1994 944T 265K

Sitting at a light I noticed that the car was idling rough. The tach was not registering but would occasionally jump up to 3500 and hold there before dropping back to zero. The gas gauge had dropped to empty. Raising engine speed smoothed out the engine as well as tack and fuel gauge back to normal.

Drove the four miles home and as soon as I pulled up to the garage and the engine dropped to idle it stalled. Very dim dash lights and no response from the starter. The battery was dead.

I put the charger on and several hours later the battery was charged. When I start the car and measure across the battery terminals I get:

600rpm 12.4v
1500rpm 13.6v
2000rpm 13.9v
2500rpm 14v

Higher engine speeds do not increase the measured voltage.

I do not know what the measured voltage should be. What do these numbers at different engine speeds indicate?

The battery is 7 years old so it doesn't owe me anything. Dragging it into AutoZone for a test seems appropriate.

Looking for other input. Thanks

Edit: The AutoZone test indicated the battery was "Good". It had just been on a charger all afternoon.

Randy