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High energy ba electric vehicle BMW’s electric car, the E1. It has a 32kW DC motor & an ABB high-energy battery rated at 120 volts & 160Ah. The E1 can easily hold its own in traffic. Fully charged, it has a range of 160 to 230km. Its top speed is 120km/h. 6  Silicon ilicon Chip hip at teries for es The development of high energy batteries is critical if electric cars are to seriously compete with conventional petrol & dieselpowered cars. In this article we report progress made by ABB in producing sodium sulphur batteries for electric vehi­cles. Electric vehicles, whether cars, minivans or buses, produce substantially less noise and emissions than their counterparts with conventional engines. In the past, electric car development has been hindered by the excessive weight of the battery; fully charged, a 400kg lead-acid battery allows a car to travel a distance of only about 50km. With a high-energy sodium-sulphur battery of only half this weight and assuming the same condi­tions, a modern electric car could travel about 150km. This means that there is now a realistic chance of emis­sion-free vehicles taking off in both private and public trans­portation. Not only does its better energy-to-weight ratio make the ABB high-energy battery superior to other types of battery. The use of sodium and sulphur as reactants has benefits which are unique to this battery, especially in the areas of design and application. The most important features of the battery are: • No self-discharge takes place in the cells. • The charging efficiency is 100%. This means that a cell needs only to be recharged with the amount of energy that it has discharged. Batteries with aqueous electrolytes, by contrast, require an excess charge to ensure that they are fully charged. This excess charge is consumed during the decomposition of water in the electrolyte. • The charge/discharge efficiency is high (about 90 percent for batteries in electric cars) on account of the 100% charg­ing efficiency. • Battery overcharging is essentially impossible. The internal resistance of the cells rises sharply at the end of the charging process, allowing them to be connected in series or parallel without risk. If a series-connected cell fails (short circuit), the internal resistance of another cell in the string will rise as soon as it has been charged by the parallel strings. In other types of battery, these conditions lead to electrolytic decomposi­tion of the water content, causing hydrogen and oxygen to form. This is why such batteries are usually not connected in parallel and why the capacity of the cells is always matched to the application. Sodium-sulphur batteries, on the other hand, can be built using cells of one standard type to obtain any required capacity. This gives the sodium-sulphur battery its flexibility and makes it economical to produce. Because of the battery’s 100% charging efficiency and the absence of electric self-discharge, its charge can be determined by simple current integration. The sharp rise in the internal resistance of the cell indicates when charging has ended. Every time the battery is fully charged, the starting point for the capacity is recalibrated. March 1994  7 The electric Cobus 200 EL, with three B17 batteries, carries 20 people. It has a top speed of 80km/h & a daily range of up to 200km. Since the cells are operated at a high temperature, the full battery charge is always available even under conditions of extreme cold or extreme heat. The thermal insulation of the batteries is very efficient, so that only a small amount of energy is required to maintain the temperature at the required level. The main advantages of the new generation of ABB high-energy batteries over their predecessors are their higher volu­metric and gravimetric energy densities. Their energy-to-weight ratio of 104Wh/kg makes them the lightest batteries available today for electric cars. This progress has been made possible by an improved cell and the use of liquid instead of air for cool­ ing. The same production technology is used for all the different battery sizes. The A08 cell has an outside diameter of 38mm and is 225mm long. Its capacity is 40Ah. A battery can contain up to 480 vertically mounted interconnected cells, arranged hex­ag­onally on 8  Silicon Chip a heat-exchanger. By using liquid instead of air for cooling, it is possible to utilise the heat dissipated by the battery at high loads for heating. A flat resistance heater heats the battery to the re­quired temperature and maintains it at this level. Operating temperature is between 300°C and 350°C The cells are enclosed, together with the heating and cool­ing systems, inside a double-walled casing. Good thermal insula­tion is ensured by evacuating the space between the walls. The only openings in the casing are for the power and measurement cables and the coolant tubing. As a result, the battery is very compact and heat losses are minimal. An insulating glass-fibre board in the evacuated space between the casting walls gives extra support. The result is a casing so strong that the battery can be mounted in the vehicle without having to use a tray. It can even be a factor in strengthening the vehicle’s body. During development of the new batteries, a large number of safety tests were carried out in collaboration with Germany’s technical inspectorate. Crash tests carried out by automobile manufacturers using their own cars demonstrated that the batter­ies meet the highest safety