How do I destroy the Harley Heads?

I'm talking about the Harley Heads in the Steel Mill where if you press the button, you hear Harley talking through the intercom. I know I have to do something to them, because there's a count for them in the Riddler's challenges under the Steel Mill, just like the Joker Balloons and TYGER cameras.

I've tried:

• pushing the button several times, but it just seems to annoy her
• using the explosive gel, but doing so makes him use it on the wall behind the head, which does nothing
• trying to use the various Batarangs at my disposal, but there's no targeting reticule for them

What do I need to do to destroy these?

Solution 1:

Shoot a Remote Electrical Charge directly at her head. It should pop off and count it as completed.