Focus

BEVs and HEVs are receiving considerable attention due to their high efficiency, low greenhouse gas emissions, and reduced reliance on fossil fuels. Nonetheless, extensive research and development is needed in battery technology in order to improve the service life, safety, reliability, maintenance, power and energy densities, and cost. Of these, the reliability, safety and longevity of the battery depend on their control strategy as batteries degrade according to their mode of operation during their lifetime.

The aging is accelerated by temperature stress, high power cycling, and over-charging/ discharging of the battery. To optimize battery operation, it is important to monitor and estimate its State-of-Health (SoH) and State-of-Charge (SoC). The SoH indicates the remaining useful life of the battery and provides a measure of the capacity of the battery that degrades with aging (it is analogous to the size of the fuel tank; except that it becomes smaller with time and usage); SoH is critically important as exceeding the maximum charge capacity of a battery can compromise its safety and result in explosions and fire hazards.

Fire incidents and range anxiety associated with Electric Vehicles (EVs) are evidence of the need for transformative research in terms of battery chemistry as well as condition monitoring. SoC is of immediate interest to drivers as it indicates the remaining energy in the battery and the travel distance achievable before recharging. However, currently these parameters cannot be directly measured for lack of sensors, and thus need to be simultaneously estimated.

Here therefore lies the importance of having accurate models that can be used along with robust filtering techniques for SoC/SoH estimation and thereby energy management. Current capability for SoC/SoH estimation allows for errors in the range of 7 to 9%. Our research targets an improvement in this figure to less than 2%.

The next five years will see a substantial increase in the number of BEVs, HEVs and other industrial applications using energy/power conversion. The emphasis for future designs is continuous improvement in component integration, power density and lifetime, factors that ultimately impact system cost and application uptake.

Power electronic converters are used to convert DC to AC or vice versa. New system concepts that have flourished during the past 20 years include the automotive sector, where HEVs have typical installed power electronic converters of the order of 50kW in high volumes. The expectation for future automotive drive systems is product lifetimes of 10 years and over, which is beyond the achievable lifetime of existing power converter topologies, in particular those employing electrolytic capacitors.

Further, electrolytic capacitors are bulky, accounting for around 30-50% of power electronic inverter volume requirements. To have greater impact on applications, it is desirable to integrate electric machines and their associated power electronics to minimise cost; for example integrated starter/alternator systems are now becoming common. However, while the new systems are compact, reliability of power electronics can be compromised by the arduous nature of the operating duties and thermal environment. Recent research has identified benefits in drive systems across all application sectors and power levels by progressing to a higher phase number from that of the traditional 3-phase systems.

In addition to a reduction in capacitor volume, machine power density can be increased by a factor of 4 to 10%, while the resulting design is highly conducive to high volume manufacture. These benefits could significantly change the form of future e-motors while improving lifetime, efficiency, consumed material and cost. This thrust aims to improve the power density of integrated e-motors as well as their reliability and longevity past the current 10 year expectation.