Introduction

The supply of fossil fuels is slowly being exhausted, and burning gasoline is causing much environmental pollution. Electric vehicles (EVs) are the future of transportation, but their batteries have some critical shortcomings: short range of vehicles, short service life, and high costs, preventing widespread consumer adoption. Regulating the battery temperature during continuous charge and discharge is a major challenge.

For most Li-ion batteries used in EVs, their temperature specification is normally as follows:

Operating temperature: - 20 °C to 60 °C

Charging temperature: 0 °C to 45 °C

Storage temperature: -10 °C to 5 °C

They can achieve their rated capacity at 20~25°C and their capacity will drop ~10% for every increase of 10 °C.

During winter, when temperatures can easily fall below 0 °C, it will be difficult or totally impossible for charging.

During other seasons, under continuous high current charge/ discharge, the battery working temperature can easily reach 60 °C, making it difficult for discharging. Higher temperatures can also cause battery degradation, shortened service life, or present safety hazards.

The specific characteristics of Li-ion cells require well-adapted and well- designed battery temperature management and control systems.

In the subsequent paragraphs, for simplification, the word 'cooling' will represent heat exchange, be it cooling or heating, and the word 'coolant' will represent a liquid that has anti-freeze, non-flammable, non-corrosive and antifungal properties for heat exchange of the battery cells, be it for cooling or heating. Drawbacks of existing battery packs

Present-day battery packs are constructed by connecting a number of battery modules, each module consisting of several cells connected side by side. The result is that cells in the center of each module are more thermal insulated, resulting in more difficult heat exchange.

Existing battery cooling solutions normally consist of the battery modules sitting on or attached to a heat sink (a flat metal plate) that is cooled by a coolant loop. The drawbacks are that the cooling efficiency is low, and the effectiveness is poor, since only a small part of each module receives the cooling effect. Also, the heat sinks are generally thick and heavy due to the coolant loop. The result is that temperatures will differ from module to module, cell to cell. Even within the same cell, different regions may have different temperatures. If heat sinks were to be used to cool each cell, it would result in a battery pack with impractical weight and volume.

Consistency and uniformity of each cell in a battery pack is very important for its durability and effectiveness. Once the cells are produced and assembled into battery packs, the only factor that is able to affect its consistency and uniformity is the temperature of each cell. Temperature changes can affect the cells' internal resistance, and changes to internal resistance can in turn affect the rate of temperature change. The existence of a temperature gradient between cells will cause different rates of cell ageing, resulting in certain cells having shorter lifespans. With the failure of a single cell, the operation of the entire battery pack is compromised as all cells are interlinked to work together.

Battery packs used in EVs are constrained by space and weight, so cooling systems for the battery packs must be compact and lightweight, yet meeting the power and energy capacity requirements.

A more compact, lightweight and longer lasting power battery pack solution

Based on the above observations, I studied and worked out a unique battery pack cooling apparatus suitable for EV requirements. Although the design below is intended for applications in an EV and is a proof-of-concept for realistic application in an EV, it does not and will not limit or affect the claims about its novel concept, principles, and structure for other applications.

The following drawings form part of this description,

Fig. 1/31 is a large-format laminated battery cell (100Ah, 3.7V);

Fig. 2/31 is a 2S-cell (2 battery cells connected in series);

Fig. 3/31 is a small clip;

Fig. 4/31 is a big clip;

Fig. 5/31 is an end clip;

Fig. 6/31 is a cooling duct plate;

Fig. 7/31 is a cooling duct end plate;

Fig. 8/31 is the cooling grid array;

Fig. 9/31 is a front rubber sheet;

Fig. 10/31 is

Fig. 11/31 is

Fig. 12/31 is

Fig. 13/31 is

Fig. 14/31 is

Fig. 15/31 is

Fig. 16/31 is

Fig. 17/31 is

Fig. 18/31 is

Fig. 19/31 is

Fig. 20/31 is

Fig. 21/31 is

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Fig. 23/31 is

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Fig 26/31 is

Fig. 27/31 is

Fig. 28/31 is

out space

Fig. 29/31 is a front view-B of Fig. 28/31 to show coolant flow direction at front rubber sheet cut-out space; Fig. 30/31 is a back view-C of Fig. 28/31 to show coolant flow direction at back rubber sheet cut-out space;

Fig. 31/31 is a detail view-D & E of Fig. 28/31 to show coolant flow direction at bottom flow duct layer.

