Two recent reviews of the HTC EVO discuss the product’s battery life. Brad Feld, one of the brightest VCs in the country, writes:

I can only find one thing to complain about – the battery life.  It’s still running Android 2.1 so I expect there will be plenty of battery tune up in Android 2.2, but out of the box the battery only lasts about six hours…Ok – that’s literally the only thing I don’t like.

Michael Arrington, a man who needs no introduction to my readers, writes:

Well, I’m an Android Fanboy, and I’m telling you not to buy this device. The battery life is abysmal…but this device routinely runs out of power while sitting on standby overnight next to my bed. You aren’t just charging this once a day. Or twice a day. You need to be thinking about your next power fix just about any time you are using it.

I think these two reviews miss something fundamental. Perhaps having worked for a hardware startup has helped me see this. Building hardware that runs software is all about trade-offs. With smartphones there are three primary trade-offs: cost, size and battery life.

While cost is a real trade-off, the upper limit to the BOM on these devices is well known to each manufacturer so this variable doesn’t change much from device to device or between generations. Size is a trade-off too, but like cost, it’s fairly constrained: the new device can’t be any thicker than the previous generation. That means the primary trade-off across all smartphones comes in battery life. Let me state this another way: producing a great phone that gets six hours of battery life, is like me saying “I love my pet tiger, but he eats babies.” That’s just not an acceptable trade-off. Eating babies makes tigers fundamentally bad pets. Unusable battery life makes a smartphone a fundamentally bad smartphone.

Why is Apple so controlling? Why do they have phones that are all nearly identical? Why do they have particular restrictions on background apps? It all comes down to battery life. Battery life is not just another feature on some specifications checklist. It is the driving philosophy behind every design decision made on the iPhone. I think Android fanboys totally miss this point. Now, you might not realize this as an end user because the iPhone’s battery life is “OK.” That’s because Apple is making an explicit decision to trade some battery life and purposefully make it “good enough” for benefits such as a super dense and bright display, background threads, push notifications, video recording, gyroscopes and a laundry list of other features. If they could make a phone with a 10-day battery and all those other features at current size and cost, they would, but that’s not possible. They do this because they know it’s those other features that sell new phones, but battery life is the fundamental currency that runs the smartphone economy.* Apple is able to maintain this balance without dipping into unacceptable territory for the majority of users.

Let’s be clear that squeezing an extra hour or two of battery life out of a phone is no small task. You must make multiple trade-offs for every hour.

Since Apple is developing iOS for a very limited number of devices running extremely similar hardware, they can tweak every bit of code to optimize for battery life. Since Android is designed to run on heterogeneous hardware, these sorts of bit-level and device-specific optimizations are impossible. As smartphone hardware continues to become more complex, this dynamic suggests that iOS’s battery life advantage over Android on average will grow over time.** What you saw at the WWDC Keynote was Apple following a Reagan-esque Cold War Arms Race approach to defeating Android. Every new iPhone hardware feature is analogous to Apple trying to outspend Google in battery life. Google will feel compelled to start mandating front and rear facing cameras, gyroscopes, etc. to stay competitive, but all this will take an even greater toll on Android battery life. Android handset manufacturers will be forced to either make their batteries larger and thus their phones thicker or raise their cost and hope the carrier subsidies will go up too…or the HTC EVO 2 will get a stellar four hours of battery life.

*I’d wager that Apple is probably striking the right balance with battery life. How do we know this? If the iPhone 4 had been announced and it was the same, except 25% thicker than the iPhone 3GS, but had an extra 25% or even 50% battery life on top of what it will have, the reactions would have been universally negative. You know this is true. Don’t lie to yourself bro.

**I say on average because there will be variance within the Android ecosystem. Some Android phones will be better designed than others, but on average I predict, holding battery sizes between iOS and Android equal, Android will become less and less efficient per watt-hour of battery than iOS. Of course, one way for this prediction to fail, is for Apple to squeeze in more battery draining features than Android because Apple has more battery life to spare.

PS- Have I mentioned in the past ten minutes how much the HDMI out on the HTC EVO pisses me off as a product manager? Features without benefits are meant to appeal to a very specific target. That target is somewhere in the basement searching for their red Swingline stapler at this very moment.