2007 Saab 9-3 2.0T Sedan Idle issues?

I've had spark plugs, fluids, and filters changed. I've cleaned the throttle body, reset ecu, and software updated but what factors affect idle? Software update fixed spark plug adaptation and stalling. Also before update car occasionally idled at 720. I could drive it for weeks and it would idle at 900. Saab tech checked for vacuum leaks, none. They did agree idle was a bit higher then norm and programmed it to idle 15 rpms lower. So now with the a/c it idles at 720. I have noticed in the recent days that car idles flawlessly on a full tank. Don't have to wait for idle to drop, but when 1/2 full there are problems. Ex. full, car idles at 720. I can turn it off as many times and it still idles at 720. It says in manual car should idle at 720 unless there is load ex. turning steering wheel. Less then full car idles at 720 but I have to wait awhile and if I turn it off forget about it. It will idle at 900 until it wants to drop. It eventually drops after a few starts or being driven for 10 or more minutes. Usually it sounds funny and my lights dim for a second before the drop. Is this the correct way the trionic 8 operates? I'm trying to pinpoint an exact cause for this. Any guesses? I don't want to replace the battery if not necessary. If the battery is low or there is a drop in voltage can that put load on the engine maybe making the alternator work harder to compensate for the lack of power? I notice that un/locking makes the interior lights dim slightly. My headlights and SID dim slightly as well. Its not very noticeable but I can tell if I look hard enough. I think its related to the battery. If I leave the radio on or a door open for awhile while cleaning the car it idles high for awhile but I've never had to get a jump and car doesn't take long to start.