The battery electric

Based on the previous Smart, more than 100 of these battery-electric prototypes have been built by Zytec in Fradley, Staffordshire, and are now being released on a four-year nationwide trial. The high-temperature, molten-salt battery is a natrium-nickel-chloride unit made by MES-DEA of Switzerland and mounted under the floor where the fuel tank sits in a conventional Smart. Its rated output is 15.5kW/hrs and it weighs 302lb. It runs permanently at temperatures of between 260 and 330 degrees centigrade and keeping the unit to that temperature takes between 10 and 15 per cent of the battery's capacity. Nevertheless, these so-called Zebra batteries are robust, with a proven long life and good resistance to overcharging.

The 30kW/41hp brushless, liquid-cooled DC motor is designed and built by Zytek, with the motor, the inverter and its controlling electronics in one housing that fits neatly onto the three standard engine mountings. A conventional transmission is retained, but it is locked into second gear; there's no need for more given the torque delivery characteristics and 12,000rpm peak operating speed of the electric motor.

The top speed is 75mph, 0-37mph takes 5.7sec (the same as the petrol version) and while the typical range in everyday use will be about 50 miles, gentle driving can extend that to 70. The key to this is gentle acceleration and braking, as the car will recycle the braking energy as current if you don't hit the anchors too hard. The battery can be recharged via a conventional 220/240 volt domestic socket - it takes eight hours for a full charge from empty, three-and-a-half hours to charge it from 30 to 80 per cent of its capacity. On the European Combined cycle, power consumption is about 12kW/hrs per 62 miles and at current values that costs about 0.02 euros per kilometre (2.6 pence per mile). The battery can be recharged at least 1,000 times, which in normal use gives a life of about 10 years.

Inside, the battery Smart is almost identical to its petrol sister; there's no gear lever, of course, and there is a charge-level indicator in the middle of the instrument binnacle, but that's about it. Starting takes no more than five seconds after a systems check, then it's a simple case of push the pedal and go.

Initial acceleration is scorching as the traction motor delivers all its 88.5lb ft of torque to the rear wheels from virtually zero revs. It sounds pathetic to say so, but the electric Smart surges off so quickly it's quite difficult to manoeuvre in tight places, so you need to learn to cover the brake with your left foot. In common with other electric cars, you also need to learn to use the horn, as pedestrians simply don't hear the little car coming. There is some noise from the driveline in the form of a high-pitched whine, but it is far quieter than some hydrogen fuel-cell cars I have driven.

The standard Smart is hardly the acme of ride quality and with an additional 220lb (110kg) on board, the electric version is possibly worse, though not by much. On smooth roads this volt-powered two-seater is fine but the individual wheel suspension rates are high; it crashes into pot-holes and sleeping policeman are an experience best forgotten. The electrically powered air-conditioning system, in addition, is a joke. But in its natural urban environment, the electric Smart is a hoot. The limited top speed is never a problem and there's enough instant surge to go for gaps in the traffic. It's also reassuring that the Smart has a four-star crash safety rating.

Fans of pure battery-electric technology always seem to have a blind spot about what lies beyond the plug, in other words where the electricity comes from; most "electric" vehicles are actually coal, gas or nuclear powered. That said, the terms of the Smart trial insist that the cars be charged from a carbon-neutral or offset electricity supply.

Companies and organisations interested in test-driving one of the electric Smarts must apply to Mercedes-Benz in Milton Keynes; costs are currently running at £380 a month for the lease plus the electricity to charge it.