A common concern for Tesla owners is how to calculate the battery degradation or establish the health of the battery as it's not that easy. This is a guide to talk you through how to take the numbers the Tesla is reporting, turn these into a max usable kwh capacity information for the car and then compare these to the figures you should expect to see for a new battery.
The following is best done at the end of a drive when the battery is warm and factors such as vampire drain and cooling of the battery have not had chance to influence the results.
It is also worth noting that day to day, the battery management system can drift depending on how the car is used with respect to charging. This drift is fairly negligible on a Model s and Model X but can be more significant on a Model 3 and Model Y. Check out our Guide to Tesla BMS calibration to the nature of the issue and how it can be resolved if you suspect it is needed.
To calculate the available battery capacity you need three pieces of information.
The projected range (ProjRange) taken from the power consumption display. In this case the car is showing a range of 285 miles. Kilometers works just the same way as the average consumption rate will also be based on km if selected. You also want to make sure the car is showing the average and not instantaneous value.
The average consumption rate (AvgCons) that is also displayed on the power consumption screen. In the example we have an average consumption rate of 258 and will be in Watts for either a mile or km depending on what is selected in the car.
The current % state of charge (%Chg) taken from either the display in front of the driver on an MS and MX, or at the top of the screen on an M3. In the example we have 60%.
Combining these three pieces of information will give you two pieces of information.
To calculate the amount of available energy in the car it is a simple matter of multiplying the ProjRange by the AvgCons:
e.g. 285 miles X 258wh/m = 73,530wh or 73.5kwh
To calculate the user available capacity of the battery you need to scale this up to 100% by multiplying by 100 and dividing by the %Chg:
e.g. 75.5 x 100 / 60 = 125.8 kwh using the example screen shots. In practice, no current car will have this level of capacity.
The example shown is just an example of the calculations and does not reflect any current Tesla.
The calculated capacity is an approximation of the maximum the Battery Management System thinks is available for owner to use. Don't immediately worry if this is much less than you expected as this excludes the buffer reserved in the battery for protection (a buffer drivers can't access so don't assume you can drive below zero miles to use this) and the battery sizes have rarely been the size stated by Tesla, e.g Tesla still maintain an 85D has an 85kwh battery when on practice it has a 81.5kwh battery and a usable 77.5kwh.
Once you've worked out the figure above, you can compare it to the usable capacity figures in the table below. The figures below are from various sources on the internet and are the theoretical limits, sometimes worked out by reverse engineering the battery management system and sometimes by tearing down the battery packs. In practice, and for a number of reasons, you can expect your car to be below these figures with 5% and more quite normal, even when new as explained below.
Tesla have applied a model 3 warranty limit of 70% retained/30% degradation after 8 years/120k miles/180km to be acceptable. They have not stated a figure for the MS or MX. Superficially it's therefore not unreasonable to assume a 10-15% degradation after 4 years is within spec, however degradation is not linear. Reports suggest there is an initial drop within the first few months and then the rate of degeneration tails off. A typical scenario is therefore a 5% drop fairly quickly and then a further 5% drop over the life of the car. If your car has between 5% and 10% degradation, a higher figure on older cars, it's quite normal. If the figure is above that, for instance the car is an 85 and have been hit by #batterygate, then react accordingly if buying or if you are an owner then consider challenging Tesla and get the issue logged if in warranty.
Please email us if you find figures that are significantly different using the approach as this guide is provided based on community sourced information.
The above calculates the available battery capacity. Range however will vary quite considerably over the course of the year due to a number of factors, most noticeably from a seasonable perspective, due to weather. When the temperature starts to fall below 10 deg c/50f there is increased demand to heat the cabin and battery, both through heaters and higher internal resistance of the battery, which consumes significantly more energy at the beginning of a journey. This is normal.