I use a Garmin Edge 500 for recording most of my bicycle rides (I do at least one ride of 160 km or more per month on my Bike Friday Pocket Rocket). One problem with the Garmin is that some of my rides will take longer than the Garmin’s battery will last, but if you try to use an external power source to top up the charge, it will instantly end the recording. Here is my working solution:

Garmin quotes “up to 18 hours” of battery life, but last May I did a 300 km brevet with a 20 hour time limit. There are also 400 and 600 km brevets with 27 and 40 hour time limits (I am not thinking about 1200 km events yet!). While the Garmin has a USB port that it can be charged from using a cable, it won’t normally operate as a GPS unit while connected to a USB power source. Any GPS recording under way will instantly be terminated when you plug in the cable. So what can you do about that?

I searched a bit on Google and found that when you plug a USB cable into the socket at the back of the Garmin, it tests pin X on the USB mini connector. On normal cables (including the one that comes bundled with the Garmin Edge 500) that pin is left disconnected. Unless it finds it tied to ground (GND) as you supply external power to the Garmin, it will switch into a passive storage device mode. In that mode it provides read/write access to a PC via the USB port with all GPS functionality and user interface disabled. The screen will display only the brand name and it won’t respond to any buttons being pressed. If pin X is tied to GND, it will operate normally.

This behaviour mirrors the way pin X works on smart phones that support the USB “On The Go” (OTG) specification. USB OTG allows smart phones to drive certain peripherals such as memory card readers, in the same way a PC can drive those peripherals. Normally when a smart phone is connected to a USB port, it acts as a passive storage device to which a PC can upload MP3 files or from which it can download photographs (JPEG files). With an OTG cable, the phone remains the active end. Pin X is the magic key that tells the phone which way to behave, active or passive. It all depends on whether the USB plug is an OTG plug or a regular one.

My cheap low-tech solution was to buy a USB mini OTG adapter (480 yen – about US$5) on Amazon. This has a USB mini plug with pin X wired to GND on one end and a female USB-A connector (like a USB socket on a PC) on the other. To get power into this I cut the USB A plugs off two old peripherals (such as an old USB mouse), stripped off the ends of the wires in the cable and connected black to black (GND), red to red (+5V). This was not too hard even for my soldering skills. A bit of insulating tape and voila! We have a new male-to-male cable that can draw power from any USB power source and feed it into the female end of the OTG adapter. When I plugged it all together, I could run the Garmin in GPS mode while running on external power from my USB battery.

Lawyer Note: Do not use a male-to-male cable or OTG adapter for any other purpose. Do NOT connect the male-to-male cable to two PCs. Do NOT connect the OTG adapter between a phone and a power source. Only ever connect the cable to the OTG adapter. Only ever connect the OTG adapter to the Garmin. I won’t be responsible for bad wiring mistakes or other stupid mistakes. Don’t sue me if your Garmin or house goes up in smoke!

Here is the photographic evidence that it all works for me:

You can find USB Mini OTG adapters and USB Mini OTG cables on DealExtreme (dx.com). They also have USB-A male-to-male cables and adapters, so you don’t have to make your own as I did (though it’s not difficult if you have at least very basic soldering skills). Any combination of a USB Mini OTG cable or adapter and a USB-A male-to-male cable or adapter should work.