European Electric Vehicle Fleet Demonstration
with ZEBRA Batteries
1999 SAE International Congress and Exposition
March 1-4, 1999, Cobo Center
Detroit, Michigan USA
Ralf Bady and
Jan-Welm Biermann
Forschungsgesellschaft Kraftfahrwesen mbH Aachen
Bernd Kaufmann
Adam Opel AG
Harald Hacker
AEG Anglo Batteries GmbH
Copyright © 1999 Society of Automotive Engineers, Inc.
ABSTRACT
INTRODUCTION
Electric vehicle technology
USERS AND OPERATING SITES
VEHICLE USAGE
ENERGY CONSUMPTION
DAY/NIGHT-CHARGING BEHAVIOR
USER SATISFACTION
Battery endurance
Conclusion
ABSTRACT
In April 1996 the European Project "Electric Vehicle Fleet Demonstration with advanced Batteries" has been started with financial support of the European Commission. Sixteen electric vehicles equipped with ZEBRA batteries have been built up as prototypes and are now operating in five major European cities. Today, after 1 years of fleet operation the fleet of 10 OPEL Astra Impuls has covered a mileage of more than 275.000 km and the project can be evaluated sufficiently exact to extrapolate the project results. Energy consumption structures, mobility structures (daily trip length) and user acceptance have been analyzed and will be discussed in detail.
INTRODUCTION
In April 1996 the European Project "Electric Vehicle Fleet Demonstration with Advanced Batteries" has been started with financial support of the European Commission. The Partners of this commonly supported project are AEG Anglo Batteries GmbH, Adam Opel AG, Renault, Citelec and Forschungsgesellschaft Kraftfahrwesen mbh Aachen (FKA).
The objective of this project is to operate 16 electric vehicles equipped with ZEBRA batteries (sodium/nickelchloride) in inner city areas. The electric vehicles to be used for this demonstration project are 10 OPEL Astra Impuls Caravan (Figure 1) and 6 RENAULT Express (Figure 2). All together 34 battery blocks with 433 kWh battery capacity, which have been produced in the AABG pilot line, are operated within the scope of this project. The high energy density of the batteries (>80 Wh/kg) enable these electric vehicles having a practical range of more than 120 km, which is enough for the majority of the daily city trip lengths. The vehicles do not need opportunity charging and can be recharged mainly over night during low power production periods of the power generation plants. The use of off-peak power improves the vehicle's economy, the power plant efficiency and the utilization of the distribution system. The vehicles are charged at the standard 230 V/16 A sockets so there is no new infrastructure necessary.
All vehicles are equipped with data recording which register all important data like driven distance, charged energy and capacity. The batteries are checked bi-annual on a battery test bench to evaluate battery aging. Questionnaires and interviews for the evaluation of the acceptance and experience of the users of electric vehicles are performed additionally.
Figure 2: Renault Express THERMIE
The innovative importance of the project is the evaluation of the mobility structures, energy consumption structures, customer acceptance and battery endurance of electric vehicles with advanced technology.
The vehicles are rented to commercial companies, municipal enterprises and private users to insure a day to day practical use.
The total duration of the project is 39 months. After one year of project preparation and vehicle build-up the fleet testing started between April/June 1997. The monitoring will be approximately 26 months.
The results which will be presented in this paper are limited to the 10 OPEL Astra Impuls.
Electric vehicle technology
The 10 Astra Impuls used in the project represent the advanced stage of the third generation of electric vehicles developed by OPEL. The "Impuls", is based on the model year 1996 Astra station wagon, an OPEL high volume production vehicle. This successful production car is a five seater with a payload up to 490 kg. Based on this car OPEL developed a vehicle that is with respect to volumetric and gravimetric payload capacity almost equal to the production vehicle.
To insure the high safety standards of the Astra series, several prototype "Impuls" had to prove their reliability on Opel's test facilities in Dudenhofen and crash tests.
The Astra Impuls (Figure 3) is equipped with two ZEBRA batteries (Z6 and Z5) which offer a range of approx. 120 to 150 km. For an evenly balanced weight distribution one battery block is under the bonnet, the other under the cargo floor, which had to be raised 60 mm. The nominal system voltage is 284 volts with an energy content of 25.9 kWh. Using the permanently installed on-board charging device (230 V connection) the charging time required is around ten hours when the battery is completely empty. The charging time is shorter if the batteries are only partially discharged, for instance about 6.5 hours are required to charge the vehicle for a range of 100 km.
Since at project start in April 1996 the ZEBRA battery system with newly designed, improved ML cells, which offer increased capacity and double power, were not available, the predecessor SL cell type had to be used. The main battery data are shown in Figure 4.
