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. Its rare that its actually a fault of the battery. We set out to explain what is happening and what you can do.
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:
As indicated above, the BMS needs a number of battery OC voltages, each requires the car to be left turned off for a period of time to allow for the voltage levels to stabilise, and across a variety of states of charge.
In the past it has been speculated that a long slow charge from near low to full would calibrate the BMS. We suspect that the slow charge is giving time for the cells to stabilise and the charging is resulting in a spread of State Of Charge (SOC) where the Open Circuit voltages can be taken. However this is not the only approach.
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.