I think I would start by looking at the keyless entry issues. That seems to be ,from what you have said, the main thing that is playing up. Battery isolator ? That will prove the battery is good if every time you connect it up , it starts ?
This Cayenne is before Porsche added battery registration to the vehicles. The dealers are correct in saying they don't do it - because they can't do it. It's not there.Just a guess that a New battery should have been registered with cars brain otherwise it may still think the battery is old. This is a dealer programming item.
Since your Cayenne is a 2004, it's possible that you have two batteries in the car. One (primary) under the drivers seat, the other under the spare tire in the rear. You may want to check if this is true. Some people have no idea they have a second battery, and when it goes bad symptoms such as you describe aren't uncommon.Any suggestions or tricks to try would be appreciated.
Turn off the others and turn them on several times to see if that happens.I think I would start by looking at the keyless entry issues. That seems to be ,from what you have said, the main thing that is playing up. Battery isolator ? That will prove the battery is good if every time you connect it up , it starts ?
Remember to pay extra attention to the matching problem of replacement parts.My 04 CTT has an ongoing battery drain issue that seems somewhat inconsistent in how frequently it occurs.
The battery isn't old (replaced late 2021) and is a correct spec AGM. If charged it can last up to a few weeks or a few days before going completely flat and not starting the car.
Some noteworthy items that could be related are;
-Headlight wiring degredation (I've ordered replacements) although doesn't look like there's any metal on metal contact between wires
-Keyless entry inconsistencies - buttons on drivers side don't work but do on passenger side. Also the drivers door sensor doesn't activate until you pull on the handle
-Aftermarket Alpine head unit - This looks like a quality installation but maybe something is awry?
Underfloor wiring is a possibility although I have seen no signs of dampness and there are no weird electrical gremlins to speak of, other than the parasitic drain itself.
I will get the battery tested to verify it is still ok although I feel like given the reputation of these cars it will be a drain and not a battery that has died in short order.
Any suggestions or tricks to try would be appreciated.