Field of Invention:
The present invention relates to the design of heat sinks for thermal management of cells pertaining to their use in high energy density electric vehicle battery packs
Background:
Most electric components and systems generate some form of heat during operation. This heat is proportional to their inefficiency and is mostly radiated out as a form of wastage. In case of batteries or cells, this heat is generated by the motion of ions in the electrolyte and the internal resistance of the cells against the flow of ions. This heat generated is directly proportional to the charge or discharge rate of the battery, i.e., when higher charge is drawn or supplied into a cell, more number of ions try to pass through the electrolyte which causes an internal resistance to be developed in the cell. This results in extensive heating of the cell. This is a major problem because a rise in temperature also decreases the efficiency and life of the cell. Excessive heat generation results in more disturbances caused in the flow of ions on a molecular level thereby raising the internal resistance of the cell. This results in a significant drop in the performance of the cell. To add to it all, cells often experience what is called a thermal runaway which is the increase of the temperature of the cell to such an extent that the electrolyte present starts evaporating thereby increasing the pressure inside the cell which may lead to a fire or a physical crack/blast in the cell. During thermal runaway, the high heat of the defective cell can propagate to the nearby cells thereby causing a chain reaction of cell failure ultimately leading to the failure of the battery pack. To prevent this, a heat exchange system needs to be incorporated by bringing a stream of air/coolant into thermal contact with the heat generating source. Heat is absorbed by the coolant/air and is then carried out by it to a heat exchanger (radiator in this case) and dissipated into the air or used in some other form such as for heating the cabin.
There are two types of heat exchange systems:
a) Passive heat exchange system.
b) Active heat exchange system.

Passive heat exchange systems refer to cooling technologies that rely solely on the thermo-dynamics of conduction, convection and radiation to complete the heat transfer process. These technologies are the most commonly used, the least expensive and the easiest to implement.
Active heat exchange systems refer to cooling technologies that must introduce energy -typically from an external device - to augment the heat transfer process. A key benefit is that they increase the rate of fluid flow during convection which dramatically increases the rate of heat removal. Their drawbacks include the need to use electricity in order to operate, the possible introduction of audible noise to a system, and the fact that they are generally more complex and expensive than passive systems.
In this case, the battery pack needs to be housed inside the cabin of the car, thus decreasing its access to fresh free flowing stream of air required for passive heat exchange. This may result in thermal runaways as passive heat exchange needs a natural circulation of coolant fluid (air or water) which is not possible in our case. Furthermore, passive cooling systems are not suitable for operations where a sudden increase in temperature is feasible as a sudden peak or rise in temperature requires more coolant fluid to be circulated which is not possible in a passive system. Thus, an active heat exchange system needs to be employed to effectively cool the cells for peak efficiency and operation.
In most active heat exchange systems, the heat source is either in direct contact with the circulating coolant or it has an interface or a heat sink which increases the surface area in contact with the coolant thereby increasing heat conduction. The direct contact method is efficient but usually the heat source has very minimal contact area in contact with the coolant conduit or coolant itself. In such scenarios, a heat sink is added to increase the surface area in contact with the coolant flowing so that much more efficient cooling can be achieved.
Summary:
An embodiment of the present invention describes a solid heat-sink to be used for active cooling of cylindrical Li-ion cells using a liquid coolant, consisting of two identical solid alloy blocks, comprising of a first set of grooves provided for the cells in an inner side of

each blocks, and a second set of grooves provided for copper coolant conduits encasing an outer surface of each block, the first set of grooves and the second set of grooves being perpendicular to each other.
a) According to an embodiment of the present invention, each block has N number of grooves for N cells with each groove having a tolerance with respect to the cell diameter.
b) According to an embodiment of the present invention, the heat-sink comprises a triangular packing of cells encased between the two solid blocks for maximum space efficiency.
c) According to an embodiment of the present invention, each block has a number of grooves for the coolant conduit running along the vertical center of the block and perpendicular to the grooves from (a).
d) According to an embodiment of the present invention, each block has a curvature at its ends which is of the exact curvature of the coolant conduit wrapped around it and continues along the groove from (c).
e) According to an embodiment of the present invention, the blocks have threaded holes on the top and bottom surfaces of the sink which are to be used for fastening and packing the blocks together or mounting electronic components with the cells as per the arrangement from (b).
f) According to an embodiment of the present invention, the blocks have grooves with a specific tolerance from the nominal diameter of the coolant conduit.
g) According to an embodiment of the present invention, the grooves in the blocks have a circular profile corresponding to the profile of the cells they encase.
h) According to an embodiment of the present invention, the grooves for coolant
conduit in each block have a circular profile, i) According to an embodiment of the present invention, the grooves on block from
claim 9 continue to take an oval shape along the end curves from (d). j) According to an embodiment of the present invention, each block has N number of
identical grooves which are attached as per the packing in (b).

