TOPIC: Battery is not holding a charge

Is the 1984 928s prone to not holding a charge? What are some things to check? Battery and alternator is new and was installed by Renntag motorwerks in Nashville. I’m guessing it may have been something I did, drove car home 50 miles parked it. Overnight temp was in low 20’s attempted to start , dash light came on but not enough energy to turn over engine. What is the safest way to jump start car and what is a good battery tender to use? Thanks!

The first thing is to clean the battery terminals and ground points around the car.

An easy method to check the leakage current is to run an amp meter in series with the ground strap between the battery and where it connects to the chassis.

With no load (and no doors open/lamps on), only 10s of milliamps should flow. If the flow is higher there may be current leak that needs to be corrected. An easy method of locating the offending circuit can be achieved by pulling fuses one-by-one and noting where there is a large decrease.

I hope that you have your charging issue solved.
I did see that you asked a couple of specific questions.
For jump starting, the manual says to put the positive jumper on the "jumper post" on the front passenger side of the engine compartment. I personally just jump directly on the battery terminals in the rear of the car. As silly as it might seem, I find that coming up with a good ground in the engine compartment can be hard, particularly when you have things spinning around as the engine starts.
As for a battery tender (I am sure that there are many good ones out there), I've settled on a Solar Pro Logic unit that I found at Napa. It's nice and small, seems pretty smart, and came with both clip-on ends and ends that fit onto the battery cable so that all you do is plug the tender into the pig tail end when you need to use or remove it.

I am curious if you have figured out what was happening. Was it a charging issue or a current drain?

It’s odd, car has a aftermarket stereo. When I first drove the car home I could not turn off radio. Has a remote! The next day there was power but not enough to turn over and start. Got the radio off and charged the battery and no issues since.