Resistors are found in many applications in standard combustion engine cars. Resistors in hybrid vehicles have additional applications in battery management (BMS) and dynamic braking systems.
A hybrid car uses two different energy sources to maximise efficiency. This usually means combining electrical energy stored in batteries, with the combustion engine (AutoExpress)
There are various types of hybrid vehicles. Some run on batteries, with the combustion engine only taking over when the batteries discharge. In others, battery power can be switched in for town driving, or to provide a backup for the combustion engine. The combustion engine recharges the batteries.
Resistors in BMS
In Hybrid vehicles, the purpose of the Battery Management System (BMS) is to maximise the performance of the battery pack, delivering reliable and safe operation over the lifetime of the vehicle.
The BMS controls input and output voltage between the battery pack and a multitude of powered elements across the vehicle. These include motor and fan drivers, heaters and driver information systems.
Resistor applications include –
- Inrush current protection
- Pulse resistors
- Sense resistors for current measurement and management
- Dump load resistors
The primary resistor application is in protection and safety cut-out applications. They protect the battery pack and powered devices from dynamic conditions. These include overload, short circuit, pulse and inrush current. Dump resistors divert excess power when the vehicle battery pack is full.
The resistor technology depends on the application. Thick film power resistors are a common choice for inrush, pulse and sense resistor applications as they are robust, relatively small and easy to install and replace. They are also lower cost than other power resistor technologies.
Dynamic Braking Resistor Applications
Hybrid Electric vehicles use a combination of conventional (hydraulic) braking and dynamic braking. In dynamic braking an electric traction motor is used as a generator. This regenerative braking process recovers some energy lost during braking and returns it to the battery. Alternatively, the energy is dissipated as heat.
Rheostatic systems convert the kinetic energy produced when slowing a motor to electrical energy. A resistor pack converts this electrical energy into heat. In Electric vehicles, the resistors are often water-cooled. The heated water then circulates within cabin heating systems.
Special Applications – Custom Resistors
High Volume automotive applications are a prime focus for the major resistor manufacturers. They produce a wide variety of resistors, qualified to demanding automotive specifications.
Specialist resistor manufacturers service lower volume requirements. They also support the prototyping and product-proving segment of the marketplace. Where a standard resistor device will not match the demands of an application (power, size, weight), custom resistor manufacturers deliver solutions.
The technology used in Formula One cars has filtered down to standard automotive applications for many years. This process is likely to continue with technology found in Formula E cars expected to find applications in road-going E-Vehicles. This will include active electronic components and passive components such as resistors.
Hence applications for resistors in Hybrid Vehicles should grow as the market matures and current leading-edge technology feeds through to the mass market. Thick Film and Wirewound power resistors will be the most common technologies. Wirewound will be the dominant technology in braking systems with Thick Film resistors more common in BMS applications.