Back up power for GPS watch

Or any other device? My Garmin 305 tends to give up after 4 hrs and as I'm doing my first Ultra in April, looking at extending it's battery life.

has anyone used one of these: http://www.portablepowersupplies.co.uk/portapow-premium-aa-usb-battery-pack/

Can't find it now, but DCRainmaker did a review of something similar.

As I recall it worked well, but I see to remember that you couldn't see the screen whilst it was charging, though it did still collect data.

in fact, i don;t think it'll work with my watch anyway as it requires a docking unit. I'll check out that review but any other suggestions gratefully received.

If your 305 is giving up the ghost after four hours then check if it's set to check the GPS every second. If it is then change it to smart tracking and it will double the battery life.

I have seen "somewhere" online someone who did a bit of jiggery-pokery with a soldering iron, a file and some sealant to adapt their 305 to affix a usb socket to it permanently for just this purpose.

I think dc rainmaker just fastened his securely to the usb cradle thing - perhaps a couple of squares of sticky back velcro would be enough to stop it falling off.

using the velcro strap you can secure it to the cradle very well, but you obviously couldt wear it whilst it charges.

Yep, saw that. Satt). How do i change teh setting. I've been through the menu but can't see it.

settings general data recording enter (change selection)

hmmm. It's already on smart recording. I'm going to try soft reset (hold lap and mode buttons down for 15 seconds) and a full power down and recharge cycle. See if that works.

I used one of these to keep my 310XT going on a 24-hour race last year. Worked like a charm, just plug it it and let it charge for a while. You'll lose the display, (it just says "Charging" but the Garmin keeps recording)

Aplogies for the long link, hyperlinks don't work in Chrome on the Mac 

ttp://www.ebay.co.uk/itm/AA-iPhone-Battery-Travel-Charger-iPod-MP3-Torch-USB-Mobile-Phone-Emergency-NEW-/271164495422?pt=UK_MobilePhones_MobilePhoneAccessories_MobilePhoneChargers&hash=item3f22a9d63e

you can actually still wear the 305, whilst it is on the cradle and using the portable charger, I tried this in readiness for a long walk, battery lasted around 7 hours then charged up in around an hour using the charger and batteries, it then went on to last a further 5 hours

So, after a soft reset and a full discharge/recharge, I have some progress to report.

I left my garmin on my desk overnight, running. It wasn't taking HRM readings and it was static for the entire time, but I'm guessing some of this saved power was possibly offset by having to hunt around for GPS signals as I'm miles from any windows. 

I came in this-morning, watch fully discharged. Charged up again, checked history. It ran for 12 hrs and 21 minutes.

I thought that was pretty good, and it probably means I don;t need back-up power.

I use a Duracell usb charger I bought on eBay for £7. Works great and holds enough power to fully recharge my 405 at least twice. Bit fiddly to carry extra gubbins about with you but cheaper than shelling out £350 for a 910xt!