Install xfce4-battery-plugin by entering the following commands in the terminal:
sudo apt update sudo apt install xfce4-battery-plugin
Description:
battery monitor plugin for the Xfce4 panel
A battery monitor panel plugin for Xfce4, compatible with APM and ACPI, using either /proc files or the new sysfs files for recent kernels. You can use this applet to monitor your battery level and be notified when your battery reaches low and critcal levels. The applet can even be programmed to execute a command on low and critcal levels.
Homepage: http://goodies.xfce.org/
Version: 1.1.0-1
Section: universe/x11