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- 2016.5 CX-5 GT AWD titanium/black 2016 Miata Club ST MT white
You bought a used alternator? Did you test the old alternator?
A simple test, if you have a voltmeter, is to check the voltage at the battery before starting the car. It should read in the vicinity of ~12.5v.
I just now checked the battery in my 2016.5 CX and it's a bit low at 12.2v. This is the original battery so it may be time to change mine out in the near future.
Now start the car and check the voltage at the battery. After a couple of minutes, it should jump to around ~14v. If it doesn't jump up, then your alternator is suspect.
An update on my low battery voltage.
When I checked the voltage of my battery at 12.2v, this was immediately after unlocking the car in order to pop the hood (CX hadn't been started for about a week). After about 30 mins I checked again and got a reading of 12.6v. I'm less concerned about my battery now but will be keeping an eye on it.