IPhone Widget Battery

The battery widget will only indicate the battery charge status for the host device - and devices that are paired with the host.

For example, an iPad with a paired Apple Pencil and AirPods will show the battery status for the iPad, Pencil and AirPods - but will not show the battery status of your iPhone.

Likewise, your iPhone battery widget will show the battery charge status of the iPhone and any devices currently paired with the iPhone - but cannot show the battery status of your iPad and associated Pencil.

I hope this clarification is helpful to you.

An iPad won't show on an iPhone battery widget.