standards. ABB currently offers two standard batteries. Designated B16 and B17, they have 120 and 240 cells, respectively. A further seven customised batteries, of different sizes and with different energy contents, are also available. The batteries feature very good voltage stability over the full discharge range. Management system Reliable battery operation and efficient utilisation of the energy content depend on the battery management system. This has three primary functions: • To monitor battery conditions and ensure adherence to specifi­cations; • To transmit data to the processor in the drive control unit; and • To regulate the battery temperature. The main components of an electric car’s drive are the high energy battery with its management unit, the electric motor with its control and power sections, a protective circuit breaker and the battery charger. When the battery is cold, the circuit breaker is open and interrupts the battery management system’s power supply. In this condition, the battery can only be started when the battery management system is connected to a socket outlet. Power from this external source is used for the initial heating of the battery which cannot be operated until it is above the lower operating temperature limit. Heat-up normally takes about 24 hours. The monitor in the battery management system authorises operation as soon as the lower operating temperature limit has been reached. However, power is still not drawn from the battery until the drive system’s processor signals ‘ready to operate’ and the protective circuit breaker has closed. During charging and discharging, the monitoring unit checks the temperature, battery current, various voltages and the insu­ lation resistance. Any deviation from the specified data is signalled to the motor control system and initiates a programmed response (eg, a reduction of the discharge current). If this does not lead to the desired result and one of the limits defined for the specified operating values is exceeded, the monitor activates the circuit breaker. The monitoring system is necessary to protect the battery from inadmissible loads. In addition, safety reasons require the entire electrical power train to remain ungrounded under all operating conditions. The management system instantly disconnects the battery if a fault occurs in the insulation. During normal operation, the battery management system signals additional information, (eg, battery temperature, charge level, battery current, etc) to the CPU of the motor control system. By monitoring the battery independently, this CPU can respond before unwanted load shedding is initiated by the manage­ment system. The battery management system controls the battery tempera­ture by activating the cooling or heating system. If the tempera­ture becomes too high due to a high continuous current being taken from the battery, the coolant circulating pump is switched on. ABB’s standard sodium sulphur batteries, B16 on the right & B17 on the left. The leads protruding from the black cover on the heat-insulated casing are the power, measurement & heating cables. Behind this cover is the flange used to evacuate the double-walled casing. The coolant connections are at the back. The heat is either transferred, via a heat-exchanger, to a cooling circuit in the vehicle or via an air cooling system to the atmosphere. When the vehicle is stationary for longer periods of time, the heating system remains switched on to keep the temperature of the battery at its required level. The energy needed for heating is taken primarily from the AC mains but can be taken from the battery itself if there is no mains power available. This is possible for about a week, after which the battery is fully discharged and its temperature will drop below the minimum operating level. In this condition, the battery is unable to heat itself up unless it is connected to a power outlet. Such cases are expected to be very rare with electric vehicles, since they will normally be hooked up every day to the AC power outlet. Trial vehicles ABB has teamed up with major automobile companies in equip­ping electric cars with the high-energy battery. Small fleets of trial cars have already run up more than one million kilometres on public roads. New developments in the automotive industry are targeting the market for electric cars which will soon open in California. By 1997, 2% of all new vehicles in California will have to exhib­it zero emissions. Only electric cars can do this. The types of car involved range from modified production-line vehicles to new, purpose-designed electric cars, such as BMW’s E1. The designers of this “urban” car have put the ABB high-energy battery at the back of the vehicle, under the seats. It has 240 A08 cells, like the B17 battery, but has different dimensions. According to BMW, the car can accelerate from standstill to 50km/h in just six seconds, has a top speed of 120km/h and a range of between 160 and 230km. Electric vehicles with the ABB high-energy battery are also being used for public transportation. Minibuses (eg, the Cobus) are used for inner-city transportation as well as in recreational resorts and other zones reserved mainly for pedes­trians. These buses have three B17 batteries for a range of more than 100km. The batteries can be charged rapidly so it is possi­ble to double the range by interim charging SC during stops at terminals. Acknowledgement Our thanks to ABB Review for the photos and for permission to publish this article. March 1994  9