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* The claimed battery lives don't hold up in the real world, with some watches going longer, others much shorter.
* If you want the maximum battery life, you have to avoid the backlight, the use of map/course displays, and don't press the buttons unless you have to.
* In my testing, the longest battery life that's available with good GPS on is 30 hours with the [[Epson SF-510]]], 24 hours with the [[Polar V800]], then 22 hours with the [[Garmin Fenix 3]], 19 with the [[Garmin 920XT]], and 17 with the [[Garmin Epix]]. * Of the watches where I've not tested the battery life, there's 20 hours for the [[Suunto Ambit3| Suunto Ambit3 Peak]], [[Garmin 310XT]] and [[Garmin 910XT]]. (I only test battery life on fairly new watches as the life degrades with usage.) * If you need much longer For ultras lasting less than 20 hours, which is common in such as 50 miles or 100 Km, you have plenty of choice and battery life should not constrain you too much. * For 24 hour races or 100 mile races, where you end up with some compromisesexpect a finish in the 24-30 hour range the [[Epson SF-510]] is worth considering though the battery life is one of its only good points. * Some watches can extend the battery life by only taking a GPS fix less frequently. This can be acceptable on a straight course like the Keys 100 or Badwater, but it can be appalling on twisty trails. However, if you need more than 30 hours then this is you only option without recharging on the go.
** The [[Garmin Epix]] gives 50 hours in extended mode (UltraTrac), but its GPS accuracy is lacking and the map display tends to eat the battery life. (UltraTrac appears to use a variable recording frequency.)
** The [[Suunto Ambit3| Suunto Ambit3 Peak]] has 20 hours with normal GPS recording, and 30 hours at 5 second sampling, which is great. It will go as high as 200 hours with 60 second sampling. There is some course display capability, but it's not as good others. There's also the cheaper [[Suunto Ambit3| Suunto Ambit3 Run]] that gives 15 hours at 5 second sampling, or 100 hours at 60 second sampling.
* You can charge some watches on the run (see table above). This is awkward at best, as you need the cable attached and to carry a USB battery pack, but some runners find this acceptable. I'd highly recommend something like this small battery pack that fits nicely in the hand - <jfs id="B005X1Y7I2" noreferb="true" n="Anker PowerCore+ mini" nonl="true"/>
* Another option, depending on the race, is to have two watches and swap part way through.
==Navigation==
For some ultras (and shorter trail runs), navigation is a problem. I've been lost on the occasional race and it's a horrible experience. Even when I've been on course, the doubts and stress of worrying can be an enormous drain. My first GPS watch was purchased so that I'd have an outline of the course I was running and there were many races where I was glad of its comforting conformation. * The [[Garmin Epix]] has a display that shows full color maps, so for navigation, nothing else comes close. (Well, the [[Leikr]] is even better, but it has a short battery life.) * The [[Garmin 310XT]], [[Garmin 910XT]], [[Garmin 920XT]], and [[Garmin Fenix 23]] will show an outline of your run and allow you to load a course outline to follow. The [[Garmin 920XT]] will also display an outline of your run, but Garmin has crippled the 920XT by removing the ability to zoom the display. ** The [[Garmin 920XT]] has some mapping capabilities, but the lack of zoom cripples this functionality. *