k) According to an embodiment of the present invention, both the blocks are solid
and made from an aluminum alloy. 1) According to an embodiment of the present invention, each block has additional
grooves for installing temperature measurement devices for monitoring thermal
condition of the pack.
The primary function of the block is to absorb the heat generated by the cells and act as an interface between the coolant and the cell surface so as to increase the surface area to improve heat dissipation from the cell surface to the coolant flowing in the coolant conduit. Special care is to be taken while interfacing the different materials and surfaces for smooth heat transmission between them and to prevent the generation of heat pockets in the arrangement. The secondary function of the blocks is to tightly hold the cells together banding them in a triangular arrangement as described above and to provide support and rigidity to the structure. This is achieved by incorporating the threaded screw holes on the top and bottom sides of the blocks so that they can be held together by screwing a bus bar or similar structure on either side. This ensures a rigid structure capable of withstanding shocks and loads and ensures that the cells are free from any kind of tension or stress on them. It also ensures that the cells do not move and come into contact with each other thereby reducing the risk of short circuits.
Implementations can include any or all of the following features. The grooves on the inner side of the blocks which are to be used for holding the cells have a minimal tolerance which can be filled in by any thermally conductive and electrically insulating pad/paste to ensure an even more smooth connection and interfacing. Furthermore, it also reduces the chances of short circuiting the cells. The grooves on the outer side of the blocks which are to be used for housing and attaching the coolant conduits are also provided with a tolerance which can be filled in by the thermally conductive and electrically isolating compound to ensure a smooth interface. It is to be noted that the shape of this groove changes from circular to oval as the groove travels along the surface and when it nears the curves at the ends of the blocks. This is done in order to ensure maximum contact between the coolant conduit and the block as the

conduits take on an oval shape when they bend. The threaded screw holes on the top and bottom sides are used to screw rails on either side of the blocks. This ensures a rigid structure capable of withstanding shocks and loads and ensures that the cells are free from any kind of tension or stress on them. It also ensures that the cells do not move and come into contact with each other thereby reducing the risk of short circuits. The blocks and all its components are made from an alloy of aluminum for maximum thermal conductivity and rigidity and minimal weight.
Brief Description of the accompanying Drawings:
Fig 1 shows an example heat sink complete with cells, the sink and the coolant conduit.
Fig 2 shows the main and major components of the example heat sink along with the three
interfaces.
Fig 3 shows the heat sink in details.
Fig 4 shows the coolant conduit in details.
Fig 5 shows the side view of the example heat sink.
Fig 6 shows the top view of the example heat sink.
Detailed Description of the Invention:
This document describes a design to effectively cool an electrical cell arrangement to be used in the battery pack by using a custom designed heat sink. The heat sink is designed taking into consideration the maximum thermal efficiency, minimal weight, volume and maximum space efficiency in a way so as to improve the performance of the cells. The heat sink has three interfaces, as shown in the diagram;
(i) Cell-sink interface, (ii)Sink-tube interface and (iii)tube-fluid interface.
The cell-sink interface is the one between the cell and the alloy heat sink as shown in FIG 2
labeled as 105. As with most interfaces, it has been designed to have maximum surface area
possible in contact with the heat generating source (cell in this case). This interface is
responsible for conducting the heat from the cell to the sink and thus lowering the cell in the
process.

The sink-tube interface is the one between the sink and the copper tube as shown in FIG 3 and FIG 4 as 106 and 102A respectively and it is from this interface that the fluid tube absorbs the heat energy absorbed by the heat sink and later, transmits it to the coolant flowing.
The tube-fluid interface is the interface between the fluid conduit/tube and the fluid/coolant as shown in FIG 4 as 102B. this interface is responsible for passing on the heat absorbed by all the previous interfaces to the coolant flowing through it so that it can be dissipated into the atmosphere via a heat exchanger (radiator in this case).
FIG. 1 shows the body of an example heat sink assembly. It is constituted of two similar blocks (103) which wrap around the cells (104) encasing them. The heat sink blocks are solid and have groves for the cells made in them (shown later in FIG 2). The sink also has small threaded holes on the top and bottom faces (101) which is used to secure them in place with each other in with the copper bus bars. As can be seen from the diagram, the ends of the sink are curved at a specific radius (100A and 100B). This is done to easily mold the copper conduits without causing any deformation or blockage to the flow of the coolant in the conduit.
FIG. 2 lays focus on the major interfaces of the example heat sink, namely the cell-sink interface (105). This interface is responsible for absorbing and maximizing the contact area of the sink with the cells and thus special care has to be taken to ensure a smooth finish on this surface. To improve the contact area furthermore and ignore the slight irregularities produced during the manufacturing of the block, the interface region can be covered with heat conductive and electrically insulating thermal paste/pad to improve the thermal contact and heat exchange between the two surfaces. A similar thing can be done with the sink-tube interface which is explained in the next part.
FIG. 3 further breaks down the assembly into its raw components leaving behind just the example heat sink. As it can be seen from the diagram, the example sink consists of two

