Battery dead?
 

Miata battery seemed dead out of the blue yesterday. Jumped it with the LR4 with no problem. Let it idle for a few minutes (we were going somewhere) and shut it off. Today, once again dead.

New jump, this time with a bit more effort and got it going. Drove for an errand about 25 minutes of very spirited back roads and high speed driving. Got there, went in for 5 minutes and again car would not start when I got back. Had to get a jump. Same return drive home.

Voltage after shutting it off when I got home = 12.05V
Voltage after 1 hour = 11.85V
Voltage after 2 hours = 11.80V

The battery had a couple of episodes of being drained early on. It (Duralast Gold) was bought a couple of years ago because I thought the previous battery was the issue, but it turned out to be loose connections. That's not the case now. Everything is tight.

I guess it has a warranty?

just bump start it:D

Just checked. 3 year warranty. I installed it January of 2019.

Sadly I was stuck in a stretch with several speed humps in front of the hospital where I had to go.

Alternator?

That's what I am wondering. The car worked flawlessly when it was running, something that didn't happen the only time I've had an alternator go bad. The battery didn't seem to charge well, but that could be the battery. I'd think that right after that long drive the battery should be at over 12.0V.

After jump starting, alternator output should be 14-14.2 volts. Headlights should be fine running from the alternator, not the battery.

Loose belt can also effect alternator output

I just replaced the belt preemptively a couple of weeks ago. Ironically, I had to pull the battery and it's cradle to get to the belt tensioner. All was fine for a coupe of weeks and a few drives.

Start by testing alternator output voltage. If it's above 13.5 or so, it's probably fine.

The battery will not stay over 12v for long once the alternator is off it.

If the alternator is good, disconnect the battery and see if it discharges overnight while not hooked up to anything.

Or, as an alternative, take it to an auto parts store and ask them to test it. The gigantic, expensive battery conditioners that they have can rehabilitate a battery that home chargers can't.

Measured voltage just now having not touched the car at all. It was down to 11.6V. It hesitated a little but started up fine.* Measured voltage while running and got 13.9V, so I guess that confirms that the alternator is working. Shut off the engine after just a couple of minutes and battery voltage read 12.2V.

*WTF? Why was the battery not able to start my car yesterday after 25 minutes of over 3.5krpm driving? Is that not enough and the return drive did the trick to top it off?

Meanwhile, the battery did drop from 12.05V to 11.59V in 24 hours.

Maybe I should disconnect the battery and track it on its own to confirm it's not some parasitic loss.

I go through this every ~2 years. In fact, the replacement unit in my 3-year old Accord is getting close to being ready for replacement itself. The heat in FL kills batteries at a rapid pace. At the point where I have to start paying for new ones, I'll do some research to find out if some brands are more durable in the heat than others. Until then...

That's not a bad plan. But in all seriousness, having the battery tested on a commercial machine (for free!) is worthwhile, too. These machines are really good because it's critical to limit warranty replacements.

Yeah, I'll do that anyway. And thanks for the heads up on those. It makes sense , to your point, otherwise they'd be exchanging batteries all the time right before the warranty runs out or new batteries obtained simply to rule out the battery.

Follow-up: Battery, all on its own, dropped to 11.6V again after 24 hours - the same value as when it was connected to the car. So I think I can rule out parasitic losses as the main culprit. A day later I reconnected it (did not measure voltage) and tried to start it and it felt just like at the Hospital (seemed just a little low). I jumped it with the LR4 without starting the LR4 (I know, not ideal) and it fired up perfectly and I drove it to run an errand.

Just now I took the battery to Autozone to have it tested and the guy came back with a new battery without checking anything. I guess that's that. We'll see if I have issues again. New battery measures 12.78V. Have not started or driven the Miata yet.

EDIT: Can I just say how nice and simple is dealing with the battery on the Miata? All the new fancy cars have all this crap and programming and other nonsense and they are difficult to get to. It took me literally 1 minute to drop in the new one.

Forget the battery, what are you doing at a Hospital ... every ok ?

Thanks for asking.

I'm ok. It's about my son. I had to drop off a specimen for analysis. He's got yet another ongoing symptom we are trying to find a cause for, but right now the specialist is not too worried.