Fishing for red herring.

[groupw]

The weekend we went to the Renaissance Festival we took the Miata. It was a great day until we got to the line for the entrance. I had no idea the line would be that long. The Miata started getting hot. I turned the heater and fan full blast and was able to keep it warm, but manageable. 
When we left, the car started ok, but the engine temperature never got up to a normal reading. The engine would also stumble at certain throttle positions. I figured the thermostat was stuck open and it was sending bad info to the ECM. I replaced the thermostat this weekend. I keep the battery on a trickle charger because those little batteries don’t have much cranking reserve after sitting for a week! Test drive all seemed good. We drove up to my office for an F1 viewing party. After the race, the battery barely turned over. Great! Now I need a new battery, too. On the way home, the low engine temp reading and and engine stumble returned. 
When I got home, I let it idle and grabbed the meter from the toolbox. Sure enough, voltage was only 10.9 volts! Looks like I need an alternator instead. Hope the battery is ok after driving on it the last couple trips!

[Airehead]

You are a good diagnostician