Automatic display brightness

Quick Q, apologies if this has come up previously.

I've just upgraded to Mavericks but I didn't know how to do this on ML either so that's frankly immaterial.

Basically, for the most part, I like to have my brightness on 50% on battery and I usually have it the same on power too.

But it's just struck me that's a bit mental. So, is it possible to set my brightness at 50% on battery but ensure my rRMP automatically adjusts the brightness when on mains power?

Essentially, if it's plugged in, I want it to manage itself in terms of brightness. But as soon as I unplug it, it defaults to 50%. And then the same in reverse, if I start on battery, as soon as I plug it in, I want it to manage the brightness automatically again.

Any help, much appreciated!

 

Anyone?

Ta!