# Voltage measurements with the alternator running fluctuate

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:

1. What can cause these fluctuations in voltage?
2. 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?

• Hi Cesar. Are the low voltages being witnessed while the car engine/starter is cranking? – mike65535 Nov 12 '18 at 19:27
• One possible explanation is a poor connection between the probes and the battery terminals... – Solar Mike Nov 12 '18 at 19:29
• @mike65535 No, the low voltages were witnessed while the engine is idling. I did, out of curiosity though, measure the voltage at the moment of cranking, and it went more or less normal, the battery right before the starter kicks in is upwards of 12 V, then when the cranking is going on it dips a little bit to 11 V, and then it goes up to the 14 V once the engine starts. – Cesar Nov 12 '18 at 21:35
• @SolarMike Well, I took the clamps off, sanded the battery terminals well, as well as the clamps, and then I connected everything back tightly. I don't think it's a poor connection between the probes and the terminals, here's why: when I measure the voltage with the car stopped, the multimeter doesn't fluctuate at all, I held it there for 45 seconds and the voltage didn't dip (other than the normal 0.01 V variation that happens sometimes). But when I turned on the car, after cleaning and sanding everything, the fluctuations came back for the first two minutes, went away as the car warmed up. – Cesar Nov 12 '18 at 21:39
• I will let the car cool down, and do the process once again, and will report what happens. I also made sure in another car that the multimeter is fine, I applied different pressures to the terminals of the other car, I jiggled the cables of the probes, and nothing happened, in the other car, the voltages remained steady. – Cesar Nov 12 '18 at 21:40