Going really really really long with your Forerunner 305

One of the most common complaints with the Forerunner 305 is that for most age group athletes is its battery life – limited to about 10 hours on a good day.  This precludes many folks from being able to use it for the entirety of an Ironman.  As well as many trail runners. While the new 310XT fixes that (up to 20 hours), it also ‘fixes’ a whole in your wallet.  Plus, what if I want to go 30 hours?

So, back a few years ago I stumbled into this site that put together a pretty creative way to use a 9V battery and some geekness to create ones own constant charger, to be mounted to a bike for long-course options.  But running would have been tricky.

I happened to be visiting the site recently for no particular purpose and noticed a link to this site, which shows a totally awesome new option that only costs about $12 (though it seems to fluctuate in price up to $16) – and requires nothing more than the ability to put two batteries in a little case.  Oh yea, it goes for 36+ hours.  Booyah!  Of course, all the credit goes to him for figuring this out.

So I clicked on the Amazon link and a day later it showed up on my doorstep – perfect for my recent Europe/Africa trip.  Here it is upon arrival from Amazon:


Next we take my normal Garmin 305 with it’s normal USB charging dock.  Go ahead and disconnect the USB cable, it’s too long for our purposes.


Then we splice our hands into 28 pieces opening the god-awful plastic packaging.  Once our hospital ER visit is complete, we have the innards removed: Two batteries, a little black charger and a tiny USB cable (technically two cables, but the other one is for cell phones, useless to me):


Next we accomplish the extremely difficult technical task of plugging the USB cable into the charger and into the Garmin base.  Then we sit back and reflect upon our accomplishment, while noting that we’re now successfully charging – according to the little Blue Light Special on the corner of the unit.


Now, if we look at the unit’s menu while it’s powered on, we’ll notice the little charging electrical bolt icon in the lower left hand corner:


See, time to go long.  Now all you ultra runners can go running all night long without fear of GPS death.  I found that using a simple rubber band held the the thing together in case of bumps.  If you look at the post I linked, he modified it to attach to a backpack/Camelbak, as well as a waistpack.  So lots of options there.

Here’s a GPS track I made, from Nairobi to Zurich (plus half a day in Zurich), then onto Washington DC.  Approximately 24 hours in length including the layovers and everything.  I used Energizer Ultimate Lithium batteries, as recommended in his post.  Worked great!


There ya go!  Like I mentioned, I use a little rubber band to keep it all hooked together, but I’m sure folks can come up with something more creative.  Have fun!

Oh, and if you’re looking for other Garmin Forerunner 305 related stuff I’ve written up, seek no further than the below:

10 Random Tips for the Garmin Forerunner 305
How to clean the contacts on the Garmin Forerunner 305
How to ski with the Garmin Forerunner 305
How to download workouts to Garmin devices (all training devices)
..and a ton of other Garmin stuff related to ‘How to guides’ is here.

[Quick update: If you have the Garmin Forerunner 310XT (which lasts 20 hours), this solution won’t quite work because of the USB connector on the 310XT charging unit is different.  In theory you could use a USB A Female to USB Mini B Female adapter, and should work…but I can’t quite seem to find such an adapter yet – so if you see one, let me know and I’ll give it a whirl.]


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  1. Brian

    I’d be interested in seeing a picture of your rubberband setup.

    Also, can it be plugged-in while on the go? IOW, say you’re out running a long run and you realize that your battery is about go out…can you whip out the little charger and then plug it in at that point?

  2. I have the 305 Quick Release zip tied to my stem. I could use a couple of rubber bands to secure the batter pack to the side, or underneath, the stem as well! Ironman Canada here I come!

  3. Beautiful! I was trying to figure out how to use the Forerunner 305 on our trip to Iceland. You have provided the answer.

  4. I did that and it works great. Kind of a hassle at times, but it does work.
    I used the Energizer brand cell phone recharger made for a Blackberry.

