Why Your iPhone Battery Doesn’t Last

By Jon McDonald
The other day a friend asked about the battery life on my iPhone. I love the phone by the way; he was disappointed with how often he had to recharge his. I responded with the one thing I had tried—turn off the Bluetooth.

With that one change I have been pretty happy with the time between charges. His question got me thinking about the battery life of the phone, and I started paying attention to how long the iPhone would run between charges. I noticed that at home my iPhone seemed to last significantly longer than when I traveled. Initially I thought the longer life might be due to the level of use, but after checking this on a few trips and comparing it to my use at home, there was not a significant difference

I started looking through the settings to try to find something that could explain this difference. After some very poorly controlled experimentation I realized if I switched off the 3G network when I traveled, switching it on only when I needed higher bandwidth for a particular application, then the battery lasted much longer. Curiously the battery life appeared to be much closer to what I was experiencing at home. At home I leave the 3G on, but I also connect to my home WiFi network. My theory is that when connected to WiFi, the WiFi connection is used for data and the 3G service is turned off, or at least put in a significantly reduced power mode.

To me this was a shocking realization—that a product with such a large number of units produced in which battery life is of crucial importance could be so poorly optimized for power consumption was absolutely amazing.

Look at the two generally known ways of improving battery life on the iPhone. First turn off Bluetooth. Why do I need to turn it off? Why isn’t the phone smart enough to know if it is being used or not and power it down when not in use? Yes, you could say it always needs to be available, but if you look at the typical use case, connecting to a headset, the phone is the master, the phone knows when it needs to make the connection, so it could turn the Bluetooth function on and off as needed. Based on the reduced battery life I see when Bluetooth is on, even when not connected, it appears to be poorly optimized for the actual capabilities that are being used.

Turning off the 3G is a similar tradeoff. For normal phone calls and even push e-mail notification, I don’t really need the 3G performance, but if I want to browse the Net, use the maps, or any other higher bandwidth application, I want 3G on. Why isn’t the phone optimized to turn the 3G on only when needed? Apparently it does a better job of power optimization when WiFi is available. From my experience turning on 3G causes significant reduction in battery life, even if I’m only making phone calls. I can manually turn 3G on and off, but this is very cumbersome. The phone knows what is being done and should be able to control this setting automatically.

This kind of problem is an example of the issues ESL modeling can help address. By creating an abstract model of the system and exercising that abstract representation under realistic use cases, we can optimize the system performance and the power consumption for the work that needs to be performed. If we don’t know what work needs to be done, we cannot optimize the system. This is a crucial problem for the hardware designer. If we only look at the hardware in isolation, we don’t know what trade-offs can be made to optimize the system. However, if we look at what the system is actually doing, we can optimize the configuration of the system to deliver the needed performance for the minimum power.

I don’t believe it is a problem with the iPhone hardware. It’s a problem with the way the hardware is being used. This is a very sobering indication of how much potential there is for intelligent ESL design. It is also encouraging. By understanding the system power and performance in the context of the work that needs to be performed, we open up the possibility for incredible optimizations that are simply not possible in other design domains. With a smarter iPhone my friend can spend more time exploring the latest applet and less time tethered to a power cord.

–Jon McDonald is a technical marketing engineer for the design and creation business unit at Mentor Graphics.