It’s possible that you have a run away process that’s eating up memory or CPU and your power settings are just putting your Surface to sleep or hibernation when you hit the power button.If so, that runaway process might simply wake up every time you wake your Surface and start chewing away on your battery some more. which suggested cleaning the air filter, MAS, and disconnecting the battery so as to reboot the electrics. So now will wait and see if it is a genuine engine fault or just an electrical one.
I can't say this will work for everyone, and don't know if the apparent improvement will continue or not, but since I use my laptop off-power for a while each day, I'm going to continue the cycle --- hopefully the improvement will continue!
I take it the laptop will not flash unless it's on DC power.
IBM support sux, I already contacted with them and they wrote:"Dear Sir Madam, You should use AC power adaptor when flashing the bios because in case ofpower cut off your machine systemboard will be seriouslydamaged so do use AC power adaptor Regards" And in the installation instructions for BIOS they say that:"Notes: - You need an AC adapter and a charged battery pack to update the system program." So I have AC adapter but not a working battery because they are in shit and I can't update BIOS!
I advise you to plug your phone to a charger and then begin with update.
During update to W10 battery percentage can go wild (my Lumia 520 went from 90% to 10% during update, and after update it had around 40% battery) or it can get stuck on last(or any other) step for a long time. When I was updating my Ativ S from WP8.0 to WP8.1 it wouldn't update if I had less than 50%, Thank you for your interest in this question.