this might be worth trying. mine is a 92 e300-24 with an oversized bosch alternator 120 amps usually used in 8 cylinder benz but still having low voltage ~ 11.5v. with lights and climate control at idle ~600rpm.

modifications was made by my electrician and it has been a month now.

even at the highest load with a/c at high blower spped, high beam and
car TV on with built in battery charging. checking voltage at battery and at ignition, here are the values

battery ignition
before modification
13.5v 13.5v no load, 600 rpm
11.5v 11.5v at full load, 600rpm


after modification
14v 14v no load, 600 rpm
14v 11.5v at full load, 600rpm


modifications: we directly connect a fused(high amp fuse as a protector ) wire from positive terminal of battery to positive terminal of alternator.
they have done a lot of these modification.

even if im having low voltage at ignition, i am sure that my battery is getting fully charged and at low risk of no start situation from a discharged battery.

i dont know the principle behind but maybe those who have knowledge in car electrics may have an answer