Chosen Solution
Is the battery replaceable on the Surface Laptop 4 15"? Also do they sell replacement batteries for the laptops?
Sonny Medrano looks like it’s possible but a royal PITA. Anyhow, go through the service guide published by Microsoft Surface Laptop 3 And 4 English Service Guideand check this video as well (yes 13" model but looks like the step are very similar) and see if that is something you want to tackle. I have not yet found the right battery. Microsoft will charge you $400 for an out-of-warranty Battery Replacement service. Dang that is nice of them :-)