  5. I haven’t had much time lately so just wanted to let you know I’ll catch up on the posts later.

  6. Nice I’ve seen this done a couple of times for a ‘on the go trail version’ check out Steve Ansell’s blog link to mountain-man-steve.blogspot.com

  7. A friend of mine recently did a 95-mile race in just under 20 hours. He used his own Garmin 305 for the first half and borrowed a second one from a friend for the second half.

    Of course, your solution is much, much sexier.

  8. Kim

    Oh very cool! This would help me maintain a pace for BR-the whole race!

  9. Can you include a picture of the whole rubber banded contraption on your wrist? I just want to see how you manage that.


  10. Brian

    FWIW – This seems to be another factor in favor of the 305 over the 405. Even though it has a USB cable charger, I don’t think you can charge the device while it’s in use.

    It certainly would be more cumbersome even if you could, what with the clamshell style charger that it uses.

  11. GENIOUS! I need one of those. Note becuase I go long but becuase I forget to charge it, or plan to do it at work and forget the charger, or plug it into the laptop and leave it but unknown to me the laptop when to sleep and somehow that turns the Garmin on and even though it was plugged in all night it is dead for a morning workout.

    ps..40+ in a 25! How did you not get pulled over? I was “yelled at” through a window of a cop car once. Me and two other guys going paceline style doing 37’ish down a small grade through a small town’s 30mph zone. He wanted everyone to slow down and be safe, how nice of him…we did until he turned. Then it was back to hammering away.

  12. This looks great. One question though, how do you charge the device without the dock? Or how do you attach the FR305 to its mount with the dock. What am I missing?

  13. Hi Uri-

    (RE: Using the dock)

    I still use the charging dock for charging, and simply just attach it to my computer/wall. Just easily disconnect it from the Duracell charger.

    Hi Brian

    (RE: Charging while in use)

    Yup, you can indeed charge while in use, it’s pretty cool. I’ve done this a few times on long flights even with my laptop and it still recording the flight. Any USB connection will work, and since the mini-charger thing is just a USB connection, it still alows you to charge it. Cool stuff! So yup, even if you realize half-way through, you can plug it in to get a bit more juice.

    I’ll try and post some pics in the next day or so of my setup.

  14. Awesome. I had this bookmarked in my stack to read after vacation, and I’m glad I did. That’s a pretty cool little gizmo. If you’re carrying a hand-held water bottle holder, the battery pack could probably fit into the zipper pocket.

  15. I guess those of us with 101s just have to carry spare AAs :-)

  16. Very nice! However, I read somewhere that the charger only works with non-rechargeable Duracell batteries. Is this correct? It would be great if we could use rechargeable batteries… Have you tested this? / What kind of batteries do you use?

  17. Hi Felix-

    I’ve just used generic AA’s, nothing special. I haven’t heard of any issues with rechargeables. Typically the reason folks don’t recommend rechargeables is that certain types don’t do well in colder weather conditions, and also have a different battery profile with respect to how they handle lower power at the end of the life of the battery.

  18. Anonymous

    Thanks Rainmaker.
    It’s really fantastic !!!!

  19. Jim

    Ray – Would this work for the 310?


    link to buyextras.com

  20. Hey Jim-

    Hmm…in theory it would work. I do know from testing that a continous charge will be honored and you can go forever on it (tried it during a long long boat ride). So, given the adapter, it should work.

    If you try it out – let me know how it works out! Would love to hear.

  21. piet

    Hi rainmaker,

    tx for this great idea. Sometimes the most obvious thing are not on the head up display, so tx for putting it up!


  22. IMHO this is one of the advantages of units like the Polar RS800CX that utilize a separate GPS pod.

    The separate GPS pod that takes simple AA batteries makes it easy to utilize long lasting lithium batteries (I have heard they last about 30 hours) and replace them during the run if necessary.

