Thanks A.J.Of course I have no idea as to where the battery will be in the CT. In other Teslas it is in the frunk - well behind the frunk actually. To get it out you have to remove several bits of plastic surrounding the frunk rim, then take out a dozen bolts and remove the frunk (its a tub). At that point the battery is pretty accessible held into place with typical hardware. There should be You Tube videos on how it's done.
As for longevity: as you may know there have been problems with short lifetime of the 12 V battery. Some owners have replaced the lead acid stock battery with a Li ion battery made by Ohmu. Quite expensive but the surprising thing is that the warranty is only 3 yrs - not that much better than what one gets out of a good lead acid.
I have seen some ICE cars where you had to remove a tire to replace the battery. Oh what fun.That's all that is required to swap the battery in the MX. Now as noted earlier you have to take out the frunk to get to the point where you can see those parts.
If you want to replace with the Ohmmu they certainly are.
LAN over power line is not the same as POE. Typically it digitally modulates a 2-50Mhz signal on the power line and then uses a high pass filter and demods it on the reciever end. There's quite a few standards for onewire, or comms over power in the microcontroller world which could be used for bus control over power in a vehicle.It is possible to send power and signals over the same conductors as is done with the Dallas One-wire system (actually the signal IS the power in this case) or in the case of the Enphase system. But I would like to point out that in POE there are 4 pairs (green, blue, orange and brown). One pair carries transmit data, anothe receive data and the third the actual power. I don't know what the 4th pair is for.
I used to have tons of battery problems on my motorcycle and wife’s scooter regularly and had to replace them frequently. It has been years since I switch to Li. I do not take care of these relatively cheap batteries at all and still zero problems. Highly recommend.