When Apple released the iOS 4 update, people noted significantly lower battery life on a number of iOS devices. In troubleshooting the problem, it became apparent that Wi-Fi usage was in large responsible for draining the battery, and initially the recommendations were to disable Wi-Fi features.
Recently we wrote another article outlining a possible issue with incompatible router firmware that could lead to similar Wi-Fi problems, but in addition you may find the device losing battery power if you have run multiple applications before putting the device to sleep.
MacFixIt reader R.T. Taylor wrote in to us about the issue:
"Each programmer is responsible for turning on or off multitasking. And evidently they are not paying attention to that tiny detail. That is how you can have a multitasking job running in the background for a flashlight app.
To see what is running in the background, press the home key twice. You will see up to four background apps in the bottom of the display, scroll right to see what else is running. Mine had about 50 apps running."
Having applications running will obviously drain the battery to a greater extent when the device is in use, but when the device is in sleep mode, the applications should be suspended; however, people have found that fully quitting them all before putting the device to sleep seems to solve the low-battery-life problem.
It may be possible that even though the applications are suspended, their having been active before the device was put to sleep may prevent some hardware components such as the Wi-Fi adapter from being fully put to sleep at the same time. This may result in items like the Wi-Fi adapter staying active during sleep, and may also be a reason why when people specifically turn off Wi-Fi before sleep, that the battery level no longer drains rapidly.
Most people run applications and then press the home button to quit them and run alternative applications, but this does not fully quit the application. Instead, if you want to fully quit an application, go to the "Recent Apps" list and press and hold it the button, then then press the close icon and the full application process will shut down.
As R.T. Taylor wrote, you can check what applications are running or are launched and in suspension on your iPod or iPhone by pressing the home button twice.
UPDATE: Some MacFixIt readers have written in mentioning iOS4 should keep all but the foremost application in suspension. Technically by doing this the iOS does not truly multi-task applications, but instead only allows one application to run at a time. Therefore even if applications are open in a suspended mode they should not use any CPU (I have corrected the article in places to be more specific about this behavior). Nevertheless, people are finding that when they fully quit these applications, the battery life during sleep notably increases.
Any bugs in the iOS that are resulting in increased activity will ultimately need to be addressed by Apple; however, the available option so far that users can try are to quit any suspended applications (or fully restart the iOS device, which will do the same thing), turn off WiFi and networking services like locations when they are not needed, or address potential router incompatibilities with router firmware updates.