FLEXTAIL Zero Power 10000 Power Bank Review 

Bottom Line

The FLEXTAIL Zero Power 10000 is one of our favorite power banks for ultralight backpacking and on-the-go use. Weighing just under 5 ounces on our scale (it’s listed at 5.1 ounces), it’s competitively light with a small form factor to match. This makes it very easy to carry and use for a variety of activities both on and off the trail. We found ourselves frequently reaching for the FLEXTAIL during testing around the Southwest due to its low-profile and intuitive design.

The FLEXTAIL is also one of the most efficient power banks we tested (measured in watt-hours per gram), packing quite a punch for its size. It’s on the pricier end, but the low weight and excellent efficiency do help justify the cost.

The only real limitation we discovered in testing was cold-weather performance, with the FLEXTAIL losing about 8% of its efficiency when placed in the fridge. Thankfully, there are ways to mitigate this, which we cover below. Overall, the FLEXTAIL is a streamlined and thoughtful design that’s well suited for anyone who prioritizes weight and efficiency. For more insights and recommendations on other options, check out our gear guide to the best power banks for best on the market.

How We Tested

We used both quantitative, side-by-side testing and qualitative field analysis to score and rank the FLEXTAIL Zero Power. Our field tests primarily took place on day hikes around Durango, Colorado, and a multi-night backpacking trip in the Southwest. We also used it extensively on-the-go as well as at home to get an accurate picture of its strong suits and shortcomings for both outdoor and everyday use.

Efficiency

The FLEXTAIL Zero Power is a highly efficient power bank. To gauge this, we used a multimeter and dummy load resistor to consistently draw power from each power bank we tested. This allowed us to calculate the total number of watt-hours available for each unit. Finally, we divided the number of watt-hours by the weight of each bank to account for variations in capacity.

After completing our testing and calculations, we were pleased to find that the FLEXTAIL scored near the top in terms of available power per gram. This is particularly impressive given the device’s low weight, making it a great choice for those who carefully consider every ounce in their pack. In the field, we were able to get multiple partial charges out of the bank, which lined up with our expectations.

Weight & Portability

If weight and portability are priorities for you, the FLEXTAIL Zero Power is a great choice. On our scale, the device checked in at just 5 ounces, which is very close to its 5.1-ounce listed weight. We also measured its dimensions to calculate total volume, and it ended up ranking in our top three for portability.

Out and about on day hikes and errands, the FLEXTAIL quickly became one of our go-to power banks. It’s small enough to fit easily in a fanny pack, hipbelt pocket, or pants pocket. In fact, we often forgot we had it stashed in a bag until finding it several days later. For outings when storage space is at a premium (e.g., ultralight backpacking or trail running), the FLEXTAIL makes a lot of sense. 

Durability

Durability is another strong suit of the FLEXTAIL Zero Power. We subjected the device to our drop test by letting it fall about eight feet into dirt, and it emerged with no signs of damage. The carbon fiber shell is a nice touch for protecting the battery, and the slightly rounded corners seem to help with overall longevity, as well. For reference, our unit showed no signs of cosmetic damage like scratches throughout testing (most others showed at least some wear by the end). 

The FLEXTAIL has an IPX4 rating, indicating it will protect against water splashes from any direction. This should be sufficient for most backcountry conditions, from rain to humidity – just avoid dropping it in puddles or bodies of water, as it’s not rated for submersion. 

Charging Speed

The FLEXTAIL Zero Power has one of the faster charging speeds in our lineup, recharging from dead to full in just over three hours. We measured this by draining the power banks, then plugging them into the same charging block and cord to replenish juice. We used a tablet to record the entire charge time to pinpoint the exact moment each device reached full capacity.

The FLEXTAIL took three hours and eight minutes to fully charge, which was right in line with most other 10,000 mAh banks we tested. When getting ready for trips, we were never amazed with the FLEXTAIL’s charging speed, but we never felt like we were waiting on it, either.

Features

Though streamlined, the FLEXTAIL Zero Power is thoughtfully appointed. First, we really liked the lanyard-style USB-C cord, which attaches to one of the corners and can be removed when you don’t need it. Despite being short, we found the cord very handy – and easy to keep track of – for powering devices on the go.

The FLEXTAIl also features dual USB-C ports and pass-through charging capabilities. The latter indicates that the device can be plugged into a wall outlet to charge while simultaneously powering electronics. It’s worth noting that you’ll need to purchase an adapter if your cords still use USB-A connectors (upgrading your cords is also an option).

Finally, the charge indicator could be slightly clearer, but we appreciated the ability to toggle between fast charging and efficiency mode. Fast charging uses more battery to charge devices quicker, while efficiency mode preserves power with longer charge times.

Cold Efficiency

Cold efficiency is one area where the FLEXTAIL Zero Power fell a bit short. In our cold simulation, which mirrored our standard efficiency test but in a fridge, the FLEXTAIL experienced an 8% decrease in efficiency. It ended up being one of the lowest performers in our testing, which may be enough to deter winter enthusiasts.

Fortunately, there are some ways to mitigate this in the field, starting with keeping the device warm. On cold nights, we usually place our electronics bag in the footbox of our quilt or sleeping bag. If you’re using the device to charge electronics on the go, keeping it close to your body (in an interior chest pocket, for example) will help maximize battery life. There are certainly better cold-weather options available, but the FLEXTAIL should meet the needs of most three-season backpackers.

Should You Buy the FLEXTAIL Zero Power 10000?

The FLEXTAIL Zero Power is a well-rounded power bank for backpackers and other weight-conscious outdoor-goers. It’s lightweight, compact, and very efficient for its size. It’s also highly reliable and built to withstand the rigors of regular outdoor use. If weight, portability, and dependability all matter to you, the FLEXTAIL is a worthy option to consider.

Skip the FLEXTAIL if cold-weather performance and price matter most. There are cheaper options available in this capacity range, although you can expect to make sacrifices in other areas. Winter enthusiasts may also want to look elsewhere, as the FLEXTAIL performed poorly in our cold simulation.

What Other Power Banks Should You Consider?

If you like the FLEXTAIL Zero Power but want to explore other options, we’ve included a few alternatives below, and you can check out our gear guide to the best power banks for a full look at the market. 

Nitecore NB10000 Gen4 Review: The Nitecore NB10000 Gen4 is similarly lightweight and efficient, plus you get handy features like a knock-to-wake power indicator and glowing RGB port lights that make connections easy to see in the dark. However, the FLEXTAIL is cheaper and more compact.

Nitecore NB Air Review: The Nitecore NB Air is smaller in capacity (5,000 mAh) but also caters to ultralight enthusiasts. Like the FLEXTAIL, the NB Air is convenient, fast to charge, and relatively durable, although it only has one charging port and doesn’t support pass-through charging.

OKZU 10000 Review: The OKZU 10000 is a budget-friendly alternative to the FLEXTAIL Zero Power. It’s heavier and bulkier but offers respectable all-around performance for the price, including a faster recharge time than the FLEXTAIL.