I'd say you might have a weak battery. You exceeded the output of the alternator for some period. This normaly should be handled by a strong battery.
There is a good chance that you were using 40-50 amps and the alternator probably only does 25-35 amps below 2000rpms (the pre1990 cars are burdened by this fact).
Bosch Master, ASE Master, L1
33 years MB technician