The batteries weight of 325 kg (2 ZEBRA batteries 310 kg plus 15 kg for additional control components) increases the vehicle weight without payload to 1,400 kg. With the use of light weight materials such as aluminum wherever possible and the help of a special rear axle designed to compensate the additional empty weight, there are practically no reductions in payload as compared to the Astra Caravan with an internal combustion engine. The Astra Impuls with ZEBRA battery has a payload of 450 kg.
The electric motor used is fitted with adapted control systems and is characterized by high torque values (130 Nm) with a wide rpm range. The comparative figures for an internal combustion engine, i.e. an Astra 1.6i with 52 kW / 71 metric hp. are maximum torque of 128 Nm at 2,800 rpm.
An inverter converts the battery DC current into 3-phase AC current. The compact 3-phase asynchronous motor develops a peak output of 42 kW based on the battery data. Power transfer is by means of a fixed ratio gearbox. The high torque from a standstill gives the Astra Impuls a very good acceleration characteristic, above all in the lower speed ranges. The car requires only 6 seconds to accelerate from 0 to 50 km/h. The top speed of the Astra Impuls is electronically limited to 120 km/h. During braking, the motor acts as a generator that returns energy to the battery (regenerative braking). The disc brakes are not actuated until the brake pedal is pushed down harder.
Slight changes were made to the interior. There is no need for a clutch pedal due to the wide rpm range of the motor.
A newly developed Electronic Shift Pad (ESP) replacing the mechanical selector lever raises the control comfort of the vehicle. The electronically controlled ESP (Figure 5) allows the integration of a lot of additional functions. It also decreases the weight compared to the former selection unit by 1.5 kg. The driver can switch between three power limitation stages. In "Maximum Mode" the batteries are able to deliver up to 240 A, this allows a power output of 42 kW. In "Normal Mode" the battery current is limited to 120 A, still giving the driver the possibility to travel at maximum speed of 120 km/h. In "Economic Mode" it is always possible to use the full range of about 150 km. In case the driver needs full power a kickdown - function is integrated switching to "Maximum Mode".
To prevent operating errors the ESP-electronics always checks input parameters against driving parameters, i.e. the reverse command is ignored at speeds above 15 km/h. The ESP changes to neutral automatically when the car is turned off. A shiftlock function allows the selection of the "Drive"- and "Reverse"-stage from "Park"- and "Neutral"-stage only when the brake is applied. A keylock-function automatically sets the parking mechanism when the ignition key is taken out to prevent the car from rolling away.
Figure 5: Electronic Shift Pad (ESP) with display showing the amount of charged energy distinguished by day (T:) and night (N:)
The integrated on-board computer also provides the driver with information such like cruising range, actual and average consumption in Watt-hours or as a sum of money, as well as information on the battery conditions.
Charging is also controlled by the ESP. The system offers the driver three different charging modes, to insure highest readiness of the car, lowest energy costs and highest efficiency of power generation. Next to the standard charging mode, that starts charging immediately after the car is plugged in, there is a charging mode optimized towards cruising range and another optimized toward "off peak power"-times of the power plants. The user just has to tell the car what range he wants to cover during the rest of the day or at what time he needs a complete charge and the electronic will start charging at optimum time.
USERS AND OPERATING SITES
To get most general results a widespread area of users and operating conditions were one demand on the selection of both. The selected cities should represent a city of european middle size, be of different topography and have different traffic conditions. Furthermore the cities should be located close to each other to facilitate easy maintenance and supervision of the project. The cities which fulfill these requirements and were selected for this project are Aachen (Germany), Maastricht (The Netherlands) and Liège (Belgium), all cities with a size of 150.000 to 250.000 inhabitants and a distance of less than 40 km to each other.
Ten users were selected in cooperation with the administrations of the three cities which represent commercial companies, municipal enterprises and private users to insure a different usage (low payload, high payload, high daily mileage, low daily mileage ...) of the vehicles. At the users the vehicles are driven by one individual person exclusive or by a couple of different drivers.
VEHICLE USAGE
After a preparation phase of one year in which the vehicles have been built up, users selected and a infrastructure for data acquisition, service, intermediate vehicle testing has been set up, the vehicles have been set into operation at the users between April and June 1997. Four vehicles are operating at Aachen (Germany), four at Maastricht (The Netherlands) and another two at Liège (Belgium). After 1 years of operation the fleet has covered a mileage of 275.000 km until end of September 1998 (Figure 7). This is equivalent to an average monthly mileage of more than 1.500 km per vehicle. The usage of the vehicles is quite different from user to user depending on the different mobility behavior and purposes. The user with the lowest average monthly mileage has driven less than 18.000 km so far, which is equivalent to an average daily mileage of less than 37 km (Figure 6 + 8). The user with the highest monthly mileage has driven more than 44.000 km, representing more than 90 km daily mileage in average. In total the average daily fleet mileage is about 65 km per day (Figure 8).
