There is some confusion about the impact of background apps on iOS on battery life. Some prominent sources advise “quitting” all background apps to maximize batter life while others say that background apps are suspended and use no battery.
Actually, both statements are true. The confusing part is that iOS does not visually indicate suspended versus running background apps. Some apps are will obviously use power in the background – GPS navigation, instant messaging, or a music player. In other cases, it is not clear.
While there are many apps in the App Store that claim to help “improve” battery life, Apple does not allow access to any API which can have any impact on battery life. The only way to do so is to jailbreak your iPhone/iPad and install an app like BatteryDetective, which can measure how much power background apps and services actually use. (It also the only app which can tell you how much charge your iPhone battery can retain.)
In my brief testing, I made a few surprises. When an iPhone is not actively being used (the screen is off), the biggest power drain are the various radios: WiFi, 3G and Bluetooth. So first tip is to turn off any service you don’t need. SBSettings is useful in doing so with one tap:
Here are some sample measurements (not intended to be typical) in milli-amp drain in the background: iPod: 50mA, Gmail App: 185mA, Skype: 265mA. I found that some apps are seeming unnecessary energy-hogs: why does Skype use so much power in idle mode? I also found that the built-in Messages app was using way too much power – which went down after I deleted some of four years of received SMS and MMS messages.
BatteryMeter has a manual mode so you can take samples when toggling background services:
Unfortunately, measuring the power usage of apps and services is not practical for most users. So if you want to maximize battery life, just (1) turn off any function/service you don’t need and (2) remember that when and only when an app is doing anything in the background, it will drain the battery.