solid alloy parts as shown by 103A and 103B. These two solid pieces engulf the cells and fix them together, making thermal contact with them. As observed here, the pieces 103A and 103B are the same in designed and are just arranged differently. This is clearly shown in FIG 6.
FIG. 4 shows the copper tube/conduit used to carry the coolant fluid to the heat sink. It is in thermal contact with the sink and thus absorbs the heat from the sink and conducts it to the coolant flowing through it. As shown in the figure, 102A is the interface surface between the example heat sink and the tube and 102B is the interface between the tube and the coolant. The tube is also bend in a way so as to allow minimum restriction to the flow of coolant and this shape resonates with the bends at either ends of the sink as shown in the diagram by 106A and 106B. They are designed in a way so as to fit exactly along the grooves provided in the sink as shown in 100A and 100B respectively.
FIG. 5 shows the side view of the example heat sink. The heat sink is designed to completely encase the cells. This ensures that the cells do not experience any vertical or horizontal stress or loads which might damage them. The heat sinks are designed to withstand the stress (if any) induced on them to prevent crushing of cells and also act as support channels to keep them in an upright position and prevent electrical shorts. It is also designed to add rigidity to the structure, preventing and saving the cells from shocks and damages.
FIG. 6 shows the top view of the example heat sink. As mentioned and shown earlier in FIG. 4, the 106B and 106A have been designed to fit and sit in the grooves 100B and 100A respectively. This ensures maximum contact between the tubes and the sink to ensure a steady temperature distribution along the whole block. It also shows the grooves 105 on either of the blocks 103A and 103B, which provide the maximum contact area with the cells to provide stability and proper heat absorption with respect to the size, space and manufacturabihty of the heat sink. It also shows the threaded grooves 101 on either of the blocks. The grooves are used to assemble and hold the blocks together, tightly holding the cells in between them, by using a copper bus bar. This bus bar can also act as a conductor to transmit the high current

and interconnect such packs in series/parallel with each other for the desired voltage, current or power.
A number of implementations have been described as examples. Nevertheless, other implementations are covered by the following claims.

We claim:
1. A solid heat-sink to be used for active cooling of cylindrical Li-ion cells using a liquid
coolant, consisting of two identical solid alloy blocks, comprising of:
a) a first set of grooves provided for the cells in an inner side of each blocks; and
b) a second set of grooves provided for copper coolant conduits encasing an outer surface of each block,
the first set of grooves and the second set of grooves being perpendicular to each other.
2. The heat-sink as claimed in claim 1, wherein each block has N number of grooves for N cells with each groove having a tolerance with respect to the cell diameter.
3. The heat-sink as claimed in claim 1 comprising a triangular packing of cells encased between the two solid blocks for maximum space efficiency.
4. The heat-sink as claimed in claim 1, wherein each block has a number of grooves for the coolant conduit running along the vertical center of the block and perpendicular to the grooves from claim 2.
5. The heat sink as claimed in claim 1, wherein each block has a curvature at its ends which is of the exact curvature of the coolant conduit wrapped around it and continues along the groove from claim 4.
6. The heat sink as claimed in claim 1, wherein the blocks have threaded holes on the top and bottom surfaces of the sink which are to be used for fastening and packing the blocks together or mounting electronic components with the cells as per the arrangement from claim 3.
7. The heat sink as claimed in claim 4, wherein the blocks have grooves with a specific tolerance from the nominal diameter of the coolant conduit.

8. The heat sink as claimed in claim 2, wherein the grooves in the blocks have a circular profile corresponding to the profile of the cells they encase.
9. The heat sink as claimed in claim 4, wherein the grooves for coolant conduit in each block have a circular profile.
10. The heat sink as claimed in claim 5, wherein the grooves on block from claim 9 continue to take an oval shape along the end curves from claim 5.
11. The heat sink as claimed in claim 2, wherein each block has N number of identical grooves which are attached as per the packing in claim 3.
12. The heat sink as claimed in claim 1, wherein both the blocks are solid and made from an aluminum alloy.
13. The heat sink as claimed in claim 1, wherein each block has additional grooves for installing temperature measurement devices for monitoring thermal condition of the pack.