I'm fairly certain that i3 comes with a percentage battery indicator by default. The default config is pretty good (except it uses `jkl;` instead of `hjkl`, but that's a simple fix).
I think that's part of a different package, i3-status or something, which is not strictly required but recommended in docs etc. Possibly it's also installed by default if you just choose 'i3', depending on your package manager.
You're right, turns out Arch's i3 package is a group of packagse: i3-wm, i3-status, and i3lock (screen locker).
For some reasons i3 was showing wrong value for battery. It also had issues with the graphic drivers, showing black screen on logging in. So I didn't consider it as a viable option for me.