*Surface Pro 4*, battery is one of its weak spots as it ages - it's a 2015 device.
The specs
• Type: Li-Polymer, non-removable, glued-in
• *Capacity*: 5087 mAh, 38.2Wh, 7.5V 
• Rated life when new: 9 hours video playback (Microsoft claim)
• Real world now: Most Pro 4 units at this age get 2-4 hours if never replaced.