The charge of the light brigade

Tony Newton reports on the belated arrival of a battery suitable for po rtable products

On Christmas Day, millions of people will tear open wrapping paper to reveal new mobile phones, video cameras and laptop computers. And then the anguished cry will go up, "We forgot to buy the batteries".

Battery technology is now the main constraint in the miniaturisation of most electronics products. Over the past few years, there has been a steady reduction in the size of computers, mobile phones and a host of other products; the batteries are now the problem in terms of size, weight and useful life.

The expense, inconvenience and environmental cost of using "primary" (non-rechargeable) batteries in these products is such that they are simply not an option, with manufacturers having to use the imperfect Nickel Cadmium (NiCad) technology for want of anything better.

A jump-start in battery chemistry is long overdue, but finding a safe, reliable, cheap and environmentally friendly alter- native to the NiCad has not been easy.

The oil crisis of the Seventies spurred interest in battery technology, but while American eyes were turned to the development of the electric car, the Japanese had foreseen the needs of the consumer market. Now Sony has launched a new rechargeable lithium battery (Energytec) that uses British patent-protected technology.

Lithium batteries have been on the market for some time. However, a major problem encountered when trying to design a rechargeable one was the tendency of lithium metal (used as one of the internal electrodes in early rechargeable designs) to react violently when in contact with moisture. After all, a battery that bursts into flames when ruptured represents something of a drawback for consumer use.

The new lithium battery overcomes this hurdle by replacing the hazardous metal with a specially designed carbon electrode, to construct what electrochemists call a "rocking chair" cell because of the way in which ions are pulled back and forth between electrodes during the charging and discharging process.

According to Professor Peter Bruce, who is responsible for much of the research at the Centre for Electrochemical and Materials Sciences (Cems) at St Andrews University, the new battery has several significant advantages over current NiCad technology.

The latest technology avoids the dreaded "memory effect" because it allows a partially discharged lithium battery to be "topped up" to full charge at any time, whatever its condition.

The battery also has a "sloping voltage profile": unlike NiCads, which tend to go flat with very little notice (in the middle of a phone call, or before you've saved your latest article to disk), the lithium battery goes through a gradual drop in output - not enough to affect the functioning of the appliance, but enough to be monitored. In practice, this means that the next generation of appliances and chargers will incorporate indicators to show remaining battery capacity at a glance.

Another significant benefit of the lithium battery is its power-to-weight ratio.

Those of us who lug around portable computers and mobile phones, and earn a living just to pay the chiropractor's fees, will welcome the fact that a "D" size lithium battery weighs nearly 40 grams less than its NiCad sibling, but has three times the operating capacity.

In practical terms, that gives one 122g lithium battery the same power output as three NiCad batteries weighing in at a total of 480g.

The lithium battery is also rather more "green" than the NiCad. Although the cobalt used in the first-generation rechargeable lithium battery is more toxic and expensive than the ideal, it is far less toxic than the cadmium used in NiCads. As most consumers simply throw out dead batteries with the household rubbish, such issues are likely to increase in importance as rechargeable appliances proliferate.

However, even as the first-generation lithium battery appears on the market, a second-generation device is already on the stocks at St Andrews University.

"What we're doing is to replace the cobalt, which is expensive and toxic, with a manganese-based system, which is very much cheaper and is already broadly familiar to the battery industry," says Professor Bruce. "Future generations of lithium battery will have a better ability to store charge - perhaps even double the capacity - and will have even less tendency to self-discharge."

This next generation will still be filled with liquid and have the conventional shape. However, Professor Bruce and his colleagues are also examining ways in which batteries can be constructed from solid polymers. This would open the way for batteries that are mass-produced as a thin laminate, which can be folded to give a lot of active material for a very small weight and volume.

Among other things, that could mean smaller and lighter mobile phones and computers, with a vastly extended battery life.

But what of the "holy grail" of battery technology, the electric vehicle?

According to Professor Bruce, the electric vehicle is "the hardest application to design for", but California has forced the pace by legislating that 2 per cent of vehicles sold within the state must be electrically powered, a figure rising year on year.

Motor manufacturers recognise that pressure to develop a mainstream electric car which performs rather better than a milk float is only going to increase, and are actively courting lithium battery technology as the best bet to date for lightweight, chea

p and safe motive power.