At last, Surface Pro 3 gets firmware update to fix battery issue
The good news is that the update is now available from Microsoft. The software giant says that there is nothing wrong with the battery and that it will eventually start functioning fine once the update is installed. You see, downloading the update doesn't immediately fix the issue. The battery will have to go through several cycles to relearn what a full charge is.
Keep in mind that full charge on the Surface Pro 3 isn't what it was two years ago when the device was brand new. But it should last for more than an hour once enough cycles have been completed. To get the update, make sure that you're plugged into an AC outlet and not the Surface Docking Station. Unplug any USB devices or external monitors and "check that Windows shows a 40% charge or greater when you hover over the icon in the taskbar."
Once you have that checked out, go to Settings > Update & security > Windows Update. Go to Check for Updates and install System Firmware Update - 8/29/2016; select Restart now and do not shut down the tablet. After the slate restarts, you'll see a black screen with the Surface logo and a message that a firmware update is being applied. This could take a few minutes. Once done, your Surface Pro 3 will return to Windows. Do another restart to get the update to take effect.
source: Microsoft via TheVerge