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Electric vehicles: one circuit, many consumers

Battery, motor and cabin want different temperatures. Once they share a circuit, the design becomes a compromise rather than an optimisation.

In a combustion vehicle waste heat is plentiful and cabin heating is free. In an electric vehicle it is not: because the efficiency is high there is little waste heat, and cabin heating comes straight off the range.

That turns thermal management from a comfort subject into a range subject.

Three consumers, three different targets

The battery. Wants a narrow temperature band — capacity and charge acceptance fall when it is too cold, ageing accelerates when it is too hot. The spread between cells must also be small. See the battery thermal note.

Power electronics and the motor. Tolerate higher temperatures and can run on a different circuit from the battery.

The cabin. User comfort; heating is the largest item.

Why heat pumps became common

A resistive heater gives one watt of heat per watt of electricity. A heat pump draws heat from the outside air and delivers several times that for the same electricity, markedly reducing the winter range penalty.

The price is complexity: depending on conditions, the same circuit has to heat the battery, cool it, move motor waste heat to the cabin, or pre-condition the battery for fast charging. The circuit now means several valves and several modes.

The job of the analysis: balance over a cycle

A single operating point is meaningless here. The critical questions are asked over a cycle:

  • In cold weather, how much energy and time does pre-conditioning the battery before fast charging take?
  • If the motor and battery both demand cooling on a climb, is the circuit sufficient?
  • When cabin heating and battery cooling coincide, which has priority?

These are transient, conjugate system analyses; component-scale CFD feeds that system model — heat exchanger characteristics, cold plate resistance, outdoor unit siting. See the heat pump outdoor unit note.

References

  1. Bernardi, D., Pawlikowski, E. & Newman, J. (1985). A general energy balance for battery systems. Journal of the Electrochemical Society, 132(1), 5–12. DOI: 10.1149/1.2113792
  2. Shah, R. K. & Sekulić, D. P. (2003). Fundamentals of Heat Exchanger Design. John Wiley & Sons. DOI: 10.1002/9780470172605

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