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. 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.
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.
The above guide covers the activities required to help calibrate the BMS. There is a second battery topic that is often discussed which is cell balancing which 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the battery down low and then charging slowly to 100% to allow the cells to balance however we think these are now all outdated on the current cars.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 individual battery cells are connected and simply leaving the car alone will allow the voltages to equalise, it is actually the foundation step before the BMS can calibrate.
Charging the battery from a low state of charge to 100% will be partially successful in maximising 100% range as the cells will have time time to equalise their voltages as the charging takes place. Secondly, if the batteries have not balanced, 100% is when the first banks of batteries are reporting the required voltage, but not all. The charging will stop, the voltages will then naturally balance over time and the state of charge will drop slightly across the battery pack (when cells balance, some voltages go up and some voltages come down). The charger could then top up the missing bit again assuming the drop is sufficient to re trigger the charger.
The 100% here is relatively arbitrary. The concept of a 'full' battery being like a full petrol tank isn't accurate. A full battery is one that has reached a design parameter voltage. You could decide you want to be safer (and kinder) to the battery and set the level to be slightly lower (which is why charging to 90% is often recommended), or push the battery harder and set the voltage to be higher. A better analogy may be an elastic band where the amount of stretch is equivalent to the voltage and relates to the stored energy. You can stretch it further to store more energy but eventually it will break. Therefore, charging to 100% does not really help the cells balance any more than charging to 90%, they are just different voltage levels. The BMS is more sophisticated but the principles outlined are the basics, either way, BMS calibration aims to help the the car accurately predict the battery capacity over the full range of states of charge, and cell balancing aims to let all the individual battery cells be at the same voltage.