At a glance

Expert's Rating

Pros

• Super-compact power bank





• Can give iPhone a 30% charge





• USB-C/Lightning/USB cables built in

Cons

• You probably will forget to charge it

Our Verdict

The tiny Tau 2 is a great and affordable back-up battery that can squeeze out enough juice to recharge the iPhone to nearly a third of its full potential while weighing next to nothing and taking up barely any space.

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We’ve seen a lot of tiny power banks go through the Macworld Labs: some super-slim, some able to recharge an iPhone nearly twice over, others with cables included and flashy displays showing you how much battery life is left. One power bank even has a fancy light that might brighten up a cloudy day.

GaN (Gallium Nitride) technology makes transistors smaller and therefore devices such as chargers more diminutive. This is now putting quite serious functionality into very small things indeed.

And we’ve never seen a power bank as small as the Rolling Square Tau 2, which wants to be the key ring that saves you just as your iPhone’s battery gives up the ghost of the sub-10% you are shocked to realize never got a charge up before you left the house.

Where most power banks are bulky—although the latest 5K magnetic power banks are positively skeletal—the Tau 2 is smaller than an AirPods case but hosts a 2000mAh (7.4Wh) battery pack that in our tests refilled an iPhone 16 Pro by just over 30%. We review the best magnetic power banks if you are willing to sacrifice some bulk for greater battery capacity.

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Simon Jary

In size it measures 2.2 x 1.7 x 0.7 inches (5.6 x 4.3 x 1.9cm). It’s a little smaller and lighter (1.7oz vs2.2oz; 49 vs 61g) than an AirPods Pro case.

The clever thing about the Tau 2 is that it doesn’t require you to always have a cable on you when you’re out and about—how many people do? If you’re canny enough to pack a cable every time you go out, you should be bright enough to have remembered to charge your phone in the first place. Most of us sometimes fail on both counts.

Remove the key-friendly hooked lid to reveal two cables: a USB-C for anything new (iPhone 15 and later) and Lightning for older iPhones and AirPods cases.

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Simon Jary

The Lightning cable even works as MicroUSB—the fiddly little connector that history should have consigned to the dustbin, but which doggedly lives on in cheap Chinese gadgets.

If you do still own an iPhone older than the 15, it will offer pass-through charging with USB-C as input and Lightning as output.

You can even charge two devices at the same time, which is where a second USB-C cable rather than Lightning could be useful to early adopters.

If you have all USB-C Apple gadgets the Lightning cable is rather superfluous. We’d have preferred an option with two USB-C so the pass-through can go both ways, but I do own an older Lightning AirPods case, so it is still useful to me. If only it could charge my Apple Watch too.

Maker Rolling Square promises that “you’ll never forget to re-charge it”, which of course is nonsense, but the company does give you options. You can plug in straight into a USB-C charger using its little cable, or power it up via its own its Charging Dock that—with the included double-sided adhesive—can be firmly stuck to a wall, desk, or any clean, flat surface that will hold it.

You can also plug it straight into one of your MacBook’s Thunderbolt ports to charge it up from the laptop.

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Simon Jary

If you keep your ke