1993 legacy wagon. Brake light and battery light come on at same time and then go away and come back again seemingly randomly. I tested battery with engine off and it read 12.7 volts. With engine on, it read 14 volts, but periodically while idling, engine would make a light click noise and the voltage would quickly go down to 13 and quickly climb back up to 14. And when I turned engine off, voltage went back down to 12.7 but after only half a minute or so went to 18 and stayed there for roughly 10-20 minutes before returning to around 12.6. Any advice appreciated. Thanks.

sounds like the alternator may be going. check the battery cable connections, make sure they are clean.

and then go to the parts store and have them test you alt.

Have them test the battery too. Could be a bad cell or something moving around in the battery cells themselves.

almost certainly a bad alternator.

Check the nut that holds the cable on the alternator. Make sure it's tight.

Also check battery terminals and the main engine ground.

If it went to 18 volts at any point that is not good, definitely sounds like something flaky going on with the alternator.

 

The clicking noise, was that maybe the A/c cycling on and off?