I don't think Tesla can invalidate the warranty on the battery simply because it was charged to 100% all the time and stored in a hot location.
Here is what Elon Musk had to say about it in a blog post:
(I added the bold.)
Creating the World’s Best Service and Warranty Program
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Battery Warranty
The battery pack in your car is obviously very important and expensive to replace. In developing the Model S, we took great care to ensure that the battery would protect itself, always retaining a few percent of energy.
If something goes wrong, it is therefore our fault, not yours.
Except in the cases of a collision, opening of the battery pack by non-Tesla personnel or intentional abuse (lighting the pack on fire with a blowtorch is not covered!), all damage is covered by warranty, including improper maintenance or unintentionally leaving the pack at a low state of charge for years on end. The battery will be replaced at no cost by a factory reconditioned unit with an energy capacity equal to or better than the original pack before the failure occurred.
The intent is to provide complete peace of mind about owning your Model S
even if you never read or followed the instructions in the manual.
Elon