The Magic Keyboard doesn't have a dedicated battery. Instead, it's powered by the Smart Connector on the back of the iPad. Meaning, the battery inside the iPad Pro is also powering the keyboard.
I noted the added battery use in my review of the Magic Keyboard, and while it's disappointing, I didn't find it to be dramatic. I expected my iPad Pro's daily battery life to be shorter -- it is powering an entire keyboard, backlighting, trackpad, and facilitating all communications, after all.
However, the iPad Insight blog has a theory: The battery drain might be due to a faulty keyboard. Reportedly, after using a second Magic Keyboard, the iPad Pro's battery life has improved and is more in line with Apple's 10-hour estimate.
In my experience, the easiest way to cut back on how much battery the keyboard uses is to turn off the backlit keyboard, which, for the majority of my day, it doesn't need to be on anyways. You can turn off the backlight by opening the Settings app and going to General > Keyboard > Hardware Keyboard and move the slider labeled Keyboard Brightness all the way to the left.
I've reached out to Apple about the complaints and whether this is a hardware issue that can be resolved by a replacement device if it's software-related or expected behavior. I will update this post should I hear back.
But I'm curious: If you've purchased a Magic Keyboard for your iPad Pro, what's your battery life experience been? Let me know in the comments what you're seeing, or not seeing.