From comment left at a blog for a battery voltage monitor project
I think this is a great project. But I’d like to relate to you all how it was done in the past, when alternators started replacing generators. Back then the rectifiers were not as rugged as now, and sometimes a rectifier diode would fail. The battery might get charged enough, except when driving at night with the lights on.
This could sometimes be heard in the car radio’s speaker as a whine sound that changed pitch with the engine speed. So someone came up with the idea to use an earphone or small speaker with a 100 uF, 25V electrolytic capacitor in series. Just connect it with the correct polarity to the battery while the engine was running. The whine with a normal working alternator was very quiet, but a bad rectifier could be easily heard. This made a good tool for finding a bad rectifier diode. Someone should make a tester using this method.
