Is my alternator dying?

[Slipstreem]

So a little back story. The battery in my Miata that came with the car when I bought it decided to call it quits a couple of days ago. No biggie. I went to Autozone and bought a new one. After installing the new battery the Miata would fire right up in a blink of an eye. A few days afterwards I had to stop by the DMV but, forgot to turn off my lights. My fault, no big deal. I was lucky enough to a get a jump from a nice stranger and went on my way.

Now this is when things start going down hill. Last night I drive to work. Miata starts right up with no issues. When I get off I try to start the Miata and...nothing. All lights come on, radio can be turned off and on, and the (power) windows roll up and down. I think to myself. "Well maybe since I didn't fully charge the battery after that jump maybe it lost it's charge because of the cold." (It was raining all day here.) I got another jump from a co worker and again was able to drive home with no further issues. I borrowed a battery charger from my dad and let my battery get a nice full charge before re installing.

Today. I started my Miata but this time it had a sluggish start. Took a few cranks before it got going. While driving around town I noticed that my rpms would bog up and down when I was at a light and IF ONLY I was in gear with the brake pressed. I do know that going in gear would cause load but a 150ish-200ish rpm drop doesn't seem natural. The only thing I can think of is maybe after the battery jumps the alternator is starting to die. What doesn't make sense is it hasn't shown really any signs of doing so. There's no check engine light, the headlights or dash lights don't dim on and off, and overall the Miata hasn't died on me while driving. I'm terrible when it comes to electrical problems so I'm hoping you guys can help me out.

-Specs-
1990 Miata 1.6 5 speed 176xxx miles

[—RotorNutFD3S—]

It could be the alternator going, you can take the car to the parts store and have them test the system because you're in an NA (for future searches: NBs cannot be tested by 99% of the parts stores because they use an external voltage regulator, it will fry the system and you will replace the alternator and your ECU).

Just in case, check the electrical 2-pin connector on the side of the alternator. I just had mine give me similar trouble, the connector is having intermittent connectivity with the alternator and the system was not always charging. On mine, there's an issue inside one of the wires or at the crimp, when it acts up I just move the wires around and it kicks back in and is good for a while. I'm going to cut that connector off and add a new one. On yours it could just be not plugged in all the way, contacts are in need of a cleaning, spade connectors could be loose, etc.

[Slipstreem]

Tried to start the Miata tonight to go to work and again it's dead. Dash lights and what not come on but again it won't crank over and the clock was reset. I don't have any after market lights, stereo, etc. So I know there's isn't a dark current. I didn't have time to check the connections since I was now running late but, just eyeballing the alternator it seems to be the original. I double check the connections when I get the time but, I'm thinking it might be time to replace it.

[— kung fu jesus —]

I have killed batteries in short order by leaving the lights on only a few times. One I killed in a week doing this. They can only be deep cycled a couple of times like that before they lose their effectiveness. Still, I would be suspicious of the alternator and they really aren't difficult to remove to have tested. The car is almost 25 years old, y'know?