FLEXTAIL Zero Power Bank Review

FLEXTAIL Zero 10,000 mAh Power Bank Review 

Spare batteries or a power bank are pretty much a guaranteed requirement in most ultramarathon mandatory kit lists and finding one that won't weigh you down, but is durable enough to survive a weekend in the mountains, is essential.

The team at Flextail reached out and sent us their ZERO Power Bank, which they claim is the world's lightest and thinnest 10,000mAh power bank on the market.

Now, we could have just tested it on a Sunday long run, but in true GB Ultra Runner fashion, we decided to properly put it through the wringer. We took it along for not just one, but two of the UK’s toughest 100 mile ultramarathons: the Arc of Attrition and Ultra-trail Snowdonia (UTS).

Here is how it held up.

FLEXTAIL Zero Power Bank Specs

Before we dive into the field tests, here is what the Flextail ZERO is packing under the hood:

• Capacity: 10,000mAh
• Dimensions: 9.9mm wide (ultra-slim)
• Weight: ~145g
• Build: Forged carbon fibre shell for extra durability
• Weatherproofing: IPX4 water resistance & -10ºC cold-proof
• Safety Net: 10% emergency reserve
• Charging Modes: 22.5W fast charging (indicated by an orange light) & 5V/1A efficiency mode (green light)
• Connectivity: Dual USB-C ports with pass-through charging capabilities

FLEXTAIL Field Test 1: The Arc of Attrition

If you know the Arc of Attrition, you know that January in Cornwall is unforgiving. With only around 9 hours of daylight, runners are subjected to roughly 14 hours of darkness.

For this race, I was using a Petzl IKO Core head torch with two rechargeable battery packs. To safely navigate the Cornish coast path, I wanted to run the torch at its full 500-lumen output. At maximum brightness, each battery only lasts about 2.5 hours. With a long, dark night ahead, I needed a dependable way to keep my spare battery charging continuously on the move.

The Flextail delivered exactly what I had hoped for. Its 145g weight and 9.9mm profile meant it easily slipped into the front pocket of my running vest (and occasionally my coat pocket) while plugged into the spare battery. Swapping out the batteries was quick and hassle-free. Because of this, I managed to maintain a 500-lumen beam all night long and didn't need to fall back on my lower-powered backup head torch once.

I also used the ZERO to top up my iPhone 15 during the race. Remarkably, even after keeping my lights and phone alive, it still had plenty of juice left by the time I reached the finish line.

In terms of durability, there couldn't have been a better test for an ultra runner. Storm Ingrid hit the Cornish coast just as we left the start line. The power bank was subjected to everything the storm could throw at us, including 60mph winds and relentless sidewards rain. Thanks to its IPX4 rating and forged carbon fibre shell, it survived unscathed.

FLEXTAIL Field Test 2: Ultra-Trail Snowdonia (UTS 100)

Heading into the UTS 100 miler in May, I was quietly hoping for better weather. Unfortunately, the Welsh mountains had other ideas.

The UTS 100 starts at around 2:00 PM on a Friday, which means running through two full nights is highly likely. This made it the perfect stage for test number two. The weather on Friday was actually quite pleasant, but by Saturday, things changed drastically. We were hit with strong winds, more sidewards rain and low-visibility clag that made the highly technical terrain exceptionally tricky.

Just like at the Arc, the Flextail didn’t let me down. It kept all of my essential gear powered up through both nights despite the dropping temperatures and heavy moisture. The -10ºC cold-proof rating definitely offered peace of mind whilst navigating the exposed peaks and ridges of UTS.

FLEXTAIL 10,000 Power Bank Features

Beyond the impressive weight-to-capacity ratio, a couple of specific features really make this power bank shine for trail runners:

The Charging Modes: The Flextail features two distinct modes. You get a 22.5W fast charge (orange light) when you need power in a hurry. However, you can also switch fast charging off and drop into a 5V/1A efficiency mode (green light). This mode uses less internal power to deliver the same amount of charge at a slower pace, perfect for trickle-charging a headtorch battery in your vest over a few hours without wasting the power bank's capacity.

The Lanyard Cable: I loved the lanyard-style USB-C cord. It attaches directly to one of the corners of the power bank and can be easily removed when you don’t need it. At first glance, the cable seems a little short, but in practice, I found the length to be perfect. It completely cuts down on the extra cable bulk that normally tangles around your other gear, keeping everything neat and together.

Verdict: Is The FLEXTAIL Zero Power Bank Good For Ultramarathons?

Yes, the Flextail ZERO is a brilliant bit of kit that did not let me down during arguably two of the UK’s toughest 100 mile ultramarathons.

If you’re on a strict budget, there are certainly cheaper, bulkier power banks available. However, if you're looking for something that is highly reliable, weather-resistant and ultra lightweight, the Flextail Zero Power Bank is a great choice.

FLEXTAIL Discount Code

The lovely folks at Flextail have provided the GB Ultra Runner community with a sitewide Flextail 15% discount code. Just use the code GBUR15 at checkout!

FLEXTAIL Zero Power Bank

15% Discount

FLEXTAIL 15% Discount Code: GBUR15

Meet Phil

Phil's an ultra runner from England's east coast who's spent the past few years tackling everything from 50k, 50 mile and 100k races to full-on 100 milers. After recently finishing the UTMB Arc of Attrition, he's now training for the huge 215 mile Race Across Scotland.