|For most applications the battery is typically replaced in a car when it can only hold 80% of it's original new capacity. |
But you could keep it for many years beyond that if you could tolerate the reduced range. Roundtrip charge efficiency remains the same but capacity drops up till cells start to internally short and cannot take a charge.
The BMS (cell management) in Tesla's car is very, very sophisticated.
The battery pack in Tesla's latest cars should last for close to twenty years and upwards of 500,000 miles if treated well by their owners.
Tesla will not reuse old cells.
They will be recycled.