There is a lot of digital ink being spilled today over the Reserve Strap for the Apple Watch. They came up with a clever design that plugs into the watch's diagnostic port and makes the strap a lot sleeker. Setting aside my concerns about a permanent connection to the diagnostic port as you wear your watch around all day, I have to wonder how many people actually need more power for their Apple Watch. Yesterday I wore my watch 16 hours and had 40% battery left when I took it off. As I write these words, it is 12:38 pm and I currently have 87% of my battery left.
If you need extra power, this may be the strap you are looking for but I'd advise to make sure you actually need that extra power first. I don't.