Now inside dash lights extremely dim. Could it be the battery or the alternator? Needed a jump start to get going this morning and engine ran fine (with 2 restarts after approximately 2 1/2 hours. Total of almost 200 miles. However, when I drove this evening, the interior lights were extremely dim. However, the engine seemed to run OK. Do I need a new battery or could it be the alternator or other cause?
Start by measuring the battery voltage with the engine off and again with it running. A fully charged, good battery will read 12.6 volts. If you find 12.0 volts, it's good but discharged. Closer to 11 volts means the battery has a shorted cell and must be replaced.
With the engine running, the voltage must be between 13.75 and 14.75 volts. If it is low, and the battery is not shorted, suspect the generator.