How to make it clear when laptop is on battery in 18.04?

Solution 1:

From the attached pics in your Question gives clue that those icons are named as

battery-good-charging-symbolic.svg and
battery-good-symbolic.svg

There are similar Images with naming which can be understood easily.

First determine which icon theme is being used.

And then copying this Icon theme into ~/.icons directory and replacing the image with your preferred one but with same name of icon will change the symbol.

There is a good documentation here.

Solution 2:

I solved this by going to Tweaks → Top Bar → Turn on Battery Percentage.

Now the battery percentage is shown as a number, so if unplugged, the number shows as declining which is more attention grabbing than the icons.