Basic Evaluation Technology Development of Battery Installation Safety for Electric Vehicles

Abstract

A common feature of electric vehicles is that they do not emit exhaust gases, such as CO₂ or Nox, have low noise and vibration harshness (NVH), and are silent as a result of their using secondary electric energy, i.e. alternatives to petroleum. The case of an electric vehicle, where a motor system can be substituted for a combustion engine comprises a very fascinating power source which is environmentally sustainable. However, due to the low power performance of the motor and battery source, the weight reduction of the electric vehicle is necessary to increase its efficiency. To research this new vehicle, the benchmarking performed by leading companies of electric vehicles was thoroughly investigated. The reinforcement degree can differ as a consequence of the motor and the battery position in the vehicle in order to satisfy the vehicle safety and it can induce the weight variation of the vehicle. Therefore, the CAE (computer aided engineering) simulation of the full vehicle (Metro by Geo Co.) model was performed. Through the battery mounts modeled by the Hyper-Mesh program and the benchmarking of the battery installation placed in the vehicle location (on front engine room, center floor or rear trunk position), the optimized position of the power train system considering weight balancing of the electric vehicle was suggested in the present study. In addition, the best mounting position and mounting method of the battery to suggest the design guidelines in order to make up a safe electric vehicle were evaluated.

1 Background and Purpose of Study Aim

The case of an EV (electric vehicle), where a motor and a battery system can be substituted for an engine system, comprises a very economic power source. Concerning fuel expenses, it is very economic because the electrical bills are a small fraction of the average petroleum ones. This economic feasibility has a tendency to expand in accordance with the rise in the value of oil price. On the other hand, there are still a number of problems for the EV that need to be solved, such as that the driving range on a full battery charge is too short and that the price of high performance batteries is very high, and the batteries’ lifecycles are very short. Furthermore, because of the large battery the electrical vehicle in accordance with the battery performance is very heavy. Therefore, design guidelines considering these safety, performance and weight issues will have to be presented.

In the present study, the design guidelines suggest to consider the high stiffness and strength of the battery mount, the optimum layout of the power train system and the lightweight design considering the mounting positions of the driveline batteries. For the research of the new electric vehicle, the benchmarking of developments from leading companies about the electric vehicle was considerably investigated first. Then the CAE (computer aided engineering) simulation of the full vehicle (Metro by Geo Co.) model was performed by using the commercially available Hyper-Mesh and MSC Nastran program. Through the battery mount modeling using commercially available programs and by benchmarking the battery installation place in the vehicle location (on front engine room, center floor or rear trunk position), the optimized position of the power train system considering weight balancing of the electric vehicle was suggested in the present study. From these results, the optimum battery location and the design guidelines were proposed considering the weight balance and the safety of the electric vehicle.

2 Benchmarking and Simulation Procedure

For the benchmarking, 24 electrical vehicles from 11 countries around the world which are being produced or are under development were surveyed. The investigated EV developing countries are Japan, USA, Canada, Norway, Germany, Austria, Italy, France, Spain, India and China. The surveyed EV developing companies were Mitsubishi Motors, Fuji Heavy Industries, Showa Airplane Industry, Tesla, EV Innovation, Coda Auto., Chrysler, Ford, Zenn, Elbil, Think Nordic, Daimler, BMW, Seat, Renault, Reva, Tata, BYD and Cherry Motor Co. and others. All the surveyed data were provided from the electric vehicle companies’ internet homepages.

The CAE computer modeling was executed by using the Hyper-Mesh program and the CAE simulation was performed by using a commercial program (Nastran Sol. 101 and Sol. 111). The simulation model was composed of 306,399 numbers of solid and shell elements plus 202,591 welding entities. Figure 1 shows the vehicle type based on the CAE model. All components were composed and mounted with 1-D welding elements [1, 2].

Fig. 1

The type of vehicle (Metro made by Geo Company)

3 Results and Discussion

Figure 2 shows the results of the EV mileage range per charge analysis. The mileage ranges of over 70 % of the surveyed vehicles varied between 100 and 250 km as shown in Fig. 2.

Fig. 2

Range distribution of the EV mileage per charge [3]

Figure 3 shows the results of the EV maximum speed analysis. The high frequency level of maximum speeds was 140 km/h except for NEVs (neighborhood electric vehicle) and sport sedans as shown in Fig. 3.

Fig. 3

Max. Vehicle speed distribution of EV [3]

Figure 4 shows the results of the EV motor power analysis. The motor power of the 20–80 kW class shows the most visible frequency. Among these, the class with the highest frequency was served to be the 60 kW one.

Fig. 4

Motor power distribution of EV [3]

Figure 5 shows the results of the EV motor power and battery capacity analysis. The motor power and the battery capacity are as factors that directly affect the performance and mileage (range) of the vehicle per one charge. As shown in this figure, these show a linearly proportional trend. The appropriate motor and battery capacity setting is important for the countries having many slope terrains.

Fig. 5

Motor and battery capacity distribution of EV

Figure 6 shows the results of the EV battery voltage analysis. Battery Voltage is 360 V class under investigation by the looks of the dominant frequency as shown in Fig. 6.

Fig. 6

Battery voltage distribution of EV

Furthermore, all analyzed vehicles, with the exception of two, were used a Lithium-ion battery and its weight varied between 250 and 350 kg. The battery mounting position of the general sedan type vehicle was on the center of the floor. The motor mounting position should be determined in accordance with the power train type (front wheel drive, rear wheel drive, all wheel drive, etc.). The process of benchmarking as outlined above, by analyzing the battery type and size, the battery and motor mounting position, power performance which has been adopted by most of the electric vehicle manufacturers, will reduce trial-and-error procedures in initial engineering.

