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I’m not saying you’re wrong about anything you’ve written here. What I don’t understand about this entire discussion is this: Your alternator under certain conditions bumps the voltage up to 14.9 or 15v. I’ve seen this on my CGTS. The electronics in the car are all 12V. The only reason to bump up the voltage (that I am aware of) is to create a faster charge on the AGM battery.

Whether this voltage is coming from the alternator or a charger makes no difference. It ends up on the negative terminal at the battery either way (because that is the whole point). There may very well be a second or third negative lead from the battery which limits voltage or current for whatever purpose on other circuits, but that shouldn’t matter one bit to how the battery is charged.

I’m probably missing something.
I think the main issue that you may be missing is that Porsche says it should not be over 14.5 volts for any or any extended period of time. If they're charging the car above 14.5, it's because their electronics know what their batteries can absorb. When you plug your ctek into it, if you're in polar mode it's going to apply 14.7 volts for some period of time that they have determined is correct for their usage. This usage is for Power Batteries not for auto batteries.
This is all a bit of guesswork because I think there's a lot of unknowns in this area. That's why I am deferring to use the lower voltage. It won't hurt as its long-term storage. If I run it at the high voltage there is potential for damage based on what I have read. That's it.
MOO & FWIW
 

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FWIW, I use the cig lighter in the console compartment and the AGM setting on my CTEK. Wire snakes out the lower door seal. I’m unaware of any problems so far. 7 years on original battery, but since COVID, I keep the car on the tender unless I’m going out the next day. Could be something fails tomorrow, though.
I just leave the window cracked and the wire out the window since its garaged. Any reason not to go that route?
 

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Just one quick word of caution for those of you using the cig lighter adapters, I too used that connection for several years without a problem since my 981 always sensed a voltage at the outlet and kept it active. Then last year we had a power outage that lasted several hours, and during that period the car sensed no + voltage at the outlet and turned it off. Once the garage power came back on, the battery tender was now connected to a dead outlet in the console and by the time I noticed it several weeks later, the battery was dead. I now have the pigtails permanently connected with the plug running up under the cowl just under the passenger side wiper. Easy to reach to connect/disconnect, and no worries about the car turning off the charging connection.
 

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It is my understanding that the only reason you have to be careful with the negative battery connection is because of sparks like when you connect jumper cables to the battery. The sparks created could damage the sensitive electronics located on the negative battery cable.
 

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The advise regarding not using the negative post has to do with jumping batteries as stated by MidLife. A battery maintainer has such a low charge capability there is no risk of sparking if said battery maintainer is connected to the negative and positive battery poles. I attached one to my car for over 10 years that way with no issues what so ever. And in fact, as I recall, the instruction that came with that unit actually said to do exactly that.
 

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There is another reason not to use the negative battery terminal to charge or jump start your car to avoid damage to the LIN Control Unit. The control unit is used for the start/stop function and the Electrical System Recuperation function among other things. I assume you are more likely to damage the control unit with a jump start than with a trickle charger but why take a chance.
Jim
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It is my understanding that the only reason you have to be careful with the negative battery connection is because of sparks like when you connect jumper cables to the battery. The sparks created could damage the sensitive electronics located on the negative battery cable.
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no annoying rattling?
Not at all. At least none that I can hear over the engine haha

I doubt it would rattle though... its doesn't stick out over the hood so I doubt the wind would be hitting it
 
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