UKC

Hi I'm after a battery pack to charge my watch while I do a long ultra - does any one have any recommendations for something small and lightweight that can do a full recharge?

The tiny Mophie ones would do the trick - you can get light ones as well as the ones that run a house for a week!

With the Fenix 3HR for instance you can run with the watch on your wrist whilst charging and a small charger in the top pocket - works well. 

Might be an idea to check if your watch can charge while your running first.

How long is the race? Most modern watches will have an ultra specific mode and will last for 30-50 hours

Which watch is it ? 

I don't think you can charge some of them while recording and activity.

Any small power bank will do the job.

Which watch? Some ideas here:

https://fellrnr.com/wiki/Charge_On_The_Run

My Garmin forerunner 235 charges while in GPS mode fyi

I also have a Garmin 235 and have charged it while in GPS on long runs. I bought this from Cotswolds. https://www.cotswoldoutdoor.com/p/outdoor-tech-kodiak-2-0-powerbank-EA63404... 

I find it very good: Its very light and can fully charge my watch in an hour or so, while not in GPS mode. Its been ages since I hard to recharge the battery bank and in that time I've recharged my watch half a dozen times. The indicator on the bank shows it as being at 80%, so it should be good for at least a dozen charges. I think they do a smaller capacity version which might be useful if all you want to do is charge your watch.  Its next to useless for charging a smart phone BTW -only  enough capacity for about 3/4 of a charge.

I guess while switched in GPS mode it would take longer to fully charge, but the once or twice that I've actually done this in a race I was beyond the point where I really cared too much about my time and had the watch shoved in a pocket in my race vest so wasn't paying much attention.

Thanks everyone - I have a Suunto Ambit 2R and it can be charged while it is in use

I am do Ultra Tour Snowdonia which is 50 miles and a load of climbing so I reckon about 20 hours. I did try the option where it only records the GPS position every minute when I did an ultra in Italy last year. The problem with this over time the distance becomes really distorted as it records a straight line between each GPS position. It really threw me as the distance on the watch was overtime getting further away from the real distance covered

So far as I can tell (not that Suunto's own specs help much) your Suunto has a battery capacity of 240 mAh. Cheap USB power banks are typically designed to charge a phone for which you need around 2000 mAh. Hence any of them will do, easily, for your watch, given that on the default 1s sampling interval it lasts for 8 hours on a charge in GPS mode. Get a light one that has a USB output so you can plug your watch charging cable into it. Some don't have that, but come with their own Android/iPhone output cables (eg mine, 3000 mAh from Lidl, less than £10), so they would be no use.

I note that your watch can be switched to 5s sampling interval and in that mode has a battery life of 16h. The measured distance shouldn't be too far out if you used that, I would think, given that you will be walking a lot of the time. But then, if it were me, I'd take a light power bank and use 1s!