How I hacked my Garmin 310XT to last 37+ hours!

I am a runner with a preference for the really long runs. Kind of runs that last 20+ hours. So I have always been annoyed by the limited battery life of all GPS sport watches. The best battery life I have heard so far for a GPS watch is in the area of 20 hours. And I wouldn't like to be without it in the last stages of a race that goes on for 24 or 28 hours. But that is what is happening.

I have a Garmin 310XT GPS watch since 2009. This is a nice piece of equipment, that is quite reliable and providing about 18 hours of operation on a single charge. Unfortunately after these 18 hours I have to plug it using the bulky connector to a power source and it takes a couple of hours to recharge.

For my short runs I always go with my iPhone which is just fine for up to 5hours of activity recording. I like having a phone with me since I am a trail runner. So why bother carrying two devices. Bottom line is that in the last two years I have used the 310XT only a handful of times and this 300+ euros device was sitting at the back of a self almost forgotten.

I was racing a 100 miles trail running event in England last July and since it is fashionable there to run on unmarked courses, I was in desperate need for a GPS to assist me in my navigation for over 24 hours. One option was to use my eTrex Hcx with AA batteries that lasts for about 22 hours and change batteries somewhere in the middle of the race. But then I saw that 310XT on the self. I like hacking things. So I thought of opening the watch to see what kind of battery is using. Caution: This is an operation that has several risks especially for those who are not inclined to this kind of tech tricks.

The Garmin opens at the connection of the top metal grey and the mid rubber part of the case. I used a blade to open it with caution so that I would not scratch the case or damage the plastic parts. It was not such a big task and minutes after the watch was wide open. I removed the LCD screen (it is connected to the motherboard with a wide cable band) Then I removed the motherboard. The GPS sensor/antenna, is located at the lower part of the case, just under and between the two button labeled "reset" and "stop". This is a useful observation, since I need to know which part of the case must be facing up with no interference towards the sky in order to lock the satellites.

Just under the motherboard I saw the battery. It is a simple Li-ion 700mAh battery, that was not so difficult to locate at an online battery-store for just 23,49 euros. The battery is similar to the one of older Garmin Forunner 305 model. Keep in mind that it does not fit the dimensions of the 310XT in case someone gets the idea to replace the battery with a new one (because batteries are failing after 3 years usually). But it is the same battery according to the tech specs.

So I got the idea that I could add a second external battery to the 310XT, connected parallel to the internal one, so that I would double the battery life of the device! I ordered the battery from BatteryUpgrade.com and after a couple of weeks I had everything I needed to start the operation.

First I did a wooden mold a little bigger than the external battery and about as deep as the battery. I wanted to enclose the external battery in silicon so that it is not exposed to moisture and water. I sprayed a bit of oil in the mold so that the silicon does not stick to it. Then half-filled it with melted silicon from a silicon gun and while it was still liquid I placed the battery in it so that it was half covered by the liquid silicon. Once it cooled down the silicon was rigid, I removed it from the mold and repeated this procedure for the other side of the battery, so that in the end it was fully covered. Silicon did not leave any open gaps and sealed the battery completely.

I removed carefully the battery from the 310XT without disconnecting anything and exposed the poles that were covered with white tape. The red cable indicated the + pole. The same applies for the external battery I bought.

With my Dremel I drilled a small hole on the case of the 310XT at the bottom left corner and away from the GPS antenna. Remember, I do not want to put anything in front of the antenna. Through this hole I passed the two cables of the external battery (which I made a bit longer by soldering some extra cables).

I used a soldering gun to connect the two + poles together inside the case. I made sure that everything was ok by switching on the Garmin from the exposed motherboard buttons. It was ok. So I started assembling the watch again. I used some glue to secure the metal grey top cover to make it as safe from humidity as possible. The external battery was hanging out in its' silicon case.

I covered the silicon case of the external battery with some epoxy in order to make it more rigid and easier to glue on the 310XT case later on. It takes about 8 hours for the epoxy to dry completely.

I would like to notice that my Garmin 310XT has the optional Garmin fast mount allowing quick fix on the Garmin bracelet or on bike mounts. This is essential, since with the standard bracelet this operation will make it more difficult to have access to the recharging points at the back of the Garmin, but not impossible!

Once everything was assembled and dry, it was time to attach the external battery

on the Garmin case. I did that at the lower side of the watch, but away from the location of the GPS antenna that was inside. I did not want to block the signal.

I used epoxy glue again to secure the external battery and seal the small hole for the cables that are going inside the case. The watch is not waterproof anymore, but it can handle rain and sweat. I secured also the cables with some cable cover, for extra protection.

I tested the watch at recording mode a couple of times and it lasted for 37hours 30 minutes or more each time. It takes twice as long to recharge though. Also Garmin software is designed to measure VA so once the official 2.5VA are reached you get the message that it is charged, but it is not! The same applied when I am using the watch, after about 17 hours I start getting the message that battery is low because the 2.5VA are almost gone. But with the external battery there are more than 5VA available. So I just ignore these messages.

I used my modified Garmin 310XT during the Lakeland 105 miles event end of July. I uploaded the whole track and used the watch with the "Do track" feature. My race lasted 25hours 38 minutes. The 310XT worked throughout the race without any problems. I had to ignore the "low battery" warning from 17 hours and on. I also had to restart it at one point because the event was very long and the Garmin could not handle so many track points. But this was happening many times even before my hack. It restarted without any problem!

So, here I am with a unique GPS watch that lasts 37+ hours of non stop operation and it is only about 50 grams heavier than the normal 310XT. And I have it on my wrist all the time giving me a vibration everytime I go off track. This was the single most important piece of equipment for this race for me. It got me safely to the finish line after 105 miles on completely unmarked trails.