Rolling Square’s Tau 2 power bank keychain is cluch in emergincies

I first learned of Swiss startup Rolling Square because of its upcoming Bluetooth smart trackers, but I jumped at the chance to first test the Tau 2, the company’s diminutive yet versatile power bank. I’d never used one so small and light, two factors that appeal to the ultralight backpacker in me.

The timing was fortuitous. Days earlier, I’d forgotten to turn on airplane mode while in the wilderness. I wandered embarrassingly lost in an exurban area after dark, my phone and digital map long dead. My sense of direction (and a helpful kid yelling from his mom’s car) eventually helped me find the train station and avoid a serious emergency, but I got lucky.

It would’ve been the perfect use case for a tiny battery billed as “the emergency power bank for every device.” So, I used this review as an excuse to hit the woods again.

Price, availability, and specs

Rolling Square is a relatively niche electronics company from Switzerland that doesn’t have many big-name retailer partnerships. For now, you can only purchase the Tau 2 directly from its website, although that could change in the future.

It comes in Ice Gray and Mercury Black, including cables and a cap (with a key ring attachment) framing the sides and top, and sports slightly offset colors compared to the body. It costs $40, with discounts starting at $5 when you buy two. Rolling Square ships to most countries worldwide.

What’s good about the Rolling Square Tau 2 power bank?

It does everything it sets out to do

The best part of the Tau 2 is that I didn’t notice carrying the 50g, roughly 6-by-4-by-2cm trinket for a second. I zipped it into a plastic bag to mitigate potential rain, squirreled it away in my pack, and mostly forgot about it.

I say “mostly” because I did toss it around a little while rummaging for snacks. It never felt like it was remotely in danger of breaking, another big plus of the form factor. The size, shape, and precise, durable construction give it an extremely solid feel. For that matter, I wouldn’t expect an IP rating on most power banks, but wouldn’t be afraid of a little rain while it’s in my pocket. The device doesn’t seem like a little moisture would damage it.

Of course, emergency gear only helps if you actually carry it. The Tau 2 is just barely compact enough for comfortable use as a typical keychain. The features are also outright extensive, with Lightning and Micro-USB plugs alongside the expected USB-C. The Tau 2 works fine for electronics stuck in the 2010s or from a certain Android competitor’s ecosystem. The USB-C output does its job even faster than I’d expected.

It supports passthrough charging, can charge two devices simultaneously, and accepts a charge from a USB-C charger, the mated charging dock, or a laptop or other device’s battery (using a simple touch sequence for mode selection). A Sherr.it-linked NFC tag embedded in the back lets you label it with your contact info in case it’s lost, or you want your digital business card always ready. You can hang it from the included, wall-mountable magnetic dock, keeping your keys in one place and your emergency battery topped off.

All those pros are wrapped up in a decidedly premium package. Its plastic body resists scratches better than keychain gadgets I’ve tested before. None of it feels cheap, and the cables offer a particularly sturdy construction.

The manufacturer seems to have thought of everything with the Tau 2. Not only is it the sole option with such a sleek form factor, Rolling Square nailed the extensive feature set. Even after accounting for my bias toward all things small and light, I’m unexpectedly pleased with it.

What’s bad about the Rolling Square Tau 2 power bank?

If you’re picky, you could complain, but only a little

Closely tracking the Tau 2’s output showed it provides up to 1,300mAh to mobile devices, out of its nominal 2,000mAh capacity. That figure remained consistent across two iPhones, an Android phone, and two Android tablets. That brought my daily driver, the minuscule Unihertz Jelly Star, to a respectable 60%, but most phones will only reach 20% or so.

However, the Tau 2 isn’t meant for additional hours of browsing the web or watching videos. It’s intended for emergency purposes, like getting lost in the woods. Nonetheless, I reached out to Rolling Square just to be sure the 1,300mAh figure didn’t represent a defect. My contact confirmed that “conversion naturally isn’t 100% efficient, resulting in a typical usable capacity closer to 1,700mAh. With standard variations in charging efficiency, it’s normal to see around 1,300mAh as an effective output, as some capacity is lost in the process.”

Physics and current battery technology limit that figure. At just 50g, I can’t argue that the Tau 2 should hold more negatively charged electrons. That would be nice, but would necessarily add weight.

Another quibble is the thickness, which some people will find too chunky for a day-to-day keychain. I’m not certain about the actual cell used, though, and I suspect Rolling Square made the device as thin as possible. Plus, I’m picky with pocket-friendly gadgets (hence my love for the Jelly Star). If your daily carry rides along in a purse, bum bag, or similar accessory, you’ll hardly notice the Tau 2. If you love skinny jeans like I do, it might not quite be your ideal keychain.

My only other possible complaint is how hard it is to open without keys or some type of lanyard attached. But, if you’re using it as an actual keychain, that turns into a positive, so it’s mostly a wash.

The Tau 2’s competition

Other ultra-compact power banks have their own pros and cons. World-class flashlight manufacturer Nitecore makes 5,000mAh and 6,000mAh batteries at the same price, double the weight, and only slightly larger bodies. They don’t offer built-in cables or an NFC tag. The TravelCard Plus costs the same, has 50% more capacity, and does integrate cables, but its credit card-like shape isn’t as comfortable.

The Aqua Vault ChargeCard is built durably but holds only 2,300mAh, despite costing nearly double. My next backpacking power bank will be the Vapcell P2150A, which is nothing more than a 21700 battery cell with a USB-C charging circuit on top. If you’re OK with a little more weight, the CukTech 10 is as streamlined as a 10,000mAh model gets. However, none of these offerings have the premium design, are lightweight, and have the extended versatility of Rolling Square’s second-generation keychain power bank.

Should you buy the Rolling Square Tau 2 power bank?

If you want to be prepared, yes

While it’s a little pricey considering the capacity, the Tau 2’s form factor has no equals and few competitors. It’s a durable, dependable, compact accessory that could save your life by powering up your phone in an emergency. It’s not the absolute peak of versatility, but it’s worth it for peace of mind.

Ultimately, my phone battery died shortly after waking up at camp the first morning. The Tau 2 did its job. I stopped taking so many pictures and remembered to activate airplane mode. If we’d ever been in real danger (civilization was never far away), it could have averted disaster. And if you’ve accidentally run your battery to zero at any time in the last several months, I’d recommend the Tau 2. It’s a neat little device.