Garmin Forerunner 935 UltraTrac mode: an incredibly fun battery test

I did something bad last weekend. Something that I'm not proud of. 

That's a total lie. I really am rather proud.

I had a half marathon coming up in my home town, but my training plan told me that I needed to do a 10KM race. I could have raced the entire thing, but it was hilly, the weather was supposed to be hot, and I'd had an idea.

What if I did the first 11KM of the race at a very easy pace, then blitzed the last 10KM? That would be amazing! I would be in disguise, having a great time in the sun, getting ready for the big speed-up ahead of me, inching closer and closer.

The only downside would be that everyone would look at me like I was, well, a massive knobhead. 

I considered this for a bit. I figured that I'd run away from them and that would make it OK.

However, I couldn't pass up the chance to test something out in the race, especially having spent an entire marathon recently without technology

I looked down at my wrist at the Garmin Forerunner 935 that I'm currently reviewing and had an idea. This thing has insane battery life (I lost the charger the first day I was sent it, and 12 days later it was still going - thankfully I found the power lead at the very last minute. Score.)

This watch was supposed to be one of the best watches out for ultra marathons... and what's another word for ultra? That's right: half*.

There's a special mode called UltraTrac on the Forerunner 935 that turns off the GPS for a few seconds every so often, using the accelerometer to work out your speed and distance during this time.

It’s supposed to extend the battery life up to 50 hours. So how would it fare if I ran for two hours straight?

The long wait

So there I am, at the very back of the pack on half marathon day, looking wistfully at the front of the snaking crowd of people waiting to start. 

It took ages to get over the line... I was at least 15 minutes away from those that could run as soon as the gun went off, but it was sunny and I was having a great time feeling so relaxed ahead of a race.

Then we were off. I instantly realised that my music wasn't working, and the familiar feeling of panic came across me... until I remembered it didn't matter. I stopped, fumbled around for a bit, waved at a child that completely ignored me and then ran off again.


I hooked onto the back of the pacer that was trying to get people across the line in two hours and ten minutes. Everyone looked relaxed... even the man wearing, what I can only describe, as a full builder's outfit. Cargo shorts, polo shirt... and expensive trainers. 

At least he got the main bit right, but surely he was going to boil up? In fairness, the polo shirt was extolling the time he (presumably) spent 2013's summer in Genoa, Italy... so perhaps it was his lucky top.

I cruised along here for a while, just loving the lack of effort needed, my watch bleeping lazily at every mile. It was an undulating course, to say the least, but I was enjoying it hugely.

I looked down again when my watch bleeped for mile three... and realised it was already about 0.15 miles behind. That wasn't good so early in the race. But I supposed that it wasn't going to be totally accurate, so 0.05 miles accuracy per ping wasn't too bad.

Oh, how wrong I was. By mile five things had gone the other way and the Forerunner 935 was registering me as having gone half a mile further than I actually had. I was running about 9:30 per mile, and my watch was telling me I was doing faster than eight minutes per mile.

This was a problem, as I now wasn't going to know when the 6.9 mile point was reached and I had 10KM left to go.

These guys - they know how to motivate.

These guys - they know how to motivate.

'Don't you worry, child of the night,' whispered Brian Harvey from East 17 into my ear. Shut up Brian... you're not trying to piss people off by suddenly trying to overtake them at an extremely specific point.

I would have to do this manually - when I saw the seven mile board in the distance, I'd just drop the hammer and go. 

(By the way, I finally worked out how to drink out the plastic cups they give out at water stations - tip a little bit of water out and squeeze the rim of the cup together to make a spout… #runhack for you all there).

There's a massive hill in this race, and the seven mile marker was somewhere up it. I hoped it was near the top, but I couldn't see it - and my watch was no use. Then I suddenly turned a corner and there it was - the yellow beacon telling me it was only muthaflippin' GO TIME!

I suddenly bolted into a sprint... and I actually heard someone scream 'WHAT?!' 

(I won't lie, I was proud of that.)

This was it. I was suddenly going about 35% faster than everyone around me, filled with energy and a caffeine gel that I'd just slurped down.

I was the world's greatest runner! I was cruising past people while they nodded in admiration! I was the hero of the enti... Oh, wait, no, I was feeling incredibly sick.

I'd overdone it badly. I'd not thought about how I would be dodging past people down narrow country lanes rather than sprinting forward in a nice straight line. 

Two miles later, I was spent. I practically crawled past the mile nine board, and knew I had a few more hills to scale before I could even think about the finish line.

Music didn’t help. I had no idea of my pace because the Forerunner 935 was essentially making up numbers on my wrist, and my heart rate was through the roof… plus I was sweating dramatically in the heat.

Thankfully, with a couple of miles to go, I began to reboot. The slower pace calmed me down, and I could slowly crank the pace back up and start passing people again on the wider roads… but this was not the euphoric sprint I was envisaging.

I trotted at the highest pace I could down the final, long mile to home - taking in the horrendous loop up and down a hill that you have to go through before you can turn into the park and towards the finishing straight.

I was thoroughly broken, just trying to make it to the end and the promise of an ice lolly. 

Yeah - not quite 13.1 miles...

Yeah - not quite 13.1 miles...

Then that thing happened that every runner loves and hates in equal measure: the finish line looms, the crowd swells and the feeling of excitement grows.

You want to run faster, cajoling the sprint out of your legs. The speed increases, you scream to the crowd and they respond in kind.

I got far too over-excited, doing some kind of floppy karate kick over the line as my fatigue-addled brain tried to do ‘something cool’. It definitely did not look good.

I grabbed my ice lolly and flopped to the ground, the familiar fear of never feeling OK again prowling around my brain. 

I checked my Garmin: over a 13.1 mile race, I’d managed to run 14.4 miles apparently. While I get that UltraTrac mode will extend battery life, if you extrapolate these findings to a trail ultra marathon you could be up to 5 miles off by the end of even a shorter one. 

What’s the point in that data, other than slightly being able to see where you were faster and slower?

Then again, in just under two hours of running I’d lost only 3% battery life. That’s immense. This watch could go forever.

Then I noticed something else: my peak heart rate at the end had hit 196. Erp. That’s a bit too high for someone of my age… was that good or bad? It would explain why I wanted to climb inside the nearest tree and pray for silence - that karate kick totally wasn’t worth it though if it meant I was about to die.

Thankfully, I’m still here. But as my ‘actual’ half marathon approaches this weekend, I’m thinking I might just put the Forerunner back into normal mode… I’m quietly confident I’ll finish it in under 30 hours.

*It's utterly not. But it makes everything tie together so well.

Gareth Beavis
Formerly Global Editor in Chief

Gareth has been part of the consumer technology world in a career spanning three decades. He started life as a staff writer on the fledgling TechRadar, and has grown with the site (primarily as phones, tablets and wearables editor) until becoming Global Editor in Chief in 2018. Gareth has written over 4,000 articles for TechRadar, has contributed expert insight to a number of other publications, chaired panels on zeitgeist technologies, presented at the Gadget Show Live as well as representing the brand on TV and radio for multiple channels including Sky, BBC, ITV and Al-Jazeera. Passionate about fitness, he can bore anyone rigid about stress management, sleep tracking, heart rate variance as well as bemoaning something about the latest iPhone, Galaxy or OLED TV.