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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.

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    Your friend was not being so helpful when they said "your car was old enough it would be okay to disconnect the battery". This is a good way to fry electrical components in your vehicle as well as your alternator. You could disconnect the battery to check the running of older cars which used generators. They haven't used generators since the 1950's-era. It's really a crap shoot to remove your connections. If no harm came of it, feel lucky. See this answer for more guidance. – Pᴀᴜʟsᴛᴇʀ2 Feb 15 '17 at 16:25
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While the car is idling, you should read ~14,2-14,4VDC on the DVM, that's right. Load shouldn't really matter. If voltage drops under load, that's usually a sign of a dying alternator.

What I usually suggest is try replacing the voltage regulator first, because it's dead cheap and can be done without removing the alternator. If it doesn't fix the issue, remove the alternator and have it rebuilt or get an other one that works.

  • With this car the regulator is built into the alternator. Replaced the alternator and the voltage was still good under load. That was the problem. – Pedram Apr 5 '17 at 6:04
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Simply put, if you can drive the car for a while (more than a 10 minutes) and the battery is completely charged when done, then the alternator is working fine.

Sounds like you may have a short/drain of some sort. This can be an interior light, radio, etc. Check for anything energized while the car is off. To search for these, you need to measure amperage, not voltage. Here is a great Popular Mechanics article on How to Stop Car Battery Drains with step by step instructions.

Alternately, you can disconnect the fully charged battery during the week and see if it holds its charge. If it is dead by the time you try to use it a week later, the battery is not actually good. If it stays charged, then it is good and you need to go back the the previous paragraph to find the drain.

  • On some models the central locking moidule can draw enough to make a decent dent in the battery charge; if the battery is only just OK, that can be enough to tip you over the edge. And in winter (lights, heater etc. on) it might take quite a bit more than 10 minutes driving to charge the battery from quite low. – Chris H Feb 17 '17 at 13:49

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