Gear – Using Garmin Forerunner 305 for an Ironman

The Garmin Forerunner is a fantastic piece of equipment for a gadget-g305_gear_1.jpgeek 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. Fortuna305_gear_2.jpgtely, 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.


18 responses to “Gear – Using Garmin Forerunner 305 for an Ironman

  1. Wow, as if the 305 isn’t techie/nerdy enough you made your own power supply for the bike. I like the looks of it, but I’ll probably have a nerd buddy of mine make something up if I ever need more power. I like your idea though.

    Good luck with your tri!


  2. Pingback: TheyThought » Blog Archive » Garmin GPS Forerunner 305

  3. chris, very creative. i’m debating using my 305 for an upcoming ironman. my training partner has one as well and often my hr will double during rides. have you had any issues w/ crosstalk?

  4. Hi Chris. Ever tried replacing your 305 battery by yourself?

  5. Hi Chris, love your battery extender.
    My Forerunner 305 battery died and Garmin wants $99.95. I plugged in an iGo 2 AA battery pack and was good to go, sort of. It didn’t fit on my wrist. Now my plan is to solder a USB cable to the back and plug it into a Datexx 4 AA battery pack. When you say “Looking at the back, the left and the right pins are -5 and +5 volts respectively” Is the metal buckle at the top or bottom? Any idea which of the middle pins is Data – and Data +?

  6. Glad you found the post of interest. Pity about the high cost. Kind of a captive audience. I’d put a multi-meter on the 4AA battery pack just to check that the output is +5 volts. When I tested on earlier it was unregulated at +6 v. I think it was looking at the back with the buckle to the top. I’m travelling so don’t have my multi-meter with me to check – so no guarantees 🙂 Not sure which of the pins is Data +/-. Why are you interested?

  7. New to tri’s and just bought this watch, but what do you do with the watch when you are swimming? Do you wear the heart rate monitor in the water and leave the watch with your bike?

    • The watch is not waterproof so a big disaster if you try swimming with it! Too clunky as well. When I set up T1 I have the watch on my bike ready to go. I make sure to turn it on initially so it gets a quick fix. I wear the HR strap in the swim. Once I reach transition I turn it on as I’m exiting with my bike. Happy Garmining. It really is a great watch.

  8. I’m an ultrarunner, this works well for me.

  9. You mentioned that the charging pins on the 305 are -5 and +5 VDC, but you get GND and +5 V from the voltage regulator. Are you using some kind of step up circuit?

  10. I think Garmin has made improvments since this unit first came out. I’ve had my third unit for a couple of years now and updated the firmware. The unit has not locked during long runs or several marathons. Amazon has announced the 310xt, just wondering if the upgrade is significant to upgrade my 305..

    • I’ve had a 310XT for a while now and it is a huge improvement over the 305 – especially for triathletes as it can be used while swimming (but the GPS are all over the place). The battery life is also something like 20h so it will last any ironman. I’d definitely recommend it as an upgrade to the 305.

  11. I like this thing quite a lot. I’ve had it for about three weeks, and have used it on 12 workouts so far. I use it primarily for running, with biking a secondary usage. The main reason I wanted it was for the instantaneous readout of distance, running pace and heart rate, the latter needed for the speed work I want to do this running season.

    The core…

  12. I used a low-tech approach to interfacing the kit to the bike stem. I cut a 1-in section of 3/4 inch PVC pipe and drilled two 1/8 inch holes in it to match the spacing requirements of the garmin saddle from the kit. The cable ties pass through these holes to hold the saddle to the PVC. I then use a velco tie-strap to hold the PVC to my stem. It is very stable despite the fact that I’m essentially holding once cylinder to another.

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