Can a bad alternator cause a battery to die overnight?

Alternator can still charge w a bad diode. But battery will drain while parkd. To test disconnect wire from back of alt..if it still drains then somethin else is runnin batt down

Andrew yes the alternator will drain your battery when alternator goes bad you running all the juice on that battery change the alternator and off you go good luck

A bad alternator no longer charges the battery, the battery will die once it is drained. This will occur more rapidly when using equipment that runs off of the battery( lights, heater blower, windshield wipers etc.) even electronic ignition will run down a battery. Your battery just went over night the alternator may have stopped working long before. Go to an auto parts store that offers free testing to make sure it is your alternator before making repairs.
Good luck.

Hi, cold will also drain your battery over night. if your alternator has not been charging your battery for a while you would run it down gradually over time. to test alternator start car, and the battery light will go off .start car and take a terminal off the battery and car should still run if it stalls then alternator is no good. another test is to start car in the dark,put the heads lights on ( point at a wall ) let car tick over then put some revs on,if alternator is ok your lights will go brighter....hope this helps

My guess is theres a short somewhere. a bad alternator will drain battery while its running because its not charging and you are running on just the battery. you might also check terminals or cables for corrosion... this may disrupt flow of electricity in or out of the battery