We Care #1 - Saving Your Battery
There have been several comments and questions on the App Store, the Twitters, and via email concerning the use of GPS and its effects on the battery life of your device. Let’s answer those questions and clear up some misconceptions. We’ll start with what you need to know, then get into the technical details for the curious.
Misconception
This app is great in theory but it leaves your location services always running. Resulting in very poor battery life. — App Store review
Running the GPS costs a lot of electrons, this is true of all location-based apps that run in the background. Task Ave. does its best to preserve as many precious electrons as possible without you having to worry if it’s running in the background.
How?
Task Ave. gives you a choice! The iPhone 3GS and iPhone 4 have a location detection mode (more details below) that saves significant battery life at the cost of accuracy. It can be better or worse depending on the density of cell towers in your neighborhood, so YMMV. When “Save Battery Life” is set to ON, Task Ave. won’t be draining your battery while in the background and you will always receive proximity alerts.
Task Ave. uses your current location for proximity alerts when you get close to a location that has incomplete tasks.
If you turn proximity alerts off Task Ave. doesn’t need location updates in the background, so the GPS is turned off in the background. Similarly if you don’t have any incomplete tasks.
The “save battery” feature is appreciated as other apps that run strictly off GPS can really drain the battery fast. Also, the ability to turn the proximity sensitivity to “off” helps with battery management when the app is not being used. — App Store review
The Technical Details
The location services on iOS are provided by a framework called Core Location. It is used to retrieve your geographic coordinates and provides two methods of doing so. The first (and default method) is to use the Standard Location Service: Assisted GPS (AGPS) - a combination of GPS, known WiFi hotspots, and cell towers.
The problem with the AGPS method is that it uses the radios of the GPS and WiFi and these consume a lot of power. When Task Ave is running in the background in this mode it will drain your battery much faster. Core Location’s solution to the battery drain problem is simple for devices with cellular radios. The cellular service knows when there is a change in cell towers and the strength of the signal. So why not have apps register to be notified when a tower change occurs? Or a reasonable estimation of the region hit-test via triangulation based on signal strength? This mode — Significant-Change Location Service — is used when the “Save battery life” switch is ON.
Power savings comes at a cost to accuracy. The Significant-Change Location Service is accurate to 500m when the app is in the background (source: The Long Weekend Website). This is why the smallest proximity alert setting is 500m. You could actually be 500m from where it places you and the location service updates occur every kilometer. That means you could have passed the location by more than 1km before it notifies you.
In addition to saving your battery life, the Significant-Change Location Service will restart Task Ave. even when it is not running (closed by you or jettisoned by iOS) with the location update. That means you’ll always get your proximity alerts!
Hope that clears up any confusion. If you have any more questions or comments, feel free to contact us at hello@taskave.com or @taskave on Twitter.
