How hot is the battery getting? Batteries normally warm up when accepting a charge, not just when overcharged.
What kind of battery is this? A lithium cell? NiMH? NiCad? A flooded lead-acid? A jell-cell lead-acid? You have to watch the temperature on some of these, but it's not all that big a deal on lead-acids unless they're getting REALLY hot rather than just warm. (In the latter case I'd wonder if something other than overcharging is wrong.)
Get an inexpensive digital voltmeter from Radio Shack or the like and measure the battery voltage. Don't unhook it or turn on extra loads until it reaches full charge voltage. Exact voltage for full charge and for a good point to stop dumping / rehook to your power source varies with battery type. When you start making that dump load controller you'll need the digital voltmeter to adjust its settings, so you're not wasting the 20 bux or so it will cost now.
Why are you unhooking the battery? Try turning on all your loads, instead. Or hook up some suitable power dumps (like a bunch of automotive headlights) and turn them on when you're approaching full charge. You can probably get one-beam-dead headlights at your local auto shop for the asking.
What is your charging device? If it's solar panels and you have a blocking diode near the battery you can just short them upstream of the blocking diode. They will only produce a few amps so an ordinary switch or relay will do the job. If it's a mill you can also short it upstream of the rectifiers (provided the alternator is sized so it will bring the blades to a near stop) but this will take a more powerful switch, such as the starter relays mentioned by another poster. (Though I'd worry about starter relays being used for continuous duty. Be sure to fuse them.)
If you know how to solder you can make a dump load controller from circuit diagrams posted elsewhere on this board using WELL under $100 worth of radio-shack parts. Use it to control a low-current automotive relay (pick on up at a junkyard) that turns on your headlights - or turns on a bigger relay such as the starter relay.)