I remember seeing a device that would shut off the power drain on the battery when critical 50% is achieved, but I don't remember what is was called or where I saw it. Try asking about this kind of device at a Radio Shack, Batteries Plus, or other such establishment. I have changed out the incandescent light
bulbs to LED light
bulb replacements, and have a 100 watt solar panel
perm. mounted, so it is more than capable of keeping up with the scant drain of an LED light. You could try hanging a sign over the exit or on the door handle to remind you to turn off the lights
(simple and cheep).
Dave & Paula