  23. Anonymous


    Since you’re charging the unit on it’s base I’m assuming you won’t be able to utilize the Hear Rate Monitor while running?

  24. Anonymous

    Mine is 3 years old and the battery is lasting only one hour now. So I need to get a new battery? I don’t think mine EVER lasted 10 hours!

  25. The docs for the 305 state that the data storage capacity is “1000 laps”. If the default value for a “lap” is one mile, the implication is that I can track 1000 miles and that’s it. In your posting about tracking for 36 hours — how did you overcome the “1000 lap” limitation?
    I want to use my 305 during a 1500 mile motorcycle ride (1500 miles in 24 hours). Could I record the entire ride simply by changing a lap to, say, 10 or 50 miles?

  26. Hi Douglas-

    Yup, I just leave autolap off, which means no laps are created automatically. Otherwise the unit will be beeping almost constantly on an airplane if you’re flying at 600MPH. ;) I learned that one the hard way once.


  27. hey. would that technique work with a motoactv? someone asked the question on the motoactv review page (stellar work, btw), but couldnt find an answer.

    • Manheim

      Did you ever get an answer to this question about the Motoactv? Seems like an amazing gizmo. But 6 hours of life isn’t going to cut it for me and is probably a nonstarter.

  28. Trent Tri's

    Very cool solution. I second Stanislas ANDRE’s question on “Does this work on the MotoActv” and if you are able to test, what kind of recharge time / run time does it add? Thanks much and LOVE the great reviews.

  29. Anders

    Does anyone know if the 310 XT will record data while charging? With the normal charger clip on i can’t get to the mode button to get into the menus.

    Has anyone had success with using extra batteries on a race with the 310?


    • Ray Maker

      Yes, it does. Though, you have limitations as you described. They key is starting it first, and then you can add a charger and it’ll charge and keep it charged.

      I did this for a long multi-day boat ride…worked well!

  30. Adnan


    I have a 310xt which seems to last only 11 – 12 hours. Is there anything wrong with it or do I have to change any settings in it ? Just a little question, can you use the watch to track the activity while its plugged in to this charger ?

    I think I have found a USB Female to Mini USB Female Adapter, is that all I would need with this charge pack to use it with 310XT? Here is a link
    link to amazon.com


    • Ray Maker

      Yup, you can leave it tracking while plugged in (I’ve done it a few times on super-long boat rides). The only trick is that you’ll need to start the activity before plugging in. And of course, your access to the buttons are a bit limited. But it works fine.

  31. Adnan

    I actually already have a torch that takes one AA battery and can be used to recharge a mobile phone. I will get a USB Female to 5 pin USB Mini Female adaptor and give it a shot.

    Just out of curiosity, is your body fat really around 5% ? I was told anything below 8% is unhealthy ?

  32. Adnan

    I just found out about another little charger which is similar to the Duracell one but the good news is it has a USB output port so don’t require any additional converters.

    Thought I’ll share it here.

    link to amazon.co.uk

  33. Brian Smith

    This is awesome! I found out the hard way that this unit will only capture full data for 3.5 hours when getting GPS every second. I’ve done 100 miles in 6 hours when in the mode that captures every 3 seconds. Do you know how long it will go in that mode? I’m planning a 150 mile ride and want to make sure it will capture all my data. I expect to be done in 10 hours. Thoughts?

  34. Jimmy

    Would the following adapter work to charge the 310xt on the go?
    link to amazon.com

  35. Justwondering...

    Hi Ray,

    Do you know if this Duracell device works with the Garmin 620?

    I’ve also heard of people using the Power Monkey Discovery portable charger (not the Garmin branded type) – here is the link: link to powertraveller.com

    Have you had any experience with the Power Monkey Discovery? And if so, do you know if it’s compatible with the Garmin 620?

  36. Griff

    Will this or any other portable charger work with the 910XT?

  37. Eugene Colon

    did you get a scar?