Sounds like your alternator is going bad. The additional load from turning on your lights is straining the alternator. Or you battery is too old causing the alternator to stain. A weak battery can damage an older alternator. Battery connections in good shape? Do you have corrosion growing on the positive or negative terminal. + side corrosion only, says the alternator is overcharging (weak battery, or going bad alternator). a (-) negative only corrosion points to bad ground connections. Corrosion on both terminals points to an end of life battery. With the engine off, what is the battery voltage? with the engine running what is the voltage, then turn the lights on and read the voltage. The engine off to running voltage should be about a 2-5 volt difference. What is the overnight voltage drop. Measure after running, with the engine off. Then check again the next day or a few hours later. anything more than 2-5 volts points to the battery not holding a charge. However, most likely your alternator is on the way out. If the alternator is bad, and the battery(s) is more than 3 years old. install a new battery as well.
