Best Practice - iPhone Background Application Mode

Question

I'm currently testing the latest iOS4 Feature to put my location aware app in the background. Well, it does work! But on the other hand it's quite hart to handle the immense power usage.

The app consumed about 50% battery power in the last four hours. It read the entire official documentation by Apple on this topic but I'm still not sure which parts of my application are still running and which functionality is suspended (beside the UI Drawing, which should be clear).

I don't use any real boilerplate code but extended libraries like ASIHTTPRequest to talk to my webservice. Tests with a friends car did you show that the Network Connectivity and and the Location Services is still running when I'm using i.e.

[locationManager startMonitoringSignificantLocationChanges];

Apples Documentation on the different application states

Background: The application is in the background and executing code [...]

Should I write a "bare metal" functions to receive and send this location data? Should I remove all other objects for the time the application resides in the background to reduce the memory footprint? It seems there isn't any best practice yet.

Any ideas? Maybe you guys can provide me with some of your insights. Thanks.

Edit: There's a new Instruments tool called Energy Diagnostics Instruments to record any power usage (for iPhone 3GS and later) with an attached device. Also there's another service on the device in the Settings App -> Developer -> Power Usage. It's great to test your power usage in field. The created logs can be pulled later in instruments.

Reference: WWDC 2010 Session 309 - Advanced Performance Analysis with Instruments

Answer 1

Sounds like your app is transmitting location data over the cellular network. Turning on the cellular radio is one of the most rapid causes of power drain, especially if the user has a weak signal connection to the cell tower.

You might want to save and package up a bunch of location data, and send the data in a quick burst as seldom as possible (twice per day, when the user stops moving for 30 minutes, only after the user gets to one of their favorite restaurants, etc.) Turning on the radio less than half as often could get you close to doubling the battery life (unless the user is doing something else with the device as well).