• • • ELECTRIC VEHICLES • • • Securing EV power systems
Simone Bruckner, managing director of dynamic braking resistor (DBR) manufacturer Cressall, explains the role of resistors in supporting the e-mobility revolution
D
evelopments in the electric vehicle (EV) market are advancing at a rapid pace, with consumer uptake on the increase and the
end of the road for newly-manufactured ICE vehicles fast approaching. But concerns around EVs’ viability for widespread usage remain. A DBR is a vital component in the EV power
chain, responsible for safeguarding the electrical system by removing excess energy from a vehicle’s braking system. This mechanism supports several applications in the EV space — regenerative braking, fuel cell power reliability and heavy goods vehicle emergency braking systems.
The regeneration game In EVs, a DBR’s ability to absorb and redirect energy facilitates regenerative braking. Regenerative braking uses the excess kinetic energy to recharge an EV’s battery. It is able to do this because the electric motor in an EV can run in two directions: one, using the electrical energy, to drive the wheels and move the car, and the other, using the excess kinetic energy, to recharge the battery. When the driver lifts their foot off the accelerator
pedal and steps on the brake, the motor starts to resist the vehicle’s motion, “swapping direction”, and begins putting energy back into the battery. As a result, regenerative braking uses the EV’s motor as a generator to convert lost kinetic energy into stored energy in the battery.
On average, regenerative braking is between 60
and 70% efficient, which means that around two thirds of the kinetic energy lost during braking can be retained and stored in the EV battery and used later for acceleration, drastically improving the energy efficiency of the vehicle and extending battery life. However, regenerative braking cannot act alone.
A DBR is needed to make the process safe and effective. If the car battery is already full or there is a failure in the system, there’s nowhere for the excess energy to be dissipated, which risks the failure of the entire braking system. So, a DBR is installed to dissipate this excess energy unsuitable for regenerative braking and dissipate it safely as heat.
In water-cooled resistors, this heat heats water,
which can then be put to use elsewhere in the vehicle to provide heat to the cabin of the vehicle or preheat the battery itself, since a battery’s efficiency is directly linked to its working temperature.
Heavier loads It’s not just in general EV braking systems that DBRs are important. There’s also an added layer of their application when it comes to the braking systems of electric heavy goods vehicles (HGVs). Electrifying HGVs is certainly more complex than
standard vehicles, but it’s no less pressing. While widespread targets haven’t been set in the same way they have for the ban on fossil-fuelled cars and vans, some goals have begun to creep in. In the UK, all new HGVs sold must be zero emission by 2040, while in Austria, the target is 2035. But there’s no denying that more of these targets will appear in the not-too-distant future. HGVs brake differently to cars, as they do not
purely rely on their service brakes to slow down. Instead, they also use auxiliary or endurance braking systems, which slow the vehicle in conjunction with the service brakes. They don’t overheat as quickly on long declines and reduce the risk of brake fade or failure of the service brakes. In electric HGVs, this braking is regenerative, which minimises wear on the service brakes and adds charge and range to the battery packs. However, if there is a failure in the system, or
the battery pack’s state of charge is unable to accept the charge, this could become dangerous. Using a DBR will dissipate the excess energy as heat to improve the safety of the braking system. To pass this test, the resistor must allow the HGV to travel six kilometres at 30 kilometres per hour on a 7% decline with the endurance braking system active and without the service brakes overheating and failing — the current standard set by the ECE R13 Type – IIA endurance braking performance test.
A hydrogen future However, it’s not just in braking where DBRs come into play. We must also consider how they can positively impact the growing hydrogen fuel cell EV (FCEV) market. While FCEVs aren’t as viable for an imminent widespread rollout, the technology is there, and it certainly has more long-term promise. FCEVs are powered by proton exchange
membrane fuel cells. FCEVs turn hydrogen into electricity by combining the hydrogen fuel with air and pumping it into the fuel cell. Once inside the fuel cell, this triggers a chemical reaction,
28 ELECTRICAL ENGINEERING • NOVEMBER 2022
electricalengineeringmagazine.co.uk
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