Apple does its best to improve battery life with each new iPhone generation. But this isn’t simply a question of engineering bigger or more efficient batteries. The company is battling against a wider trend towards more demanding features (such as the always-on display added to the iPhone range this year) and slimmer designs. As things stand, iPhones last for around a day between charges, if you’re reasonably careful about usage. That’s enough for some. But if you’re on holiday, travelling for work, or otherwise unable to easily reach a charging point, it’s woefully inadequate. The iPhone has …