Polymer Composite Battery Case

Design and Manufacturing of Polymer Composite Battery Case for Solar-Powered Vehicle

Abstract

The purpose of this study is to manufacture a composite battery case that will protect the Li-Ion battery pack of the Solaris 11 solar-powered vehicle produced by the Solaris Solar Cars Team. The objective in composite manufacturing is to strengthen one or more properties of materials that are not individually suitable and are immiscible in each other according to their application area. In this study, strength was provided using carbon prepreg fabric and foam, and insulation was achieved using fiberglass surface veil. In addition, the weight of the battery pack manufactured for the Solaris 11 vehicle designed by the Solaris team should be kept as low as possible so that it does not affect the vehicle's center of gravity. This study addresses the composite sheet manufactured to minimize weight and ensure strength, as well as the assembly of the battery pack.

1. Introduction

With the proliferation of electric vehicles, the importance of rechargeable batteries has increased. Li-Ion cells, which have become widely used due to their fast charging capability, are quite lightweight. Li-Ion cells are preferred in electric vehicles due to their high rated voltage and rated current. The best storage conditions for cells are ambient temperatures between 10°C and 35°C. Storage of these cells at extremely low or high temperatures will shorten the battery's lifespan. An external shock to the battery also shortens its lifespan. In addition, internal cell cable/connections may be damaged due to moisture, or electronic components may experience excessive oxidation and corrosion issues due to liquid leakage. To meet all these requirements, our battery pack must be sturdy and have liquid insulation. Additionally, it should be as lightweight as possible to avoid affecting the vehicle's center of gravity. Since it would be difficult to meet these criteria with a single material, composite manufacturing was necessary. High-strength-to-low-weight ratio prepreg carbon fiber fabric, lightweight foam, and insulating fiberglass surface veil were used in this manufacturing. When used alone, these materials cannot meet all criteria, but when combined, they create a case that protects the battery pack.

2. Li-Ion Cells

Li-Ion cells are lightweight batteries with long lifespan and high power density that charge quickly. Their average lifespan is 5 years and they need to be protected from sunlight and direct heat. The electrolytes used in Li-Ion cells can cause fires when overheated. The most important feature of the case required to protect the Li-Ion battery pack is to provide air circulation inside the case.

3. Characteristics of Li-Ion Cells

These most commonly used and efficient cells consist of four main components: the cathode, which determines the product's voltage and capacity; the anode, which allows electric current to flow from inside the cells to an external circuit; the electrolyte, consisting of solvents, salts and additives; and the separator, which acts as a physical barrier between the two charged electrodes. The battery pack contains 35 modules connected in series. Inside these modules are 14 Li-Ion cells connected in parallel. Cell weight: 43.1 grams Module weight: 820 grams Li-Ion cell capacities are in the order of 2000 mAh, with a nominal voltage of 3.6 volts and safe operating ranges of 2.8–4.2 volts. Rising above 45°C during discharge shortens cell lifespan.

4. Material Selection

To increase the strength of the battery case without increasing its weight, it was decided to use prepreg carbon fiber fabric with a low weight/high strength ratio. Pre-resin impregnated reinforcement materials kept under special storage conditions with a specific shelf life are called prepreg (resin-impregnated fiber). Because prepreg fabric is used, there is no need for separate resin impregnation during manufacturing. Prepregs are produced as unidirectional tape, fabric, or twisted yarn. Prepreg carbon fabrics are more costly than carbon fiber fabrics but offer advantages in manufacturing time and resulting production quality. In the composite material composition of the battery case to be manufactured, along with prepreg carbon fabric, foam was used as an intermediate/core material to increase the strength of the sandwich structure without increasing weight and to provide elasticity. Fiberglass surface veil, the final layer of the sandwich structure, was used on the inner walls of the battery case to achieve electrical insulation inside the case.

