Battery light on?

[Bobthebuilder]

I drive long miles AM and PM with my 85 300SD, so most of the time my lights are on. This AM, after 20 mile warm-up, I hit the freeway and got up to about 75. After only 4 miles the Battery light began a dim pulsing which got brighter for about a mile and then stayed on. I pulled off to make sure the belts were intact even though it wasn't heating up. No noises, no smells and pulleys were spinning fine. I continued for another 100 miles and the headlights are still bright. Starts fine and went another 30 miles tonight, headlights normal. The Battery light gets very dim at idle and bright red when running. Is the alternator out or am I getting some other message?