For the battery cell type and cell specifications, I have selected laminated cells. Compared to cylindrical cells, laminated cells have lower internal resistance and therefore lower heat generation upon charging and

discharging. Also, it has a higher energy / power density. Because of its flat geometry and higher exposed surface area, it is easier for heat exchange to take place.

I have designed the battery pack to have 86 pieces of large-format laminated cells (100Ah, 3.7V, as shown in Fig. 1/31). The battery cells are connected by means of a novel clip system, outlined below, which provides for electrical connectivity under space constraints.

Two cells are connected face-to-face in series (Fig. 2/31 ), with one of the terminals connected to the opposite terminal of the other cell, by a small clip (Fig. 3/31 ). This will be one 2S-cell. Both sides of the 2S-cell will be in contact with the cooling duct plate (Fig. 18/31) for heat exchange.

2S-cells are connected in series, with the terminals of each 2S-cell in identical orientation. A big clip (Fig. 4/31 ) connects the terminals between each 2S-cell, straddling the cooling duct plate (detail-D of Fig. 18/31), which is between the two 2S-cells once they are inserted into place.

End clips (Fig. 5/31) are used at the positive terminal of the first cell of the first 2S-cell (Fig. 20/31 ), and the negative terminal of the last cell of the last 2S- cell, for connection to main power cables. All clips are made of metallic materials with electrical conductivity.

This will achieve the voltage (320V) and energy capacity (32kWh) for purely electric driving for a range of 120~150km (the range of 90% of daily urban commuters).

To lower the cost of the battery pack box, extruded aluminum alloy

construction will be used for the box construction plates wherever possible. Fig. 6/31 depicts the cooling duct plate. The divided hollow flow ducts are for the coolant to pass through and for heat exchange to take place with the 2S- cells.

Both ends of the cooling duct plates are inserted into the cooling duct end plates cut-out slot (Fig. 7/31 ), resulting in a flushed outside surface. Friction Stir Weld (FSW) will be used to make a leak-proof joint, forming a

homogenous and regular structure - the cooling grid array, depicted in Fig. 8/31 , consisting of multiple cooling duct plates arranged in a row and attached to cooling duct end plates. The cooling duct plate also supports the battery cell and keeps the battery cell in its position and maintains its shape.

The front rubber sheet (Fig. 9/31 ) will sit in the recess of the front cover (Fig. 10/31 ). The final configuration, as viewed from the side where the rubber sheet is, is shown in Fig. 11/31.

The back rubber sheet (Fig. 12/31) will sit in the recess of the back cover (Fig. 13/31). The final configuration, as viewed from the side where the rubber sheet is, is shown in Fig. 14/31.

The cut-out patterns of the front and back rubber sheets are slightly different. The front rubber sheet has narrow cut-outs on its left and right (with different layouts for the left and right narrow cut-outs) and wide cut-outs for the rest. The narrow cut-out will open out to two flow ducts laterally (1x2 or 2x2 configuration) while the wide cut-out will open out to four flow ducts (1x4 configuration) (Fig. 24/31 and Fig. 25/31 ). The narrow cut-outs exist in two configurations, namely the large narrow cut-out and the small narrow cut-out (Fig 25/31 ). The large narrow cut-outs can open out to four flow ducts (2x2 configuration), while the small narrow cut-outs can open out to two flow ducts (1 x2 configuration)., The back rubber sheet has only wide cut-outs throughout (Fig. 26/31 and Fig. 27/31). This arrangement of cut-outs will facilitate the directional change of the coolant flow (Fig. 28/31 to Fig. 31/31). The rubber sheets also have the function of sealing the space between flow ducts.

