Why Tesla Warms the Battery Before Fast Charging
In BMS engineering, we usually try to keep the battery at a moderate temperature, often around 25°C, to limit aging. Fast charging changes that trade-off. At high charging rates, raising the temperature can reduce lithium plating and help mitigate degradation.
The figure below shows how battery temperature and current change before and during six Tesla fast-charging sessions.
What the charging sessions show
The red curve is the highest module temperature, the orange curve is signed C-rate, and the gray dashed curve is outdoor temperature. Positive C-rate means current entering the battery. Yellow shading marks the reported heating state, purple the reported preparation state, and blue the DC-charging phase. The red dotted line marks 2C on the right-hand axis; it is not a temperature target.
In L4, the highest module temperature rises from about 31.5°C to 41.5°C before charging begins. In N2, it rises from about 27°C to 35.5°C. The temperature increases overlap with reported heating periods. Because they occur before DC charging starts, they cannot be explained by losses from DC charging.
Once charging begins, current increases sharply. In several sessions, particularly N2 and N3, current then decreases while battery temperature continues to rise. Peak current and peak temperature occur at different times. The temperature reached later in a session should therefore not be treated as the temperature required before charging can start.
Why heating can mitigate degradation
At high charging rates, lithium transport and the reaction that inserts lithium into graphite can become limiting. Lithium can then deposit as metal on the anode instead of being stored in the graphite. This lithium plating can cause irreversible capacity loss.
Raising temperature improves transport and reaction rates, which can reduce plating risk. However, higher temperature also accelerates side reactions. The benefit depends on controlling both the temperature and the time spent at that temperature.
Yang et al. demonstrated this balance by combining elevated-temperature fast charging with a short period of thermal exposure. Their experiments showed that this approach could improve fast-charge cycle life while limiting degradation from prolonged heating [1]. The research explains the physical benefit; the six sessions here illustrate charging behavior rather than directly measuring lifetime degradation.
From cell science to vehicle control
Putting this into a production vehicle is not easy. The cell supplier and OEM need to agree on temperature limits and lifetime requirements, and the vehicle needs to coordinate heating, charging current, and cooling.
In a patent filed in 2012, Tesla described raising battery temperature before fast charging to mitigate degradation [2]. Tesla later announced navigation-triggered battery warming for Supercharging in March 2019 [3].
These charging sessions show why the period before plugging in matters. Battery temperature is already changing before the charger supplies current, and it continues to evolve as current rises and tapers. Understanding fast charging means looking at that whole sequence.
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References
[1] X.-G. Yang et al., “Asymmetric temperature modulation for extreme fast charging of lithium-ion batteries,” Joule, vol. 3, no. 12, pp. 3002–3019, 2019. doi:10.1016/j.joule.2019.09.021.
[2] C. Dangler, S. G. Stewart, and C. H. Kishiyama, “Fast charge mode for extended trip,” U.S. Patent 9,728,990 B2, filed Oct. 31, 2012; granted Aug. 8, 2017. Patent text.
[3] Tesla, “Introducing V3 Supercharging,” Mar. 6, 2019. Tesla announcement.
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