The Garmin Forerunner is a fantastic piece of equipment for a gadget-geek triathlete. This watch with a built in GPS and a heart-rate monitor, lets you monitor your performance, especially your pace. With a battery life of about 11 hours, the problem is that it does not last long enough for an Ironman race if you are duffer like me. However, there is a way around this problem …
The 305 recharger is powered by a USB connection to a computer. I connected my 305 to a computer and then started the watch logging data by pointing the watch at a window. It worked fine. This means that if one was able to supply +5 volts to the watch during the race, theoretically it would last 16 hours – the life of the race.
The first challenge was to decide how to mount the 305 on the bicycle. Fortunately, Garmin solved that one for me with their ‘Quick Release Kit (Bike to Wrist)’. This consists of a mount for your bicycle and a replacement wrist strap. The 305′s standard strap is removed and the 305 body snaps into the bicycle mount, or the new velcro wrist strap.
Unfortunately, the bicycle mount is designed to be used on road handlebars, not a triathlon bike with wing aero bars. I solved this one by cannibalizing a bicycle reflector holder which was designed to sit on the handlebars with the reflector facing forward. I drilled a hole in the Garmin bicycle mount and used a small screw and nut (with locktite!) to hold it in place.
The back of the 305 has four contact points. Looking at the back, the left and the right pins are -5 and +5 volts respectively. You can check this by putting a mulit-meter on the USB cradle. To be safe, I put some electrical tape over the centre two contacts which are used for data transfer.
My first attempt was to purchase a USB charger which ran from AAA batteries. However, I found that the charger put out +6 volts, and I wasn’t brave enough to test my 305 with a 20% voltage overcharge. A much better solution was to use a LM7805 voltage regulator. This takes in a voltage and drops it down to +5 volts. Connected to a 9 volt battery, it would probably last 6+ hours. Enough time to have the regular battery in the 305 last the distance.
The LM7805 is a 3-pin IC — Pin 1 (left) is the +9 volt + in from the battery, Pin 2 (center) is a ground, and finally, Pin 3 (right) is the +5 volt output.
UPDATE 1: I have since found several USB batter power units – even at Home Depot – which do give the +5 v output so you may like to try those instead. Much more convenient than and LM7805 if you are not technically inclined. However, I would advise checking with a multi-meter that they do indeed have +5 v output before you connect your 305.
UPDATE 2: Stephen Foaster noted that there is an error in the above drawing I did. It should read +5v and ground, not +5v and -5v.
I used a standard 9 volt battery connector. For the 305 I ran two wires and soldered flat terminals on the end of the wires. Fortunately, the 305 bike mount has holes in it already which are perfect for running the wires to the mount.
In order to keep pressure on the terminals I put in a couple of small springs out of ball point pens. The photo below shows the bike mount, with the 9 volt connector and the LM7805. Note that the +5 volt terminal is on the left as you look at the bike mount.
The photo below is the same, but with the 305 in the bicycle mount.
To supply power, I connect the 9 volt battery and then tape the battery under the arm rests on the aero bars. Insert the 305 into the bike mount, having remembered to put tape over the center two terminals first. The 305 display should say ‘Battery Charging in Progress’. Press the ‘Mode’ button and the display will change to being ready to record data. You are ready to roll. The life of the battery will depend on the type of 9 volt you are using. I go for high quality ones like the Energizer.