Last updated 15-Aug-2021
Owners often comment about the indicated battery range not being what it was, that charging stops short of 100% even when set to 100% or that they have heard about rebalancing the batteries. This is all controlled by the battery management system (BMS) and is typically caused by the BMS being unable to recalibrate itself successfully. The other thing is a forced battery balance as the individual cells can drift and leaving will not correct. It's rare that it's actually a fault of the battery. We set out to explain what is happening and what you can do.
Calibration is required to help the BMS accurately read the status of the battery. If it only sees high states of charge, or low states of charge fleetingly (ie the car is driven to reduce the state of charge and then relatively quickly put on a charger) then it has to start assuming where zero is.
Cell balancing is essentially trying to get all the individual battery cells to the same level of charge. There are a number of guides on the subject about running the battery down low and then charging slowly to 100% to allow the cells to balance however we think these are now all outdated on the current cars.
The Tesla Battery Management System (BMS) is responsible for looking after the battery. As well as managing charging it also works out the available amount of energy stored in the battery and in turn the number of miles that energy can drive the car for. It does this by using an algorithm that adapts over time, periodically calibrating itself. In order to do the calibrate it needs a number of voltage measurements taken at different states of charge and when the car is in a known state. While this sounds easy, it is harder than you may imagine.
The challenges the BMS has are:
This process was relatively ok but is now taking longer for the following reasons:
The BMS needs a number of readings at different states of charge when the car has been left turned off for a period of time to allow for the voltage levels in the battery to stabilise. The steps are fairly simple and not required to be done that often.
The goal is to build up a number of occasions where the car totally asleep for 4-6 hours and across a variety of charge levels.
Over time the car should recalibrate itself using these reading and should correct any under reading of the available range plus you are giving the battery time to sort itself out and the various battery groups to stabilise between themselves.
While the battery cells will sort themselves out up to a point if the car is simply left, there can still be some residual imbalance in the cells. It used to be sugegsted that the cars was charged slowly from a slow state of charge to 100% to do so. We certainly think it is a waste of time using a rapid charger to charge to 100% because the car will slow the charging right down to a crawl.
You can contact Tesla but first it is worth doing a combination of the steps above. We would suggest: