If you're looking to extend the life of your smartphone, there aren't too many options. You can turn off all the features and basically not use it, or you can find an external battery source. Mophie is well known for its iPhone power cases and now you can get a Juice Pack for your HTC One too.The premise is simple: the case houses a battery and you slip your device into the Mophie Juice Pack. The USB connector of the case slots into the bottom of your HTC One and that's it, your battery life is longer.The case itself is sturdy enough, so we've no doubt that the Mophie Juice Pack will keep the aluminium body of your HTC One safe from knocks and scratches and the black plastic of the sample we've been using is nice and grippy.Once in the Juice Pack, your handset is going to be quite a bit bigger. That's possibly the greatest barrier that Mophie faces with the Juice Pack. Where the iPhone 5's svelte design means the complete package isn't too bulky, the HTC One is a substantial beast on its own.\u00a0Measuring 153mm, the Mophie Juice Pack adds 16mm to the height of your device, as well as an extra 7mm to the thickness. In terms of weight it brings 143g extra to your phone, meaning you're looking at closer to double the weight. In the hand it feels fairly hefty and even with respectably sized man hands, it can be a bit of a stretch around the screen when using it one-handed.READ: HTC One reviewBut that might be a trade-off you're prepared to make, given that you're adding a 2500mAh battery that will in theory double the life of your HTC One. We've been getting around 16 hours from the HTC One battery on light days, so having the option to extend that to a full day is appealing. You'll probably choose to use the Mophie Juice Pack on days of heavy use and we'll report back how we get on as we use it further.On the rear of the Juice Pack is a push-button LED indicator to let you know how much charge there is in the case and a switch to move the Juice Pack from "charge" to "standby". This means you can wait until your HTC One battery is starting to run low, flip the switch and recharge your device. Mophie recommends that around 20 per cent battery is the best time to do this.Charging remains the same as normal, as the case's USB connection means you can connect and charge the phone and the case at the same time, from the same charger. Of course, things like the camera and the buttons are all accessible.The Mophie Juice Pack for HTC One doesn't come cheap, however. At \u00a389.95, it's going to make something of a hole in your pocket.We're going to be using the Mophie Juice Pack for the HTC One for the next few weeks to see exactly how it performs, so stay tuned.