Get a drop-in adjustable one from here https://www.240turbo.com/AdjustableVoltage.html

Thanks for that. I can solder fine, but the mod would be totally arbitrary to me.
As in, I don't really understand the parameters, safety, etc.

Have you done this yourself, or installed one that guys modded regulators?

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1981 240D 4sp manual. Ivory White.

Me thinks that charging the battery shouldn't be an issue/ It was an issue when I had a faulty regulator. Once I had a working regulator. the batteries charged and no more issue. If battery technology changes and warrants a mod, then it's good to have. So far, lead batteries work and are less expensive than AGM. Old car, old technology.

I saw a thread for my 280Z where the "upgrade mod" is to replace a perfectly good working external fuel pump and move it to inside the gas tank. Why anyone would want to make the pump harder to service in 8 or 10 years is beyond me.

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85SD 240K & stopped counting painted, putting bac together. 84SD 180,000. sold to a neighbor and member here but I forget his handle. The 84 is much improved from when I had it. 85TD beginning to repair to DD status. Lots of stuff to do.

Because AGM batteries are better in every way. Charging them at 14.5v when its cold ambient temperatures just maximizes their capacity.

With headlights, blower on full, and stereo running, my alt dips down to about 13v at idle.
With nominal load, the highest I'm seeing at idle is 13.8. This would be arbitrary except for the fact that most mornings I've been finding my battery around 12.37-12.43.

It's either discharging at night, or not getting a proper charge while running.

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1981 240D 4sp manual. Ivory White.

When you put it to bed at night disconnect the Negative Battery Cable and check it in the morning. Or you could also do the same but use a battery charger to be sure it is charged and disconnect the charger and check it the next morning.
After that you will need to reset your clock which of course draws a tiny bit of current all of the time when everything is connected.

It is well known that the colder it is the lower your battery voltage is going to be. I know on some of the mornings last week it was down to 51 degrees F here in Long Beach, Ca.

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84 300D, 82 Volvo 244Gl Diesel

You're thinking there may be a parasitic draw?
I'll try that tonight.

Unfortunately, using a charger requires me to unmount and carry my battery into my apartment... bit of a drag.

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1981 240D 4sp manual. Ivory White.

Voltage this morning was 12.54.
I decided to clean up the battery ground terminal and ground cable.
Doing so made a surprising amount of difference. 13.8 to 14v at idle.

I say surprising as there was little noticeable corrosion. Not bad gains.

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1981 240D 4sp manual. Ivory White.

12.6 is full charge on lead battery. There is also the issue of how accurate your meter is which is unknown. I say unknown because I've seen difference between quality meters reading the same battery and also same house plug a few seconds later.

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85SD 240K & stopped counting painted, putting bac together. 84SD 180,000. sold to a neighbor and member here but I forget his handle. The 84 is much improved from when I had it. 85TD beginning to repair to DD status. Lots of stuff to do.