<<BACK TO HOME

Tesla’s electric-powered Roadster targets supercar owners with a conscience. Its battery performance is everything, reports Tristan Honeywill

In places like London, Frankfurt, Dubai, LA and Moscow you see a lot of sportscars out of their natural habitat. Bankers, oil magnates and the super-rich like to drive fast, exclusive cars around cities. People stare, point, take photos. The car is the real celebrity and there’s always an audience as they race away from the lights.

Straight into the next queue of traffic, watching all the time the temperature gauge rise on their Murciélago, F430, DB9 or GT. Waiting. There’s relief when the fans do start when they’re supposed to and then regret that they are louder than the idling engine. Sometimes there is guilt: such machines are not built for this sort of life.

Many just don’t care, but those who do might fit the customer profile for a Tesla Roadster. Because it’s electric, there’s no highly strung engine to torture in traffic jams. It’s guilt-free sportscar ownership. And, for those rare few who care about their contribution to the greenhouse effect, they can buy the electricity from a renewable source.

Tesla has delivered its first 17 cars to US customers. While the firm’s engineers are ramping up production, Daryl Siry is in charge of creating a dealer and servicing network in Europe for half the 2,000 cars a year it aims to produce.

For €99,000 you can buy one of the first 250 special-edition models, capable of 0-100km/h in 3.9 seconds. The silence is a little disconcerting when you first start it up. It lacks the soundtrack and sense of occasion that a well-tuned exhaust gives, but the instant 380Nm of torque and resulting thrust provide exhilarating acceleration. An hour’s drive around town proved it has plenty of go and grip. The 360km or so range would not last many hours on the open road, but you could probably keep pace with many supercars and be able to talk to your passenger without shouting.

It has a good chassis: the tub of bonded extruded aluminium is Lotus technology. But not just a carry-over from the Lotus Elise, says Siry: “We have a 450kg battery pack behind the passenger cell. The greater mass means all the structure must change.”

The battery, motor and power electronics are all Tesla-designed. “We contract manufacture the power electronics module,” says Siry. “We assemble the battery pack ourselves in California and our factory in Taiwan makes the motor.” Tesla has also designed the car’s single reduction transmission.

“With no engine, there’s no vacuum to run the servo for the brakes, so electric pumps are used,” says Siry. “You can’t use waste heat from the engine, so we have a heater coil that’s similar in principle to a hairdryer. You can’t use belt drives for the air-conditioning compressor. The small things add up to quite a design challenge.”

Tesla’s most important intellectual property is the 53kWh battery pack. Custom automotive cells are expensive and Tesla gets around the problem by using what is just a commodity in electronics.

“The industry’s already made more than 1.2 billion small lithium-cobalt cells,” says Siry. “They have enough power and energy density to give our battery pack the right performance.”

Tesla has patents covering the cells’ assembly, connections and control.

Thermal management is the big worry for lithium-ion cells: extremes of hot and cold cause the cells to degrade more quickly. The battery pack is liquid cooled to balance temperature throughout its 11 sheets of 621 cells. Each sheet has its own ECU monitoring and sensors in the pack detect pressure, smoke, inversion, moisture and acceleration.

There is no “best” battery chemistry. All have trade-offs between energy which gives you range, power which gives you acceleration, and cycle life which gives you longevity. Tesla’s is designed for range and power. “Around 500 cycles will get you about 160,000km. The iron-phosphate batteries often used in hybrids would last longer but would have less range on each charge,” says Siry.

Recharge times are more or less fixed, because domestic electricity can only supply a certain number of volts and amps. With a common 240V, 12A electrical supply, you can get 2,880W of power. Filling a 53kWh battery then takes 18 hours.

Tesla says most customers will fit a 32A circuit to their garage, similar to the one fitted in kitchens. A recharge will then take just under seven hours. It’s not everywhere that you can fit a 63A circuit, but it’s becoming clearer why OEMs need to talk to electricity suppliers about this.

By May, Tesla hopes to produce 40 Roadsters a week. It’s hard to say how closely matched supply and demand will be.