Tesla roadster

Batteries struggle for greater power

Electric vehicles and devices such as Internet tablets need higher-capacity batteries.

Formula One drivers know all about the performance-sapping features of fuel load. Until the latest season started, race strategy was all about the fuel - how little could each driver run with to get enough speed to make the extra pit stops worthwhile? Fuel is heavy, so the less you carry, the faster you go.

Road vehicles are not nearly as sensitive to fuel weight as a Formula One car where every kilogram counts but it's going to be a big factor in the future of electric power.

The problem with electric vehicles, says Carnegie Mellon University researcher Jeremy Michalek, is that you are always towing the equivalent of a full tank of fuel with you. And what a tank of fuel.

The 55kWh battery in the Tesla Roadster, a sports car sold in the US, can sustain a driving range of 400km, roughly the same as a petrol-driven vehicle. But it weighs some 450kg - ten times more than the weight of a tank that could power a petrol-driven equivalent. The support for the fuel, which is always there, is one-third the total weight of the sports car. That does not include control electronics, structural reinforcement and cooling. Much of the bodywork is made from carbon fibre to save weight that has to be devoted to the battery.

'With a long-range electric vehicle, you have the problem of how long it takes to recharge the battery,' says Peter Miller, director of electrical and electronic engineering at Ricardo. 'You could drive 200 miles over a weekend but then find it takes nearly a day to recharge with a domestic 13A electric socket. To get a vehicle that has the same range as a petrol or diesel model is not viable for the foreseeable future. The debate in the industry is what are the best cost-performance tradeoffs to make. It's generally going to be better to sell a vehicle that is cheaper than to push the range up.

Miller adds: 'If you look at the distances people do drive, using European statistics, 78 per cent of daily driving during the week is less than 60 miles. For 120 miles it's 88 per cent. Doubling the range only captures another 10 per cent of journeys.'

Batteries for electric and hybrid vehicles are not just bulky but expensive. They cost $1,000 per kilowatt-hour of useful capacity today - a 15kWh battery, which would provide a driving range of 100km, on its own costs more than a Ford Fiesta.

Over time, costs should come down as volumes increase. The US Advanced Battery Consortium has set an ambitious cost target of $250 per kWh. 'A lot of those figures are reverse-engineered from what it needs to be to hit volume rather than a technological roadmap to get to those prices,' says Miller. 'The chemistries are by no means standardised. So the volume for any particular battery chemistry is not that high, which reduces the economies of scale.'

The Boston Consulting Group (BCG) reckons the consortium's figure will not be reached before 2020 - a price to the manufacturer of $400 to $500 is more likely.

Massimo Russo, a partner at BCG, says the cost of the battery in 2020 will still be a significant part of the purchase price of the car, even though the cost of a 15kW battery should drop to around $6,000. But if you try to scale up the battery to provide the range of a petrol-driven vehicle, costs become prohibitive. The extra mass demands even more energy so that the capacity has to scale faster than the desired driving range. To get into the 600km range, a battery-powered Ford Focus would need an energy source at least six times larger and that does not take into account extra support needed to mount the battery and a longer chassis to hold it.

Subsidies might help the progress of the electric vehicle but those are far from guaranteed unless they can demonstrate a benefit in CO2 emissions. A Royal Academy of Engineering report released in May claimed the typical CO2 emissions of a vehicle powered with electricity from a domestic supply would be slightly worse than a turbo diesel injection Volkswagen Polo. 'To have a major effect, the introduction of electric vehicles must be accompanied by an almost total decarbonisation of the electricity supply,' the report said.

New battery chemistries could provide significant leaps in storage capacity and improve the overall efficiency of electric vehicles. But progress is slow. The key to improved performance is surface area, as that largely controls how much charge a battery can hold and usefully release when it's needed. Researchers are looking at anything that can help: even the pesky, smelly green algae that needs to be cleaned off beaches when pollution causes them to bloom in huge numbers.

'If you take these algae and extract cellulose you get a cellulose with a unique nanostructure that is not found anywhere else,' says Albert Mihranyan of Uppsala University. 'It has a surface area of up to 100m2/g.' This is a hundred times more than ordinary paper pulp.

The team coated the algae paper with a conductive organic material and used that to build an experimental battery with a surprisingly fast charging time. Although the energy density is currently not much greater than a lead-acid battery, it is flexible and easy to make.

The conductivity of the electrodes also plays a key role. Angela Belcher's team at the Massachusetts Institute of Technology used metal-coated viruses and nanotubes to build percolating networks that are much more conductive than conventional designs. This may open the door to the use of battery chemistries that are not viable today because they aren't conductive enough.

The problem for the nascent electric-car industry is the amount of time it takes to convert promising research into useful, manufacturable, reliable products. Although commercial nickel metal-hydride batteries (NiMH) were shipping around five years after their chemistry was first explored, it took many more years for them to compete with their predecessor, the nickel-cadmium battery.

Battery life

'For the next five to ten years, the volume in the market is going to be lithium ion,' says Miller. 'The big problem with any new battery chemistry is how do they finance the scaling up of production? There are lots of potential chemistries out there but the expectations in automotive are for long life and to be able to deal with high stresses. In a mobile phone you may accept a life of a few years. If you are replacing a £10,000 vehicle battery every few years, you would get pretty upset.'

Any company providing a warranty would want to know that they would not be paying out time and again over the lifetime of a vehicle to replace the battery, and this will stretch out the time it will take to introduce new, potentially lighter or cheaper battery technologies. 'Ideally, you would want to test for ten years before you can be confident about providing a warranty,' says Miller.

Shai Agassi, the founder of Better Place, wants to replace the internal-combustion engine with batteries but said last year at the TED conference he wanted to do it without waiting for some super-battery technology to come along. But the high upfront cost remains a big stumbling block.

'Affordable is not a $40,000 sedan,' says Agassi. 'Convenient is not something you can drive for an hour and charge for eight.'

Agassi came up with the idea of renting batteries to consumers instead of selling them. Because it's cheaper to charge a battery than to buy the equivalent amount of fuel, especially in Europe, Better Place expects consumers to choose electric vehicles over petrol or diesel vehicles, even with the battery-rental cost taken into account. To overcome 'range anxiety', Better Place aims to build quick-change battery stations that 'look like a car wash', according to Agassi, that will recharge a car in about three minutes.

Better place charging

Renault has adopted the Better Place system to sell a 160km-range vehicle for roughly the same price as an equivalent diesel car: the purchase price does not need to include the battery because that will be rented to the consumer. The Fluence ZE that the French carmaker unveiled last month will be fitted with a 250kg, 22kWh battery. The basic Fluence chassis has had to be stretched to accommodate the battery - the ZE is 13cm longer than its fossil-fuelled equivalent. The boot is also smaller - at 300 litres it is about the same size as the smaller Clio and about 60 per cent the capacity of the conventional Fluence. The car weighs a total of 300kg more than the diesel-driven model.

A Renault spokesman said the cost of ownership will be similar to that of a diesel-engine model once the rental and government subsidies for low-emissions vehicles are taken into account.

Miller says companies may try many different business models to promote electric, pointing to the range of contracts that the mobile-phone market has used. 'We will probably see all the permutations being tried.'

As no-one is expecting a sudden breakthrough in battery technology, rental schemes such as these may be the only way to improve the chances of success for electric vehicles in the near term. And in electronics more widely, it is all about designing around the limitations of batteries rather than hoping new forms will arrive that rewrite the rules on how much electrical energy you can get into the same space.

Recent articles

Info Message

Our sites use cookies to support some functionality, and to collect anonymous user data.

Learn more about IET cookies and how to control them

Close