Figure 6: Driven mileage per vehicle
Since at the most users the Opel Astra Impuls has replaced the before used conventional vehicle, the users don't use their vehicles only for city driving but also for much longer trips. As a result the highest driven daily mileage is much higher than the range per charge. More than 300 km per day (Figure 9) have been realized so far using intermediate charging.
Figure 8: Average daily mileage (without stand still days/ stand still days enclosed) [666-30 replaced 666-44 in first 2 months]
Figure 9: Average and maximum daily mileage per vehicle
A very important aspect is the percentage of stand still, because the battery has to be kept at operating temperature using electric energy. Regarding only the days of 100% stand still (more than 24 h) the most frequent usage represents a stand still of 3,9% whereas another vehicle's proportion of stand still is 36%.
ENERGY CONSUMPTION
The energy consumption reflects the ratio of recharged electric energy to driven mileage. Therefore the energy consumption is influenced by the vehicle usage (driving profile and style, payload), the efficiency of the drivetrain and battery and the efficiency of the on-board charger. Since the ZEBRA battery is a so called "hot" battery, also energy is consumed during stand still for keeping the operating temperature. This required additional energy, in the most efficient way directly taken from the net, increases the energy consumption especially at low daily mileage.
Figure 10 shows the specific electric energy (from mains) versus the average daily mileage. Every dot represents the specific energy consumption per vehicle and month. At low mileage the specific energy consumption can increase to the double of the average value of the complete fleet (100%, 53 km/day). At high daily mileage the losses at the internal resistance of the battery will heat up the battery so that normally no external heating is necessary.
Because of a high energy efficiency of the ZEBRA battery (>90%) low energy consumption can be monitored at frequent daily usage (>70km/day) where no or only low additional external heating is necessary.
Figure 10: Specific electric energy consumption vs. average daily
mileage per month
DAY/NIGHT-CHARGING BEHAVIOR
One objective of the project is to increase the percentage of night charging during low power production periods of the power generation plants. The use of off-peak power improves the vehicle's economy, the power plant efficiency and the utilization of the distribution system. To assist the vehicle user a charge control has been integrated into the ESP. The results show on one hand a significant increase in using electricity produced during the night (Figure 11). On the other hand at the users which don't use the charge control the main energy is still charged during daytime. In average more than 63% of the required energy is charged during low power production periods.
Figure 11: Ratio of off-peak-power charging
USER SATISFACTION
User questionnaires and interviews show excellent user satisfaction. The users were impressed of the good acceleration and quite operation of the Astra Impuls. None of them had problems with the limited range. On the street driven range tests have allowed to drive between 165 and 180 km per charge. The functionality of the power and charging modes have been praised and are used by mostly all users.
Battery endurance
The ZEBRA batteries show best operation behavior in an electric vehicle. No battery failures have occurred up to now. In total 20 batteries (10 Z5 + 10 Z6) are in operation with a total of 3.300 cells. After 18 months of operation 6 batteries have one ore two cell failures. In total 13 cells failed (5 cells in Z6, 4 twin cells in Z5) which represent a failure rate of less than 0,4%. These cell failures are due to a crack of the ceramic electrolyte, which is always caused by imperfections in production never by aging. Since with a cell failure the failed cell gets conductive the batteries can be operated with no noticeable decrease in capacity or power but with a reduced voltage. The batteries can be operated without maintenance with more than 5% cells failed. Additionally the bi-annual executed battery investigations on a battery test bench show little increase in internal resistance at high state-of-charge due to slow changes of the cathode morphology at increasing cycle numbers (Figure 12). As result a decrease in power and energy can be recognized.
Figure 12: Internal resistance vs. DOD at 2h discharge
On this background the ZEBRA battery system is designed for no maintenance lifelong of 10 years and more of 1000 cycles corresponding to more than 100.000 km.
Conclusion
The project is presently the largest EV field test run in Europe. Total mileage will reach more than 200.000 km for the Renault Express Thermie and more than 400.000 km for the Opel Astra Impuls.
The innovative importance of the project is the evaluation of the energy consumption structure, mobility structure, customer acceptance and battery endurance of electric vehicles with advanced technology.
From the available intermediate results follows that:
• energy savings can be monitored beyond a minimum daily mileage,
• excellent user satisfaction can be monitored,
• the battery system provides safely sufficient range for most daily trips in cities and the life-time extrapolation of batteries is satisfactory.
The availability and demonstration of these results are new and necessary for major decisions on the market introduction of electric vehicles equipped with this technology.
