Acid test

UK engineers are poised to road test a lead acid battery designed to make hybrid electric vehicles more affordable for the mass market.

Within the next few weeks the battery will be fitted to a Honda Insight hybrid car and put through its paces at the Millbrook Proving Grounds in Bedfordshire.

The planned 50,000-mile test is the culmination of a three-yearproject, Rholab, to adapt traditional lead acid technology for the hybrid engine era.

Rholab hopes to demonstrate that valve-regulated lead acid battery can be a viable alternative to the more expensive nickel-metal hydride (NiMH) power packs used in the first generation of HEVs such as the Insight and the Toyota Prius.

Conventional lead acid batteries are poorly equipped for the unique demands of hybrid cars, prompting the Rholab consortium’s bid to overcome key technical barriers and create a viable alternative.

This included developing a modified power cell and chemistry, and designing a battery able to interface with the complex electronic systems needed to manage a hybrid car (see sidebar).

Allan Cooper of the European Advanced Lead Acid Battery Consortium, project co-ordinator for Rholab, said: ‘We are confident that the battery will run the car. The question is whether we get the life out of it.’

The team hopes to run the Insight for 50,000 miles, subjecting it to a range of conditions including urban, high speed and hill climbing.

Cooper said the project partners would be ‘cracking open the champagne’ if the car went the full distance. ‘That would be a very good start. It represents about five years’ worth of standard driving.’

He claimed NiMH batteries can be up to six times more expensive than lead acid power packs. ‘If we can go down the lead acid route it could be significant step towards making HEVs more widely affordable,’ he said.

The other Rholab partners are Hawker Batteries, vehicle electronics specialist Provector and the universities of Warwick and Sheffield. The project operated as part of the UK Foresight Vehicle Programme, which helps R&D into new automotive technologies.

Even if the Rholab battery can run the Honda hybrid, Cooper admitted there would be some way to go before convincing the car industry to take the lead acid route.A follow-on project, called Isolab, is already underway to build on the Rholab developments.One particularly tricky issue facing lead acid is likely to be weight – currently a major bugbear for the automotive industry.

Although the Insight Rholab battery takes up a roughly equivalent space in the car it is heavier, adding about 4.4 per cent to the overall vehicle weight.

Cooper said it would be possible to shed a significant amount of the excess weight in a production version of the battery. ‘This has to be seen very much as a prototype,’ he said.He conceded, however, that it was unlikely to reach weight equivalence with nickel-metal batteries.

Sidebar: How the 19th cell comes into play

The 144V Rholab battery pack was designed to meet the unique and complex demands of hybrid electric vehicles.

To operate as the auxiliary power source in an HEV, a battery has to exist in a partially charged state and to be able to deliver or receive high current pulses according to need.

Under these operating conditions a lead acid battery’s capacity can quickly decline due to an effect known as negative plate sulphation.

The various Rholab partners have developed a range of solutions to these and other potential limitations of the lead acid battery.

A major focus has been the individual battery cell. Rholab uses a new, spirally wound, dual-tab cell. This has a current take-off for both the positive and negative plates at each end, helping to get high power in and out of the cell quickly and improving its life cycle.

To help overcome negative plate sulphation, as well as changing the negative plate’s chemistry, Rholab regularly brings each cell up to a full state of charge. This means temporarily removing one cell from service. To this end Rholab uses 19 rather than 18 cells in each 36V battery module. At any one time the ‘spare’ 19th cell will be undergoing conditioning, or can even be removed from service altogether in the event of failure.

This degree of monitoring and control of individual cells requires a sophisticated battery management system (BMS). Within Rholab, Provector, with the help of Warwick Manufacturing Group, has designed a BMS that fulfils these requirements without incurring prohibitively high production costs.