Battery life of an HP laptop

Got a 2nd hand HP laptop was 8.0, now W10, from a friend in 2018. A few days ago I noticed it was UNplugged, I plugged it back in & it showed 73% plugged in. Now its showing 63% plugged in. NO access to battery or HDD so I will not take it apart to replace battery.
Will it continue to work even if battery goes completely dead? TIA, I'm guessing it will still run, but asking anyway

F.W.I.W., mousing over the battery icon, it simply says "battery meter", no % shown, Thinking about resetting it to factory & ????

Same laptop different situation. I decided to put it to use so I booted it up, typed in the passcode, said it was wrong, clicked ok & typed it in again, same message, I knew it was correct. Shut it down, pulled power cord out, pushed power on button for a few seconds,released , plugged it back in & booted it back up.Entered my passcode , same message, got angry, pulled power cord out pushed poweron button, plugged back in & booted it up again. It did a disk check & other things too fast to know. As of now, it will not go to desktop. It did a BIOS thing. I'm currently "trying" to do a system restore to yesterday, taking a few hours. If that fails, I have a W10 repair thing saved on a flash drive. Been working on this all day. Did try a factory reset but did not work. Any suggestions?? Hoping