I was thinking that if a battery was replaced by Tesla under warranty, it is warranted for the remainder of the 8 year warranty, or 90 days, whichever is longer.
"Repaired or Replaced Parts
Any Parts or Used Parts repaired or replaced under this Tesla Parts, Body & Paint Repair Limited
Warranty continue to be covered by the applicable original warranty period of the repaired or
replaced Part or Used Part (mentioned above in "Coverage Period") or for a period of 90 days,
whichever is longer."
If the owner bought a battery from Tesla out of warranty, it is covered for 4 years, 50K miles.
"Coverage Period - Parts
This Tesla Parts, Body & Paint Repair Limited Warranty begins on the
Part(s)’ purchase date and
provides coverage for the period of 12 months or 12,500 miles (20,000 km), whichever comes first,
with the exception that specific categories of parts have unique warranty coverage periods as
outlined in the table below:
Sheet Metal Limited Lifetime
Drive Unit* 4 years or 50,000 miles (80,000 km), whichever comes first
Vehicle High Voltage Battery* 4 years or 50,000 miles (80,000 km), whichever comes first"