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If I have location services enabled and only Find My iPhone checked, will it use my battery constantly or only if I go to icloud.com and try to locate my iPhone?

I noticed that disabling location services saves a lot of battery but I still want to keep Find My iPhone enabled. I tried to do this but it looks like it's using location services all the time.

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The location services are only used when specifically required to do so. You should see a small compass symbol in the menu bar when the services are being used. You can see what apps have made use of the service recently by checking in the settings app.

For Find My iPhone, what happens is that the device maintains a connection much like being logged into a push mail account to the iCloud services. This allows the iCloud service to communicate with the phone, and when you log into iCloud and request a location for the device in question, it is able to send a message to the device over this IP data connection, and request that it turn on the location based services, locate the device and report back the findings, as well as tell it to display a message, play a sound etc etc.

So you are quite free to leave it turned on all the time and it will only use it when you need it. You can test this by setting yourself a reminder with a geofenced alert - if I am at work and I set an alert for when I get home to feed the cat, the icon will display constantly as it needs to know exactly when I arrive at a location, and the services and as such battery drain associated with it will also be used during that time. Clear the reminder, and it will turn off.

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Although there were probably some bugs initially that caused the location to go off more than needed, it now runs only in response to a program on the phone needing a fix or a push message indicating that you have signed into the web app or another device and pushed a request out to the device to show it's current location. It clearly doesn't keep the GPS engaged continually since that would drain the battery in 2-4 hours for most devices. –  bmike Jan 31 '12 at 22:22
    
I'm sure that the services for the reminders app are staggered in some way, if I have a reminder to do something when I go near a store, and I don't go for a week, I would't want a nonstop check going on, as it would clearly be detrimental to the operation of the device in terms of battery, and yet the icon shows the whole time just the same - although it's important to note the colour, grey for used recently, purple for used now. Whether it checks every 10 seconds or 2 minutes I'm not sure, but I have had it go off anywhere from the next road, to at my door, suggesting intermittent polling. –  stuffe Jan 31 '12 at 22:28
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Reminders are triggered by ambient cell tower changes and not GPS locations so you get this location accuracy for free on the device as long as it has a data connection to any cell towers. You can see this in action when you log into the web app. It will draw a rough mapping based on the last checked in value and then send a request to the device for a fix. The device shoots back it's last rough fix and engage the GPS for a better fix, reporting first the new approximate and finally the accurate fix to the web app. –  bmike Jan 31 '12 at 22:58
    
@bmike Are you sure about the reminders? Seems geofenced reminders drain my battery faster than normal. –  zigg Aug 3 '13 at 12:33
    
Of course they drain the battery faster. There is a little extra check each time cell tower changes on a long interval sampling. However, this drain is nowhere near the drain of GPS on and CPU on all the time in the foreground. –  bmike Aug 3 '13 at 12:53
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I do not believe this is enabled constantly - You can confirm this by going to Settings -> Location Services -> Find My iPhone and turn on "Status Bar Icon". That will indicate and confirm when the service is being used.

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I believe if I understand how it works, it only uses power when called. You can look in your location services menu, and any purple direction arrows mean that app is currently using it. Otherwise the arrow is grey, or not there.

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