Based on your description, I don't see any "issue" here. The car is acting normally. When you plug in at night, it'll charge up until the cells reach their configured charge level (90% or whatever you have it set at). Once it reaches this level, charging stops (let's say it's done at midnight). However the car has electronics that still run 24/7 that check on battery state and other various things (like your internet connectivity so u can control remotely). Electronics use power. You check the next morning at 7am. Well between midnight and 7am the car used a little power. Hence a few miles range lower.
The car isn't going to continuously recharge your car to stay at 90%. That would be harm long term battery life. Everything you describe is normal behavior.
What's your model. Mine is 60KWH...
I have been talking to Tesla service, they claim that the battery does continuously/periodic intervals charge. I.e. if the battery uses power to run the fans et al, it should recharge. that's why you can set the climate in the car while plugged in...so that you don't lose implied range. this could very well be a battery issue. Many posters have had their batteries swapped.
I am rather concerned about the behavior of the car while plugged in and with the low miles that I have on the car. My fear is that this issue may be amplified. I cannot make compromises or settle.
As for service comments, my concern is with the substantial lead time required to get service. I got a date 3-4 weeks later when I called to bring in my car for service. I am not sure about other posters here, but I am used to maximum lead time of a week Now this could very well be an isolated issue in the NE, considering the car is very popular in my area.
the responsiveness of the service team is excellent while the car is in the shop. I have to say that there has to be a wider culture of considering and managing exceptional issues as one sees them. The oft response I have heard while talking to folks at tesla about my issue is " the car should not be losing range/rated range while its plugged in". its almost incredulous, so now I have been taking screen shots from my app to track it...I feel like a beta tester for the car.
Bottomline... I love my car and would not trade it for anything else ... except maybe a p85...
- - - Updated - - -
FYI. here is an interesting corollary
when you leave your laptop plugged in to charge/ or using it while plugged in... Do you ever see it go from 100% to 95%. I am no battery expert but my understanding that the battery tech in the car is similar to modern laptop batteries... so I do not understand "implied" loss of charge while plugged in...
