Tesla battery management system (BMS) calibration

Last updated 10-Apr-2024

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 still relatively unusual to actually have a faulty battery, and the guide we set out below will give you some techniques to restore some of the lost indicated range.

It is, however, 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 and there will be some small loses in capacity through natural and reasonable degradation. The value should however be reasonably close and generally no mode than 5-10%.

For countries where the car is rated using the WLTP standards, it is also worth noting that the car displays the range using the EPA standard which can differ more significantly. This is one of the most common queries from new owners where their factory fresh car does not show the range the spec sheet shows, however this entirely normal and correct. Don't forget, the battery stores energy, not miles, and the mileage figure is based on a conversion factor applied by the car i.e. the car assumes it can do 4 miles per kwh. The petrol equivalent would be to show how many gallons of petrol are in a car using the US definition of a gallon and not the imperial gallon resulting in a different number, in practice the car holds the same amount of energy/petrol.

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.

How do I know if I have a high state of degradation?

The most obvious way is if the range at 100% has significantly reduced from previous values. This is one advantage of shows miles/km rather than %, because % is always a fraction of what the battery could hold, even if that amount has reduced. You can also use our battery capacity checkers which will allow you to calculate the battery capacity. You can either manually calculate the battery capacity which works for most cars, but you will need to know what the normal value would be for your battery, or you can automatically calculate the battery capacity via the API if you are happy to proviude your Tesla token. We will also benchmark your car against many other cars.

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

Firstly, don't panic. If there is a genuine fault with your battery the car will typically be giving you a warning message. That said, you probably still want to recover that lost capacity and make it useable. So 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.

Advanced level only! Checking the health of the battery

Within the car there is a hidden service menu. One of the options here to perform a battery health test. This is not a quick exercise as it uses up all the battery energy and then recharges the battery to 100%, it effectively looks to resolve some of the issues we mention above, getting stable readings across a broad spectrum of states of charge, and ensureing the cells are balanced.

We've elected to not cover this in detail as entering the hidden service menu can cause problems with the car if you do not know what you are doing, and the actions take a long time to complete. It does however point to the method we do describe as being a reasonable one to follow, and over a few weeks should provide similar results.

Car range accuracy and other factors

One of the common complaints is the displayed range at thee top of the screen doesn;t reflect the stated range on the website, as a consequence many do not trust it. The reality is people tend to just not understand what its saying.

  • The range displayed is the available battery energy converted to a range figure using the EPA test cycles results on how much energy is required to travel 1 mile (or km). In theory, at 100% state of charge the figure shown should match the EPA rating.
  • The range on the website may be based on the WLTP rating system as used in Europe. This will yield a different figure on the website to that in the car, the figure in the car will be lower.
  • The battery will lose some capacity fairly quickly before levelling off. The range will therefore drop slightly by up to 5% fairly quickly.
  • As mentioned elsewhere in this article the BMS calibration may be off or the cells may require balancing which can add further to the drop.
  • However, results using the real world range reqported by the cars over the last 30 miles and the rated range displayed in the car, the long term average is remarkably accurate. Standard range M3 and MY show almost 100% accuracy over many data points across many cars, and the Long range and performance cars about 95% accuracy. Of course your use of the car may result in better or worse than indicated, but as a guide, the range displayed on averag is pretty much achievable in the real world.

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