If your iPhone is an extension of your arm, then chances are you're often stuck without a phone when your battery conks out before the end of the day.

For the past couple of weeks, FHM have been testing out the MiLi Powerspring 4 and 5 and, to put it simply, it's a great, simple bit of tech.

This slim and stylish gadget essentially works as a second battery for your iPhone. You put the case on, connect it up to your power source and it'll charge both your iPhone and the Powerspring together.

You disconnect it, carry on with your day and, when you're battery is running low, just push the little button and the power from the Powerspring will bring your battery back up to 100%. Sounds good, right?
Mili Powerspring
With festival season coming up, it's a pretty handy bit of tech to have on you. And that's why we're giving away 20. That's 10 for the iPhone 5 (above and also available in matte grey) and 10 for the iPhone 4 (below.)

All you have to do is email comps@fhm.com by Friday June 7, telling us your name and which model iPhone you have, and you could win a free external battery case worth £55.

Mili Powerspring

One more thing. If you don't win, we'll send you a 20% discount code to use on the MiLi website. Check it out for more info on the Powersprings and all other MiLi gadgets.