Running out of phone battery is a pain, but lots of portable chargers simply aren't portable enough. Not only is LinearFlux's upcoming LithiumCard charger about the size of a credit card, it promises to charge phones at a rate of one percent per minute. Gizmag took a look to see if it lives up to the hype.

As someone who's phone regularly runs out battery, the idea of a portable charger that is small enough to slot in to my wallet without adding excessive bulk is an appealing one. The LithiumCard is, therefore, of particular interest. So, without beating around the bush, I'm pleased to say that it's really rather good.

There's very little to it. As described in our previous article, it has a USB connector that swings out of its body and plugs into a computer for charging the device itself, a short micro-USB cable that also swings out of its body to charge a phone and a power button for switching it on. The chassis is a sleek aluminum and a light on the front indicates charging status.

Although it's the same height and width as a credit card 54 x 86 mm (2.1 x 3.4 in), it's about five times thicker at 6 mm (0.2 in). This is to be expected, of course. The innards have to go somewhere. As a result, getting it into a card slot in your wallet may be a bit of a squeeze – or indeed impossible. I managed it, but only just. Fortunately, like most wallets, mine also has a larger back compartment and the LithiumCard slots in and sits there comfortably without too much added bulk.

It comes in handy too. I played around and tested it at home first, but then needed to charge a phone whilst out and about one night. On the promise of adding one percent per minute to a phone's battery the LithiumCard fell short, although I should say that the device being tested was a prototype so that may be why. We charged a Galaxy S3 at a rate of about one percent every two minutes. That's still quite reasonable in terms of speed, but if you need to charge your phone then frankly any power source is usually welcome, regardless of charging speed. Of course, you're not shackled to a plug socket with the LithiumCard and, in fact, you could feasibly charge your phone in your pocket.

I could detail the swish packaging that the LithiumCard turned up in or the ease with which it slots into a computer USB port for charging, but really all that matters is this: it's dead simple to use and pleasingly convenient. Charge it up, pop into into your wallet or bag and feel satisfied when it comes in handy.

The LithiumCard is due to be shipped to its Indiegogo campaign contributors in May and will be be commercially available after that. It's expected to cost $39.99-$49.99. Keep an eye out for it.

Product page: LinearFlux LithiumCard