How did you try this, and did you search this forum?
Again, this is basic knowledge: you can only compare if both sides have the same type. Considering templates always return strings, and you need a float, you have to add that to the setter:
{% set audi_level = states('sensor.audi_percentage_level')|float(-1) %}
Note that you Also have to set a default value inside the brackets, or otherwise this can throw an error if the entity is not yet initialized. (again this is pure basic templating knowledge you need to read up on, as Ivce asked you in the other thread)
Personally I set those defaults to impossible values, so you can even set a catch for that iff so required or useful, hence the -1
But, as I asked on the other thread: doesnt the integration that you use have a correct battery sensor entity? that should already have the correct battery-icon if following the device_class: battery logic in core HA