I own a 1994 Mazda Protege. On Sundays, I measure the voltage at the battery terminals, and register it. First with the car turned off, and then with the car idling. The voltage with the car turned off is usually between 12.76 V to 12.8 V. With the car idling and the alternator running, it usually goes up to 14.46 V to 14.5 V. But yesterday something weird happened: the battery is ok, but when I start the car, and measure the voltage at the battery terminals, it does go up to 14.5 V, but it drops suddenly for about half a second to very low voltages, ranging from 10 V to as low as 5 V! Then it goes up again to over 14 V, holds there for 6 to 15 seconds, and then dips again. The multimeter is fine, because I tested it on my other car, and there is no problem, and I tested it on another person's car just to be sure and it also worked well. So the problem must be on the Mazda's side. My questions are the following:
- What can cause these fluctuations in voltage?
- This second question is just out of "scientific" curiosity: even if the alternator was dead, shouldn't there be a reading of at least 12 V? How is it electrically possible for the voltage to drop so low, if I'm measuring the voltage at the battery terminals and the battery is ok, meaning it's charged and has a voltage over 12 V?
Thanks in advance for your help!