EV regenerative braking and the essential role of resistors

01 November 2011

The blunt truth is that the all-electric family car is still a long way away. However, an affordable, practical and desirable hybrid family vehicle isn’t quite so distant, thanks to developments in battery, braking and resistor technologies. Peter Duncan focuses on regenerative braking and the role that resistors must play in this technology

The crux of the electric vehicle (EV) issue is deciding what is both affordable and practical in light of the engineering compromise forced on vehicle designers by the cost, size and weight of batteries. It’s the battery and the necessary stations for re-charging it, or simply exchanging it for a charged unit, that mean widespread adoption of the all-electric car is unlikely to happen soon.

However, an affordable hybrid with a battery that has sufficient capacity to be recognised as useful by even the most ardent Jeremy Clarkson fan is realistic. A battery of reasonable size, complemented by a small diesel engine operating at peak efficiency and a regenerative braking unit is all that is required. Furthermore, this combination would add value to the car by decreasing running costs and reducing carbon emissions, providing that the majority of the power added at the point of charging was from a renewable source.

The vehicle would use the battery’s capacity to smooth out the peaks and troughs in demand, meaning that the diesel engine only has to be sufficiently powerful to provide for the average power requirements, not the peak. Regenerative braking would make the process even more efficient.

However, the common misconception is that it’s possible, in a regenerative braking system, to use or store all of the electricity produced in the battery. This isn’t the case; most existing electrical cars use mechanical brakes because practical batteries can’t be charged at the power generated during braking. The answer is a power resistor that is compact enough to fit in a small vehicle, coupled with a cooling system that would make using the resistor practical. This would allow the excess power to be dissipated safely as heat.

It’s for this reason that Cressall has developed its EV series of liquid cooled resistors for the braking systems of hybrid and all-electric vehicles. Manufactured using advanced materials to achieve reliability and low weight, the units are among the first working examples that are compact and light enough for use in an electric car. They have already had two years of R&D time devoted to them and will be the subject of two patents for the technology they contain.

The EV series employs a system that totally separates the resistor elements from the coolant. Their simple, modular construction means the resistors can be combined to handle any power inputs from 10kW upwards and voltages from 12V to 6kV. The coolant pressure is up to 3bar and the coolant liquid can be water or water glycol, meaning that the resistors are suitable for the most common types of liquid cooling.

So, next time you hear Jeremy Clarkson claim that a usable electric or hybrid vehicle is a pipe dream because of low battery capacity, think of the power that will eventually be produced by braking. The small amounts of electricity being generated in every one of the thousands of batteries in thousands of cars slowing down or coming to a halt millions of times a day across the globe will eventually add up and prove him wrong.

Peter Duncan is managing director of Cressall Resistors