Figure 7 shows the CAE model including boundary and load condition for the strength analysis for a weight of 752.5 kg because of battery weight increase when the batteries are located in the engine room.

Fig. 7

CAE model for strength analysis when the batteries are located in the engine room (Metro by Geo Company)

Figure 8 shows the design of the mounting position for the batteries and the stacked position when the batteries are located in the engine room in order to suggest the best design method in present study. The vertically stacked structure of upper and lower batteries is shown in this figure. The upper stacked batteries are mounted with 2-points position and the lower stacked batteries are mounted with 4-points position as shown in this figure.

Fig. 8

Batteries mounting position and stacked position in engine room

Figure 9 shows the stress contour after the strength analysis of the model with batteries which are vertically stacked and located in the engine room with the loading condition z = −4 g. In the model, the maximum stress was shown to be 696 MPa in this case.

Fig. 9

Strength analysis results with batteries which are vertically stacked structure with the loading condition z = −4 g

Figure 10 shows the stress contour after the strength analysis of the model with batteries which are arranged in parallel with structure and located in the engine room with the loading condition x = −20 g (=case 2). In this model, the maximum stress was shown to be 853 MPa. The loading condition with z = −4 g case (=case 1) was lighter than the x = −20 g case. Therefore, the loading condition with x = −20 g was comparably important and strong condition than that with the z = −4 g condition. Therefore, the remaining analysis was performed only for the x = −20 g case.

Fig. 10

Strength analysis results with batteries which are arranged in parallel with structure with the loading condition x = −20 g

From these analyses, it was found that the number of battery mount can be reduced by using a battery mounted not with floor panel but with a cross member and section.

Figure 11 shows the CAE model including boundary and load condition for the strength analysis when the batteries are located in the rear floor.

Fig. 11

CAE model for strength analysis when the batteries are located in the engine room (Metro by Geo Company)

Figure 12 shows the stress contour after the strength analysis of the model with batteries which are arranged in parallel with structure and located in the rear of the floor with the loading condition x = −20 g. In this model, the maximum stress was shown to be 1132 MPa in this case and its value are comparably higher and better than the model with batteries located in engine room. Therefore, the modification or design change in order to make up a safe vehicle is required in the case.

Fig. 12

Strength analysis results with batteries which are arranged in parallel with structure in the rear of the floor with the loading condition at x = −20 g

Figure 13 shows the CAE model including boundary and load conditions for the strength analysis when the batteries are located in the center of the floor.

Fig. 13

CAE model for strength analysis when the batteries are located in the center floor (Metro by Geo Company)

Figure 14 shows the stress contour after the strength analysis of the model with batteries which are arranged in parallel with structure and located in center floor. In this model, the maximum stress was shown to be 1034 MPa. Therefore, the design changes such as the application of reinforcements should be required in this case. The reinforcement is needed in the floor and battery mounting area with batteries. Therefore, the design of the battery mounting bracket requires considering the location of the battery in the initial design stage in order to make up a lighter vehicle.

Fig. 14

Maximum stress analysis results after the strength analysis with batteries which are arranged in the center of the floor

Figure 15 shows the comparison of the reaction force analysis results when the battery pack was placed in the center floor (model 10), engine room (model 11) and rear trunk (model 12). There was an increase of front axis load of more than 50 % when the batteries were mounted in the engine room (model 11), however, the rear axis load was decreased. Therefore, in this case, the stiffness of the front suspension must be increased and the body reinforcements and stiffness distribution should be improved following the load concentration. In addition, there was no variation of the front axis load when the batteries were mounted in the rear trunk (model 12), however, the rear axis load increased more than 115 %. Therefore, in this case, the stiffness of the front suspension must be increased and the body reinforcements and stiffness distribution should be improved following the load concentration. In addition, it was found that it is possible to suppress the increase in thickness to increase the strength of the suspension parts that can be expected to remove a concentrated load applied to the tire by placing the battery in front floor.

Fig. 15

Results of the reaction force analysis in various models when the batteries were placed in the engine room, rear trunk and center floor

4 Conclusions

In the benchmarking investigation, the mileage ranges of over 70 % of the surveyed EV vehicles varied between 100 and 250 km. The high frequency level of maximum speeds of the EVs was 140 km/h. The motor power of the 20–80 kW class shows the most visible frequency. Among these, the class with the highest frequency was shown to be the 60 kW one. The motor power and the battery capacity are factors that directly affect the performance and mileage (range) of the vehicle per one charge. These show a linearly proportional trend. The appropriate motor and battery capacity setting should be required for countries having many slope terrains. Battery Voltage is of the 360 V class under investigation by the looks of the dominant frequency.

It was found that number of battery mounts can be reduced by using a battery mounted not with a floor panel but with cross member and section.

By using CAE simulation, the reaction forces were calculated when the batteries were placed in the floor center, engine room and rear trunk. From these results, the position of center floor exhibited excellent and predictable load distribution. Therefore, the optimum battery location is found to be the center floor from the view point of the vehicle weight balance. Additionally, this kind of approach could be useful for EV design engineers. In addition, it was found that it is possible to suppress the increase in thickness to increase the strength of the suspension parts that can be expected to remove a concentrated load applied to the tire by placing the battery in the front part of the floor from the perspective of weight balance.