The novel structure of the apparatus, having a cooling grid array, a front cover with a front rubber sheet having a special cut-out pattern and a back cover with a back rubber sheet having a special cut-out pattern, is what allows the coolant to flow in a path that results in even and effective cooling throughout the apparatus, at the same time keeping the apparatus compact and lightweight. Friction Stir Weld (FSW), or any other suitable connection method, can be used for these connections:

- Front cover with front cooling duct end plate (must be a leak-proof joint)

- Back cover with back cooling duct end plate (must be a leak-proof joint)

- Bottom plate (Fig. 15/31 ) with front & back cooling duct end plate

- Two side plates (Fig. 16/31) with front & back cooling duct end plate and bottom plate.

The construction of the final empty battery box is shown in Fig. 17/31.

After the 2S-cells have been inserted in the spaces between the cooling duct plates (Fig. 18/31 ), with the laying of necessary insulation sheets and spacers and the connection of the small, big and end clips (as described above), battery management system (BMS) connections and cables can be laid.

The top cover (Fig. 21/31 ) has slots that can accommodate a BMS. It is also equipped with various sockets like a 12V DC connection for the BMS, a main power socket and two Can-bus 2.0 terminals. It also can be equipped with a quick-release coolant connector at the coolant inlet and outlet. All these make it easy for plug-and-play operation with any EV.

With a top rubber seal (Fig. 18/31) between the top cover and the battery box secured by fasteners, the entire battery pack box (Fig. 22/31) will be an IP65- rated enclosure, suitable for EV application.

Fig. 23/31 shows the schematic flow of the coolant in the cooling grid array in the battery pack. For simplification, the single cylindrical pipe-like structure represents the path of two flow ducts. The locations of the flow ducts are depicted in Fig. 24/31 to Fig. 27/31. With the connection of both the front and back cover with their respective rubber sheets, the flow of the coolant will be able to change direction at the rubber sheet cut-out space. How the front rubber sheet is able to change the flow direction is depicted in Fig. 29/31 , and how the back rubber sheet is able to change direction is depicted in Fig.

30/31. Fig. 31/31 shows the flow direction at the bottom flow duct layer.

Conclusion

This apparatus is able to carry coolant to each individual cell evenly, effectively and efficiently in a simple design, extending the lifespan of the battery and enhancing its safety.

The total weight of the battery pack (Fig. 22/31 ), including ~13kg of coolant, is ~ 350kg.

Its dimensions are L=1031mm, W=509mm, H=445mm.

The battery pack is lightweight but strong, with a voltage of 320V, energy capacity of 32kWh, energy to weight ratio of ~91Wh/kg, and an energy to volume ratio of ~137Wh/L. These specifications will be able to meet requirements in most EV applications.

Possible future implementations

The present-day EV battery packs are produced by the various carmakers in a variety of forms. This results in higher costs from a lack of economies of scale, and no interchangeability of batteries between cars from different manufacturers.

If battery packs were standardized, it would be able to be easily adapted to fit different vehicles. The state grid can possibly produce a standardized battery pack rather than the carmakers themselves, and there are benefits to be reaped in a number of ways.

Centralized facilities for producing, charging and maintaining battery packs will result in economies of scale, giving cost savings to both consumers and producers. Battery packs can be charged at power stations during off-peak hours and delivered to petrol kiosks. In place of filling up petrol, consumers can merely replace their batteries at petrol kiosks, leaving their flat batteries to be picked up and charged by the state grid. This will also extend the range of EVs.

With a central charging facility delivering fully-charged battery packs to petrol kiosks, governments need not spend money to build charging stations at different locations, resulting in substantial savings. Also, consumers need not worry about the serviceability and maintenance of their batteries, as the state grid will handle these.

This approach will reduce the price of EVs dramatically and hence promote market growth. Carmakers only need to design future cars to be able to accommodate a standard battery pack.

The concept and structure of this power battery pack cooling apparatus can also apply to other future implementations and applications requiring compact and lightweight battery packs, or applications requiring effective cooling systems under space constraints.