Tesla battery management system (BMS) calibration

Last updated 06-May-2022

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 one of two issues:

It's rare to be a faulty battery. Before we go further, it's also worth noting that the range displayed on the screen is unlikely to ever show the rated range given by Tesla when charged to 100%. There are a couple of reasons for this, temperature will not be optimal, there will be some small loses in capacity, and for countries where the car is rated using the WLTP standards the car will differ as the values used are based on the EPA standards which are more pessimistic, i.e. for a given amount of available energy in the battery, the car will assume it can cover fewer miles. The range displayed is also based on an assumed efficiency and real world driving will yield different results, possibly further but more typically lower.

Calibration v cell balancing

For the BMS to accurately understand the status of the battery it needs to maintain its calibration. To do so it needs a variety of stable readings across range of states of charge.

To get a stable reading, the battery needs to be left for several hours without being used or charging, ideally without sentry mode running. If a car is typically recharged straight after a journey then the car will not provide a stable reading until the charging has finished. Consequently the car will have few, if any, readings at lower states of charge. As the BMS extrapolates through these readings it has, if there are few low down it will start to be cautious and assume a dropping capacity. If readings are however available at lower states of charge, the BMS will be able to know where both zero and 100% are more accurately. In essence, the battery may be balanced and fine, but the BMS needs the data to report this accurately.

Cell balancing is essentially trying to get all the individual battery cells to the same level of charge. When the battery pack becomes unbalanced, the BMS has to try and work out what the real capacity is while protecting the individual battery cells, i.e. keeping them all within their working range.

The fix for each of these problems is slightly different, and both may be needed if you feel your car has lost some of its expected range.

Tesla Battery Management System Calibration

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, constantly updating itself based on data points it collects from stable battery readings. In order for it to maintain an accurate calibration it needs accurate measurements taken at a variety of states of charge. While this sounds easy, it is harder than you may imagine if the car is always being either driven or being charged.

How to improve the battery management calibration

As said, the BMS needs a number of stable readings at different states of charge. To get a stable reading, the car needs to be left in it's sleep state for several hours. The following steps are fairly simple and are not required to be done that often to help the BMS achieve a good level of calibration.

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.

Cell balancing

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. To address this, the battery benefits from a 100% charge.

What to do if your car is showing a high state degradation.

Before contacting Tesla we suggest you follow a few simple steps to see if this is either a calibration or balancing issue:

We have seen up to 6% of the lost range being recovered this way. If you are still worried after doing both these things, then contact Tesla.

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