You are right, but they are writing in a relaxed, informal way, and may also want to save space. They might also have written:
Remove batteries when not using air conditioner for a long time.
This kind of language is typical of instruction manuals and user guides, where they often want to use as little paper as possible.
As you indicate, the correct formal style would be:
Remove the batteries when not using the air conditioner for a long time.