I have the same computer. I have Micromats Checkmate software installed an it shows that my computer doesn't have a internal battery.
I also have Checkmate installed and it, too, shows that I don't have a battery. BUT I DO (and so does your iMac)! I believe that Checkmate refers to the rechargeable battery in a laptop, not the internal backup battery.
Johan,
I want to make sure that we're on the same page (and I should have asked this earlier): What exactly do you mean by a "freeze"? I interpret this to mean that the entire computer becomes unresponsive and you have to hold the power button to force a restart. Is this what's happening?
Or, is one program freezing and you have to force-quit it? If so, which program? If that's the case, you may be running out of RAM, particularly with memory-intensive apps like Photoshop. 4GB may not be enough.
It happens when I'm on the internet, usually watching YouTube. The pointer will move, but I get the spinning ball. I can't force quit so I have to do a cold restart. If it happens again, I will note the time and look at the logs and report what I find.