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Tesla’s Real-Time Battery Impedance Determination

Photo by Jp Valery on Unsplash

Tesla’s market cap has exploded in the last 3 months. Many credit Tesla’s meteoric rise to the company being more than just a vehicle company. In particular, their success is often credited to being at the forefront in battery technology.

Tesla’s patent application US2019/0283617, titled ‘Determining Battery DC Impedance’ relates to determining impedance (e.g. resistance) of the vehicle’s battery during driving. This information can be used to modify how the vehicle’s motor draws from the battery – for example, by limiting power or current. This patent application claims priority to other allowed patents, one of them dating back to 2009.

Without diving too deep, a relationship is derived between impedance and degradation while the batteries are charging. This relationship can be described as the battery’s state of health.

Then, during operation, a reference impedance is determined by a look-up table using state-of-charge and battery temperature. The reference impedance is applied (e.g., multiplication) to the previously derived state of health, to determine the operational impedance of the battery, at the current moment. This real-time effective impedance is then used to generate a ‘feed-forward’ parameter that can limit how much power/current is pulled from the battery.

Tesla’s method helps measure DC impedance in real-time (while in use). These values can be used to monitor the battery and control how the batteries are used by the motor, as the battery degrades over time. While not particularly sexy, Tesla’s battery management is a critical technical asset which puts them ahead of the competition.

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