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.