I'm trying to do something similar, but instead of one light to indicate "charging" or not, I wanted to have a *series* of lights indicating the charge status of a series of batteries. When charging only, or under load only, it's able to detect an empty (or full) battery and switch to the next one in the line, but detectors do not have the ability to tell if a battery is actually full, just if current is flowing, so if you are in a situation where your power generation is higher than your load, it will never think the battery is 'full' and switch to the next one, or if your load is higher than your charge, it will never think the battery is empty and will not switch to the next full battery.
What I'd love to see is a module you can stick on the side of the batbox/mfe/mfsu that triggers a redstone signal when battery is within %5 of full or empty... It would easily be able to tell what role it's playing by looking at the side of the box it's attached to: If it's on the output of a battery, trigger redstone on depletion (within a small margin of error - twice the output rating of the battery would be enough, to account for charging operations being done at the same time), if on one of the box's inputs, signal on completed charge (again, within a margin to account for the system being under load).
Just make sure not to let the battery itself see the redstone signal. I've seen some interesting side effects of putting cable detectors right next to battery boxes.
Such a module would allow me to make a system that would let me serial-charge an array of batteries, and would also allow me to build a power station capable of bringing backup generators online if all but the last battery are permitted to deplete themselves.
Unless someone can think of a way to do that using the current cable detectors and cable splitters...