Following on from its B-class F-cell fuel-cell car, Mercedes-Benz has introduced a fully electric A-class called the E-cell. The smaller car draws its electrical power from the grid rather than a fuel-cell, but some design principles are shared — notably, the mounting of the battery-pack inside the car’s ‘sandwich’ floor. Because of this, the electric A-class comes with no space penalty compared with the more conventional versions.
The A-class E-cell is Mercedes’ second electric car — after the B-class F-cell — to be built in what the Company describes as ‘series-production conditions’ — a dry run using real production tooling. A total of 500 electric A-class cars are set to emerge from Daimler’s Rastatt plant, with production beginning on 15 September. The cars will be leased to a variety of corporate and private customers in several European countries, including Germany, France and the Netherlands.
Mercedes-Benz A-class E-cell.
Two separate lithium-ion battery-packs are fitted, providing a combined energy capacity of 36kWh and a range of around 120 miles over the New European Driving Cycle.
A single permanent-field synchronous traction motor delivers peak outputs of 95PS and 290Nm to the front wheels; the continuous power output rating is 68PS. A kickdown function is used to deliver maximum acceleration, depending on the current charge level and battery temperature. Maximum speed is electronically set at 95mph.
A thermal management system looks after the high-voltage battery-packs: lithium-ion batteries have a relatively narrow optimum temperature range of around 10°-40°C. The battery-packs are cooled by a low-temperature liquid cooling circuit using a water-glycol mixture. In very high ambient temperature conditions, the battery cooling system is boosted with the coolant circuit of the air-conditioning system.
The electrical drive system and two on-board chargers are cooled with a high-temperature cooling circuit. The car’s power electronics supply the 12-volt vehicle electrical system via a DC/DC converter from the high-voltage system.
The car’s electronics also control other functions, such as the heating and air-conditioning system, to minimise the load placed on the lithium-ion battery packs.
Charging is by means of a public charging point or an ordinary household power socket. At home, the car can be fed from a single-phase 230V socket or from a three-phase socket in a household wall-box. Using a single-phase 230V supply, the battery packs take around eight hours to accumulate the energy required for a range of 60 miles of NEDC driving. This time is reduced to three hours if you charge from a wall-box or a roadside recharging point.
Daimler and a number of other European automakers and energy utilities have agreed on standardised charging plugs and sockets, with an ultimate aim of worldwide standardisation. This makes the technology more customer-friendly, and is seen as a key prerequisite for the rapid spread of electric mobility.
Like the Smart Fortwo electric drive, the A-class E-cell is fitted with an intelligent charging management system based on Mercedes’ ‘Smart Charge Communication’. Using the vehicle electronics system, all relevant information — such as the electricity supply contract identification data — is exchanged with the charging point. Benefits include a convenient, simple and allegedly totally reliable billing system.
Other ‘intelligent’ features of the electric A-class include pre-start climate control, which we have seen elsewhere. While the batteries are being charged, the interior can be pre-heated or cooled to a temperature set by the driver. Another function enables the driver to monitor charging progress and therefore the present range available, by way of the internet or a smartphone.
The electronics also provide for proactive control of the charging operation: if the electric car is parked for a long period, recharging can be set to take place at times when electricity is cheaper. The human interface in the A-class provides a range of billing, control and monitoring functions. The vehicle is automatically immobilised on connection of the charging cable to the socket, preventing any possibility of driving off by mistake while charging is in progress.
The multifunction display in the vehicle shows the battery charge status, trip data, travelling time, average speed and average fuel consumption, along with the remaining range and residual electrical energy capacity. Another detail specific to the system is a display of the currently available drive power. The rev-counter normally to be found on the instrument panel is replaced with an indicator showing the instantaneous drive power demand or recovery power. Green, white and red-coloured segments show whether the drive system is currently operating in charge, economy or boost mode.
The lithium-ion batteries and high-voltage system of the A-class E-cell are protected by a number of safety measures:
All cables are clearly colour-coded, and safety warnings are placed on high-voltage components.
Generously dimensioned insulators and special plugs provide full contact protection for the entire system.
An electronic controller continuously monitors safety requirements and immediately alerts the driver to any malfunctions in the high-voltage system.
All high-voltage components are connected in an electric loop. This means that the high-voltage system is automatically completely disconnected in the event of a malfunction or an accident.
The high-voltage system is also automatically completely disconnected when the ignition switch is turned to the ‘off’ position, or in the event of possible faults.
The system is continuously monitored for short circuits.
An automatic drive disabler is activated when the charging cable is connected to prevent any possibility of the car being driven off by mistake.
In line with the Company’s strategy for hybrid vehicle development, Daimler has developed a modular system for electric vehicles, including fuel-cell cars. The benefits include the use of shared parts in all the brand’s electric vehicles. All the key components of electric cars are appropriate for a modular approach — for example, the electric drive of the A-class E-cell is the same as that used in the B-class F-cell. The two lithium-ion battery-packs used in the electric A-class are the same as that found in the Smart Fortwo electric drive.
Mercedes-Benz A-class E-cell
Battery — type — capacity
Charge time *
Track: front Track: rear
(1) Single-speed reduction gear.
* For 100km range under NEDC conditions: 230V single-phase / 400V three-phase.