Q: imac keeps saying mouse batteries need replacing even after inserting new ones

Check if the batteries you inserted were full.If yes, there must be a problem with the battery sensor on the mouse.

Nothing to worry.If you continue to have the same problem and its disterbing , go to your nearest apple store and buy a new mouse.Not the imac's fault.

Check to confirm that the batteries are in properly.  Unlike many other items, both batteries face the same direction.