Battery light on 115 240d

[NC240D76]

After this last little cold spell (10's & 20's for 10 days or so) and the car only being driven 2-3 days a week instead of 5-6 the battery light came on yesterday morning but it was very dim and went out as soon as the rpm's came up as I started driving. Last night the car cranked a little harder than normal and the light came back on but dimly, I drove home (20 miles) with headlights, radio and heater blowing and no battery light on but when I pulled in the driveway and put it in park the light came back and when I cut the headlights off the battery light glowed very brightly (that is the only reason I know it was dim before). If I turned the headlights back on the battery light would glow dim and brighten up again when the headlights were turned off.
The car started fine this morning but the light is still there (dim).
The battery is only 9 mths old and the correct size for the car.
I will of course check everything tonight or tomorrow but I am hoping that the battery light getting brighter when I turn the headlights off will mean something to someone with more Mb experience.
Thanks for the help