A team of enterprising engineers have begun work on a Reserve Strap accessory for the Apple Watch, which they say can boost the wearable's battery life by 125 percent. Apple has also released official guidelines for third-party straps, opening the doors to companies who want to develop new looks and functionality for the timepiece.

The Reserve Strap team (part of Mutual Mobile) is only at the prototype design stage right now, but the design has already become more efficient and streamlined since the Apple Watch was launched. The latest design uses a direct connection to the device's hidden diagnostic port rather than a wireless inductive charging method, which should guarantee faster charging times and higher charging capacity.

The first rendering of the Reserve Strap wrapped around the back of the Apple Watch, leaving holes for the sensors and LEDs, but the new incarnation simply clips in and leaves the back of the device unobstructed. Mutual Mobile founder John Arrow says the 125 percent estimate is based on initial testing on both the 38 mm and 42 mm Apple Watch models.

At this stage it's not clear if Apple will allow third-party strap makers to access the 6-pin diagnostic port on the Apple Watch. There's no mention of it in the company's new band design guidelines but it's difficult to see how Apple could stop determined users from accessing it – the Reserve Strap team is planning to provide a special tool to enable users to flip it open.

Based on the documents supplied by Apple it seems the Cupertino company wants accessory makers to focus on the style and fashion of their straps rather than battery charging or additional functionality (like extra sensors). We'll have to wait and see how much control it can exercise over what third-party manufacturers do with their straps. Environmental requirements are also set out in the guidelines.

Pebble, meanwhile, is actively encouraging third-party developers to build all kinds of weird and wonderful smartstraps for its watches. As for Reserve Strap, the team is already taking preorders for $249 through its website, even at this early prototype stage. As yet there's no estimated shipping date or production schedule.

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