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* You need to have the new HRM for the some of the features. The Fenix 3 will display Heart Rate and calculate [[Firstbeat| Training Effect]] with the older Garmin HRM, but not [[Running Dynamics]]. There is a version of the Fenix 3 with an [[Optical Heart Rate Monitoring| optical HRM]], but it's the same technology as found in the [[Garmin 235]] which I found to be rather useless.
=Charge On The Run=
The Fenix 3's 20-hour battery life is not enough for longer ultramarathons; most runners in a 100-mile race will need a little longer at least. One option is to reduce the GPS polling interval, which bumps the battery life up to 50-hours, but you could also [[Charge On The Run| consider charging the Fenix 3 during your run]]. The Fenix 3 has about a 350 mAh (milliamp-hour) battery based on my measurements (others put it at nearer 300 mAh), so it doesn't take much to recharge it. In fact, the problem is generally finding a battery pack that's small enough, as batteries tend to be heavy. My two preferred options are below, the Anker and the PulsePak.
{| class="wikitable" style="margin-left: auto; margin-right: auto; border: none;"
|- valign="top"
|[[File:Charge On The Run - Batteries (3).jpg|none|thumb|200px|A close up of the PulsePak battery.]]
|}
The Anker is much bigger (3.75"/9.5cm) and heavier (2.9oz/82g), though it's shape makes it easy to carry in your hand and it will slip into the pockets of Race Ready shorts. It holds 3,350 mAh, which is way more than you need, recharging the Fenix 3 over ten times. It's reasonably priced at <jfs id="B005X1Y7I2" noreferb="true"/>. The PulsePak is far smaller (2"/5cm), lighter (0.5oz/15g), and cheaper at <jfs id=" B00WBIK25C" noreferb="true"/>. It only holds 500 mAh, but that's enough for a full charge, and it's small enough to carry a couple. The obvious approach is to use the charging dock and cable that came with the Fenix 3, holding a charger battery pack in your hand, which is shown below. I'm not a fan of this approach as the battery occupies your hand and the cable is rather long. You can tuck the battery in a pocket, but then the cable really annoys me. (Putting the watch and battery in a pack or pocket seems to defeat the object of having a watch if you can't see it.)
[[File:Charge On The Run - Fenix3 (1).jpg|center|thumb|300px|The Anker and the Garmin cable.]]
You can use an adapter to use the PulsePak with the Garmin charger which requires a full size USB plug, which makes the batter lighter, but still isn't ideal.
[[File:Charge On The Run - MicroAdapter.jpg|center|thumb|300px|]]
Inspired by [https://dejswa.wordpress.com/2016/03/21/garmin-fenix-3-on-the-go-charger/ David Jackson's blog]], I decided a better approach is to use an aftermarket dock. The advantage of the aftermarket dock is that it has a microUSB socket rather than a cable, which is <jfs id=" B017SDY9BM" noreferb="true"/>. While the quality of the aftermarket dock is not quite as good as the Garmin original, it's surprisingly close. Because the dock sits under the watch, it naturally stays in place.
[[File:Charge On The Run 7.jpg|center|thumb|300px|Original Garmin dock and an aftermarket alternative.]]
Here you can see the microUSB socket with the dock worn under the watch. The underside of the dock does not cause me any problems, such as skin irritation or pressure points.