Prodrive cuts battery-pack weight

A new approach to the configuration of DC-DC converters could help reduce the cost, weight and size of electric-vehicle battery packs.

The system was developed by a consortium led by technology consultancy Prodrive to address the spiralling costs of energy storage.

Its solution substantially increases the voltage available at the traction motors.

Electric motors require a wider speed range to improve efficiency.

Speed is proportional to voltage so electric vehicles and hybrids are progressing from 300 to 600V systems.

The increase in voltage has presented significant challenges for engineers because battery chemistry dictates the voltage per cell.

By doubling the voltage with the same battery chemistry, one must double the number of cells and incur massive cost, weight and packaging penalties.

More cells, or a switch to an alternative chemistry, may also exacerbate challenges such as thermal management, availability, environmental protection and end-of-life recycling.

A team of engineers from Prodrive, ManchesterUniversity, HILTech Developments, Sloan Electronics, International Transformers and NewcastleUniversity has developed a new type of bi-directional DC-DC converter that gives a 600V output from a 380V source.

The system maintains its output even when the source-voltage drops to as little as 200V.

It is claimed the key solution is a new topology that delivers up to 97 per cent efficiency and, unlike alternative solutions, is also cheap and compact.

The power-handling capability is also claimed to be scalable, with development units rated at 50kW for full hybrid applications.

‘DC-DC converters are widely used to drop battery-pack voltage, for example 300V, down to vehicle-system voltage of 13.8V, but we believe this is the first time that a substantial step the other way has been possible from a realistically priced and packaged technology,’ said Pete James, a technical specialist at Prodrive.

‘With a unit that is a quarter the size of a conventional converter, we can eliminate 20 per cent of the vehicle’s batteries and possibly more.’

James said the move to higher voltages is the EV equivalent of engine downsizing.

The technique, he said, is important for the reduction of emissions from internal combustion engines.

‘It’s expensive and inefficient to carry around the huge battery pack that you only need for maximum motor speeds,’ he said.

‘With our solution you don’t have to, yet maximum voltage is available whenever it is needed.’

Another aspect of the converter design is it constantly presents the full voltage required by the motor.

Usually the voltage of batteries (and fuel cells and supercapacitors) fluctuates with changing load.

James said the other benefits of the system are size, weight and cost savings.

‘A normal DC-DC converter would cost more than the saving in batteries but our unit potentially saves four times its own cost in batteries,’ he added.

Prodrive has worked in the past on improving the integration of powertrains.

James said the consultancy’s concentration has been on control systems and techniques for enhancing efficiency.

‘It’s all about managing the energy flow through the vehicle,’ he said.

‘There are so many efficiency pitfalls when moving energy between chemical, electrical and kinetic states.

‘Our whole-vehicle perspective means that we work well with specialist technology suppliers.

‘That’s the only way to get real value from innovation.’