Power on/off time. Battery test. How to know?

Hi guys! I want to know how long my laptop will working without charging. I plalling turn it on and wait to full discharge and auto power off. How can I know the time was when it turned on and turned off with logs or any other method?

Try this (boot with battery allready):

systemctl |grep startup 
systemctl |grep poweroff

No info

use start and power instead to grep.

No useful info too.

systemctl |grep power
upower.service loaded active running Daemon for power management
[@laptop ~]$ systemctl |grep start
plymouth-start.service loaded active exited Show Plymouth Boot Screen
rpc-statd-notify.service loaded active exited Notify NFS peers of a restart