A few months ago, Holux released a new firmware for its M-241 GPS logger. This latest version v1.12 allows to turn off Bluetooth whereas before it was always on – whether used or not. Turning off BT is expected to lower the overall current consumption of the device. But does it really?

To find out more, Oliver and I set out to measure the power consumption. In theory, this is quite simple: Get your hands on a fairly accurate current meter (in our case a Fluke 87), put it in series to the power supply circuit and measure the current in various states.

In practice, there were a couple of problems. First, attaching wires to the device, wasn’t that straight-forward. But that’s where a PhD in physics and a bit of sticky tapes come in handy. The second problem is the fact that the current consumption of the device is not very stable. This means that the measurements weren’t very accurate at all. Here is what we found out:

• The base consumption is about 100-110 mA.
• The backlight draws about an additional 40-50 mA.
• Using an active Bluetooth connection (to laptop or mobile phone) adds another 40-50 mA.
• If there is no satellite fix, the device has an increased power consumption of about 180 mA. The power consumption is a lot more stable without satellite fix.

Overview over some of the results:

• Fix with 8-9 satellites without Bluetooth-Traffic: 115 mA
• Fix with 8-9 satellites with Bluetooth-Traffic: 160 mA
• Without fix and without Bluetooth-Traffic: 180 mA
• Without fix and with Bluetooth-Traffic: 225 mA
• Turning off Bluetooth in the Settings menu didn’t seem to make any different compared to leaving it on – as long as there is no client communicating via BT with the device.

Using a rechargeable battery of 2100 mAh and assuming an average current consumption of about 150 mA, this should lead to a battery lifetime of approximately 14 hours.