Terry, I'm not a mechanic, but ...if your mechanic had not already looked at it I'd say place a battery tester on the battery and with the engine off it should read around 12.6 volts. Then start the motor and check again and it should read around 14 volts (I don't know the precise number for this model).
If it's much less than 14 volts then your lights are draining the battery faster than the alternator can charge it and you may need a new alternator. If you get a reading around 14, but below 12 with the engine off then it is probably your battery. If both your readings are good then you don't seem to have a problem at all except for the light coming on. Do you have a battery tester. They don't cost much.