Final answer:
Overcharged batteries are often due to a defective voltage regulator, which fails to maintain the correct charging voltage.
Step-by-step explanation:
When you have overcharged batteries, the cause could be a defective voltage regulator. The voltage regulator is designed to maintain a constant voltage level to the battery.
When the regulator fails, it can result in an incorrect charge being supplied to the battery, leading to overcharging. Defective rectifier bridges, lack of ignition excitation of the regulator, and loose drive belts typically lead to undercharging rather than overcharging.