5. Sheet Manufacturing Process

• Mold release film was used during the manufacturing process to allow the product to separate from the surface without damage before placing the fabrics. • Subsequently, 2 layers of fiberglass surface veil (Kortel) were used to insulate the battery pack's inner surfaces. Due to the thinness of the fiberglass surface veil, this fabric was used on top of the easily removable mold release film. • It was decided to use a total of 8 layers of prepreg carbon fiber fabric to achieve high strength. After the mold release film, 4 layers of prepreg carbon fiber fabric were placed. During this process, the direction of the fibrous fabric is important to avoid upsetting the fabric's balance. Since the fabric tends to fold inward during curing, placing the fibers in opposite directions will reduce this tendency and the part will be less distorted. It is advisable to pay attention to the colors on both sides of the fabric during this process. • Subsequently, 10 mm thick foam as the core material was placed and 4 layers of prepreg carbon fabric were placed paying attention to their directions. • After fabric placement, peel-ply fabric was placed to allow easy removal without damaging the product. • A vacuum blanket was used to absorb excess resin in the mold. • After the structural composition of the sandwich composite sheets planned to be manufactured by the vacuum bagging method was completed, a vacuum bag was applied, the edges of the sheets were sealed with sealant, and contact with air was cut off. • Before entering the oven, compatible connectors were attached to the vacuum ports in the oven and mounted on the sheets. Subsequently, before the oven was operated, a seal check was performed by connecting to the vacuum ports inside the oven. • Cured at 110°C for 720 minutes. Battery case composite structure composition:

6. Mechanical Design

It was decided to make the pieces cut from the sheets produced for the battery pack puzzle-shaped. Thus, the pieces being shape-locked to each other would make assembly more advantageous. During assembly of the cut pieces, a mixture of epoxy resin and epoxy hardener was used as adhesive. The mixture, which adheres tightly to the surface, also provided water insulation. The rear surface of the battery pack is 4 cm higher than the front surface. The reason for this height is to prevent the metals in the opening and closing mechanism from contacting the modules. Additionally, during the manufacture of the battery pack, the regulations of solar-powered vehicle competitions that our team participated in were taken into account. In these regulations, since it was desired that the battery be inspectable, transparent plexiglass sheet 3 mm thick was used for the cover.

7. Strength Test

For the strength analysis of the sheets manufactured for the battery pack, material definitions of the composite structure compositions were made separately for each layer and solutions were obtained in 3 different scenarios. In the first scenario, a load of 28.7 kg, which is the actual battery weight, was applied from the bottom surface of the composite battery case, and the case was fixed from the top surface of the holding channels on the side sheets.

8. Electronic Design

Our battery pack consists of 14 parallel 34 series modules. For battery health, voltage and temperature data are collected during charging and discharging. For the health of a battery module, it should not fall below 2.8 volts or rise above 4.2 volts. In addition, the temperature of a battery cell should not exceed 45°C. In the electronic design process, ventilation is the most important criterion for battery health. If the battery temperature rises above 45°C, it will reduce the battery's lifespan. For this reason, 4 units of fans with dimensions of 800×800×250 mm, rated 24V, 0.10 A would be sufficient, but adjustments are necessary due to our modules' design. The external protection of our battery packs was preventing heat from escaping and causing our test data to be incorrect. For this reason, it was decided to remove the protection from our package, but this decision caused the battery modules to move considerably. As a solution to this problem, the 3 mm plexiglass we used on the cover was also used for internal stabilization. During this design, battery modules were first modeled. Subsequently, the plexiglass cutting locations were determined according to the module placement and cut on a water jet. Bolts were used to secure these parts so they would not move inside the case. To isolate the bolts inside the case, easily shapeable parts were manufactured from a 3D printer. The distance between the two plexiglass sheets was chosen to allow the fans to provide air flow as easily as possible. The gaps between the modules facilitated air circulation inside the case. These gaps served as a tunnel for Battery Management System (BMS) cables.

9. Electronic Assembly

The Battery Management System is an electronic card that monitors the voltages and temperatures of modules during charging and discharging. The BMS card used by our team can monitor 12 modules. One card has 38 cable inputs: 12 voltage, 24 NTC, and 2 BMS communications. To ensure these cables reach the BMS card without damage and are not crushed under the modules, EVA foam sheet was used. This foam also prevented vibration. Our 34 modules are connected to each other with metal busbars. The general + and general − cables are connected to the contactor for safety reasons. A contactor is an electrically controlled switch used to switch electric power circuits. Thanks to this feature, the high voltage exiting the pack can be controlled. Additionally, when the battery voltage or temperature goes outside the safe range, the BMS sends a signal from the processor to the relay, disconnecting the contactor connection, allowing charging and discharging operations to be terminated.

10. Conclusion

As a result of the manufacturing of the battery case consisting of composite sandwich sheets for the S11 solar-powered vehicle and based on the data obtained from strength tests considering the mechanical properties of the composite material, it was determined that the data is within safe ranges and the usability of the manufactured case is appropriate.

11. Acknowledgments

We thank the Solaris Solar Cars Team, our valued advisor Assoc. Prof. Aytaç Gören, and our dear team members for their valuable support during the writing and publication process.

12. References

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