Where is battery widget

See the following instructions from Add widgets on iPad - Apple Support

(the Batteries widget is in the sidebar in step 3 below)

Add a widget to your Home Screen

1. Go to the Home Screen page where you want to add the widget, then touch and hold the Home Screen background until the apps begin to jiggle.
2. Tap  at the top of the screen to open the widget gallery.
3. Scroll or search to find the widget you want, tap it, then swipe left and right through the size options.
4. The different sizes display different information.
5. When you see the size you want, tap Add Widget.
6. While the apps are still jiggling, move the widget where you want it on the screen, then tap Done.