Portable USB batteries are nothing new, but they vary in price and quality more than you'd expect. Some don't have enough capacity to fully recharge your smartphone, while others are ludicrously expensive.
The PowerMonkey Discovery costs about £40 online, putting it in the middle ground for price. Capacity, though, is pretty impressive at 13Wh (that's 3,500mAh if you prefer).
As you might expect, the battery is fairly large and resembles a tin of sardines thanks to a moulded aluminium case that has a stick-on aluminium top. On our sample, the top wasn't particularly well stuck on and rattled a bit.
Being aluminium, it was also easily scratched and marked and the back looked pretty beaten up after just a couple of days in our rucksack.
Six green LEDs show the remaining charge, and the charging progress when connected to the mains or a USB port.
No mains charger is included, but you do get a 30-pin Apple cable. A second cable comes with interchangeable tips for micro and mini USB, plus Nokia, Motorola and Sony Ericsson phones. These cover virtually all bases, but you can use the USB cable that came with your device if it has a different connector. In the box is a small pouch for keeping all the tips together.
Without a tip connected, the bundled cable is used to recharge the Discovery. Although this sounds clever, it's a pain. We'd have preferred a standard connector such as micro USB, so it doesn't matter if you lose the bundled cable or don't happen to have it with you.
In our tests, the PowerMonkey Discovery charged up an iPhone 5 and showed two LEDs of remaining capacity. Connecting an empty iPhone for the second time, the Discovery managed to charge it to 37 percent - quite a way short of the 100 per cent that PowerTraveller claims.
We also charged an empty iPad with Retina display, and the fully charged Discovery could only get this up to 21 percent. It's enough to get you by, but not ideal. Since there's only one USB port, you can't charge two devices simultaneously as with some USB batteries.
The Discovery's output is limited to 500mA, so it takes around twice as long to charge an iPhone compared to using Apple's mains charger, and four times as long to charge an iPad. If you use a standard USB port to charge the Discovery, it will take around seven hours - we'd recommend using 1A mains adaptor to halve this charging time.
If your budget can stretch a bit to £60, we'd recommend Innergie's PocketCell which delivers 2.1A from its 3000mAh battery. It's smaller than the PowerMonkey Discovery and has a standard micro USB charging port.