I have a 2007 Mazda 3 2.3L. The battery is about 1 year old. I should note right off the bat that I usually only drive my car on weekends so that might be the problem, though it's been like that for about 4 months and only now am I having an issue.
OK, so about two weeks ago I went to turn on my car, and it was completely dead. I got a boost and got going again and drove away, but if I let the RPMs get low, dash lights would start to come on and the engine would sputter and I had to give it some gas to stop it from dying. About a minute later it died at a stop sign, and I had to get another boost. This time I kept revving it high to keep it from dying. I drove around for 15 minutes and came back home and everything seemed OK.
I came back to it after it was sitting for a week and tried to start it but it was dead again. I got another boost and this time I decided to test things. First, I tried disconnecting the battery and it kept running for about 20 seconds, so I connected it again. I heard that might not be good, but a friend told me with a car as old as mine it should be OK.
Then I decided to test with a multi-meter on the battery terminals. It had a steady 14.4V when idling, and around 12.5V with the car off. I did a few load tests (turned on heater, wipers, rear defrost, radio, seat warmers, fog lights, head lights) and it dipped back down to 12.2V-12.6V with all that on. With just the lights and heater, it was around 13.5V. Those are all with the engine idling though. I'm guessing it would have been higher if I revved it.
I drove it to the auto shop and they tested my battery and said it was good, so that shouldn't be the issue.
So after all that testing, what I'd like to know is if my alternator is on its way out, or is it the fact that I hardly drive the car that's causing it to not start? Are those voltages normal, or are they signs of a dying alternator? I'd like to avoid getting another alternator and having the same thing happen, but I'd also like to avoid being stuck on the side of the road with a dead alternator.