12 hours, the battery capacity was still at 57%! That’s incredible! And the irony is that this best and most impressive usage of the new iPad was run with the fancy new Retina display OFF.

Next time, I’ll look at power efficiency of the 4G LTE network vs. 3G. All that speed has got to cost something, right??

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