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I generally turn off Bluetooth on my Macs and iOS devices when I'm not actively using a Bluetooth device. My assumption is that this saves battery life, but I realize that I have no fact to back that up.

Does having Bluetooth enabled draw extra power when no paired devices are within range? What about when paired devices are in range but not connected?
Does turning off 'Discoverable' except when I need to pair to a device save power?

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The answer is yes, as the radio needs to be powered on in case devices try to connect, but I have no idea how much. –  Kyle Cronin Mar 24 '12 at 23:51
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up vote 3 down vote accepted

Yes. When the bluetooth radio is active it not only turns on the receiver (which consumes power) but also periodically transmits to see if any known devices are in range.

When paired devices are on and within range, they do communicate occasionally even if not being used, and thus consume some power.

Turning off Discoverable will save some energy as it transmits less frequently than when it's not discoverable.

However, bluetooth is such a lower power communications standard that it's hardly worth worrying about - active bluetooth use (headsets in particular) do use a noticeable amount of power, but almost any other use, or just leaving it on, is negligible.

Further, for iOS devices the bluetooth is often part of the RF chipset, and turning it off doean't have much impact because it shares resources with the other RF functionality of the device, and without turning the other radios off (Wifi, GPS, cellular, nike+, etc) very little actually turns off when you disable bluetooth.

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It is worth noting that the new iPad is Bluetooth 4.0 standard, making it even less of a problem: en.wikipedia.org/wiki/Bluetooth_low_energy –  Adam Eberbach Mar 25 '12 at 10:28
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If Bluetooth is enabled, it’s listening for connections. So yes, having it enabled will require more power than not having it enabled, even if no devices are connected.

The same goes for WiFi — OS X is constantly looking for new wireless networks in range unless you disable it.

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In case of WiFi and cell networks, I think they even draw more power when no service is available, because they will constantly try to get a lock onto something. Another reason to switch on Airplane mode on an airplane. –  Thilo Mar 25 '12 at 1:19
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