Here's the best use we've seen yet for the Apple Watch's secret port

Though the Apple Watch's display stays off most of the time to preserve battery life, the Apple Watch's battery still only lasts about a day.

So in the same way accessory makers sought to extend the battery life of smartphones with battery cases and other energy-saving accessories, some went to work on extending the battery life of the Apple Watch.

The problem here is that battery cases add sizeable bulk and weight to your smartphone, which could be forgiven for a smartphone because battery cases also usually offer protection. 

But Apple Watch owners will be less likely to add bulky and heavy battery cases to their new smart time pieces. The only room for customization here is with the watch band, not a cheap plastic case around the Watch's body. Not even Apple sells cases for the Watch - it only sells bands, and it offers a way for third party accessory makers to design additional bands for the Watch with its design kit.

Unfortunately, most accessory makers who strive to make battery bands are seemingly limited to add battery life through the conductive charging port on the back of the Apple Watch. As you may know, the Watch's conductive charger is fairly bulky, and it would be extremely uncomfortable, if realistically impossible, to wear the Watch with a device that used the same chunky charging system. Not to mention that it would block the heart rate sensor, too. 

Enter the Reserve Strap.

The team behind Reserve Strap has has figured out how to use the diagnostic port found in the Apple Watch's bottom lug (the part that connects the Watch's body to the strap) for charging the Apple Watch, saying that "it's way more than a diagnostic port."

According to their video at the bottom of this article, charging through the port is actually faster than Apple's standard conductive charging, too.

We spoke with the two-person team behind the Reserve Strap, Lane Musgrave and John Arrow, who claimed that six small batteries built into a custom in-house strap adds up to 125% battery life to the Apple Watch, which could give it an additional day's worth of use.

The Reserve Strap's batteries are charged via micro USB, so you can keep the reserve Strap, and therefore your Watch, plugged in and charging on your desk while you work, for example.

It doesn't sound ideal to tether your wrist to a charger, but it beats having to take off the Watch if you want to use one of Apple's inductive chargers.

While there are renderings on its website, there is no prototype of the Reserve Strap yet to see what it actually looks like and how it'll function. Musgrave and Arrow say they're expecting to have one ready in the next few weeks and will release a video to give Watch owners an idea of the final product.

The Reserve Strap will apparently use the same materials found in the Watch's Sport band, and it will maintain the same water-resistance as the Apple Watch. 

According to the team, "there's no explicit mention of the port itself being used in the band category" in the warranty, so using the port won't affect the Apple Watch's warranty.

The Reserve Strap is expected to ship this fall. It's available to pre-order now for $249.99.

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