This killed my Die Hard in my SD just recently... well the fully working HVAC system did. The puny alt was just not able to charge the car at idle/my city driving with the lights on, blower on max, HVAC relays on, clutch engaged, etc

Bought a 150A, problem solved. I never encountered this issue on my other stuff as I have upgraded the alts in all of those already... did not dawn upon me to be proactive and swap it. Oh well.

I was told the big issue with the small alts and these batteries is that they are almost all not Maintenance-Free which does not allow them to charge well.

varta also makes batteries for wrist watches

the earliest 150amp mercedes alternators i'm aware of started in '94 with the 500e. possibly earlier with the r129.

which alternator did you use on your w126 300sd? would be curious since my 300dt has essentially the same engine

UPDATE:

I am driving the BMW this week. I pulled the battery out of the 300SDL and have been charging it with a trickle charger for two days now. I charged it for 6 hours on Sunday. It went to 13.04 Volts and then on Monday it settled at 12.5 volts. I charged it Monday for 6 hours and it went to 13.1 volts and I could hear it " cooking". I unplugged and this morning it was still at 12.5 volts. I will let it sit all day then charge it again tonite. I know it would be a whole lot easier to just go buy a new battery, but I do not need the car this week and I want to play with the battery and charger. I am going to do this till Friday night and then re-install it on Saturday and see if it works.
If it does not work, Saturday it is new battery day.

... walmart now carries the white MB style battery and the group 49 black 1000CCA MB style battery.
both 75.00 and both have 3 year free replacement...

Is this at all WalMarts or online?

it is at all the walmarts in my area... like 8 of them around charlotte... no idea about online.