"Can I leave my Surface plugged in all the time?"
We seem to get this question at least once a month so I'll post the official recommendations from the Surface Team:
Surface is designed to work between 32°F and 95°F (or 0°C to 35°C). Lithium-ion batteries are sensitive to high temperatures, so keep your Surface out of the sun and don’t leave it in a hot car.
Recharge any time.
The battery doesn’t need to be empty or low before you recharge. You can recharge the battery whenever you like. However, it’s best to let the battery run to below 10 percent at least once per month before you recharge it.
If your Surface battery is found to be defective during the warranty period, it is covered under the standard Surface Warranty.
Microsoft Surface battery and power | Recharge Surface