There may be a CMOS discharge jumper near the battery. Most of them have three pins. Moving the jumper form one set of pins to the other will discharge the battery. Be sure the computer is off with the power cord pulled when you do this. Some motherboards can be damaged if power is applied with the jumper in the discharge position.
If you can't find this jumper, The best thing to do is to try and track-down the motherboard manufacturer.