How to choose the right power resistors for electric vehicles

Simone Bruckner, managing director of Cressall Resistors

(Credit: Shutterstock)
(Credit: Shutterstock)

Most electric vehicles will use regenerative braking, which recuperates energy lost through braking and returns it to the battery to increase the range.

However, when the battery is full, the vehicle must rely on its mechanical friction braking system to slow down or stop. 

Instead, it would be more beneficial to use a resistor to discharge the excess energy. Not only would the resistor be useful in the event of an electrical fault, it would also reduce servicing costs owing to reduced wear on the friction braking system. 

The energy dissipated during dynamic braking into the resistor can also be used to heat the cabin or trailer, reducing the requirement for electrical heating, which places an extra load on the battery. 

Heating the cabin

The first step for automotive manufacturers looking to purchase a resistor is to analyse its purpose. If it is to be used for discharging excess energy produced during dynamic braking and heating the cabin, I would advise a liquid-cooled resistor. Discharged energy is transferred as heat to the liquid coolant, which can then be circulated to the vehicle’s heating system. 

This is an effective use of energy that would otherwise be lost. It also reduces the need for an electrical heating system that uses the battery’s energy and cuts the vehicle’s range. Based on an average estimation of the braking power needed for a car, a liquid-cooled resistor used for both applications will require a power rating of 7-10kW. Larger vehicles such as trucks or vans will require a higher rating. 

Pressure drop

In addition to ensuring a sufficient power rating, a vehicle manufacturer buying a liquid-cooled resistor must take pressure drop into account. Within the resistor, liquid flows between the inlet and outlet to remove excess heat. 

Often this flow will be through channels that may have additional obstacles inside. This causes the pressure of the pumped fluid to drop between the inlet and outlet and, if the pressure becomes too low, the flow will not be sustained. 

A trade-off exists between efficient cooling, which allows a resistor to have a higher power rating, and pressure drop. Resistors with a higher power rating will also experience higher power drop. 

For larger vehicles, which require the highest power rating, it may be possible to use an additional auxiliary pump. However, smaller vehicles are often limited in space and the pressure drop experienced by a resistor with a higher power rating might be unacceptable. 

Design aspects

The design of the vehicle and placement of resistor could also dictate the choices. 

Another consideration is the level of maintenance required. A liquid-cooled resistor such as the EV2 requires flowing coolant but has no moving parts. Servicing the coolant is as simple as avoiding contamination and ensuring the reservoir is filled.

Content published by Professional Engineering does not necessarily represent the views of the Institution of Mechanical Engineers.

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