Configurable thermal conditioning of battery cells

A battery cell thermal conditioning system for a vehicle includes a battery pack having multiple battery cells. Multiple foam layers are individually positioned between first successive ones of the battery cells. A first carbon nanotube sheet is positioned in direct contact with one of the battery cells on a first side of the foam layers and a second carbon nanotube sheet is positioned in direct contact with a different one of the battery cells on a second side of the foam layers. A cooling plate is positioned between second successive ones of the battery cells. A controller directs current flow to the first carbon nanotube sheet and the second carbon nanotube sheet when a temperature of the one of the battery cells contacted by the carbon nanotube sheet drops below a predetermined threshold temperature.

INTRODUCTION

The present disclosure relates to vehicle battery cell charging and battery cell design, and in particular lithium-ion battery cell charging and lithium-ion battery design for electric powered vehicles.

Vehicle battery packs, particularly for lithium-based batteries, such as lithium-oxide batteries are susceptible to plating of metallic lithium (Li) during a battery charging process. Lithium plating occurs during fast charging of the battery pack and at low temperature environmental conditions when a temperature of the battery pack is below an optimum operating temperature range of approximately 25 to 35 degrees Centigrade. Lithium plating during charging is of particular concern when battery temperature is 10 degrees Centigrade or lower. Lithium plating is also of concern during battery fast charging operation, defined as charging in approximately 30 minutes or less. Lithium plating is particularly prevalent during fast charging operation and when the battery pack is at or below 10 degrees Centigrade. Plating of metallic lithium can induce lithium-ion (Li-ion) battery cell degradation or battery cell failure, which are a failure mechanism and an operating concern for an electric powered vehicle.

Methods to pre-heat the battery pack to avoid low temperature charging using heated air or fluid require additional components such as heating elements, pumps to induce flow and sensors. These systems have the potential to leak and create other failure events, and also add vehicle weight, negatively impact arrangement space, and increase cost.

Thus, while current lithium-ion battery designs achieve their intended purpose, there is a need for a new and improved system and method for allowing fast battery charging and charging at battery temperatures below 25 degrees Centigrade while mitigating lithium plating of individual lithium-ion battery cells.

SUMMARY

According to several aspects, a battery cell thermal conditioning system for a vehicle includes a battery pack having battery cells. A carbon nanotube sheet is positioned in proximity to one of the battery cells. A controller directs current flow to the carbon nanotube sheet when a temperature of the one of the battery cells drops below a predetermined threshold temperature.

In another aspect of the present disclosure, a power source independent of the battery pack provides the current flow upon command from the controller.

In another aspect of the present disclosure, the power source defines a 12 VDC battery or a 24 VDC battery.

In another aspect of the present disclosure, the carbon nanotube sheet includes: a positive terminal connected to a positive terminal of the power source; a negative terminal connected to a negative terminal of the power source; and a thickness of approximately 100 microns.

In another aspect of the present disclosure, the carbon nanotube sheet is positioned in direct contact with one of the battery cells and the controller directs the current flow to the carbon nanotube sheet until a temperature of the one of the battery cells increases to a minimum optimum temperature.

In another aspect of the present disclosure, the minimum optimum temperature is approximately 25 degrees Centigrade.

In another aspect of the present disclosure, a second carbon nanotube sheet is positioned in direct contact with a second one of the battery cells on a second side of the foam layer, the controller also directing the current flow to the second carbon nanotube sheet.

In another aspect of the present disclosure, the controller defines a pulse width modulated controller; and a voltage signal from the controller defines at least a 0% current flow, a 25% current flow and a 100% current flow.

In another aspect of the present disclosure, the 25% current flow is provided when the predetermined threshold temperature is at or above approximately 10 degrees Centigrade and below approximately 25 degrees Centigrade; and the 100% current flow is provided when the predetermined threshold temperature is below approximately zero degrees Centigrade.

In another aspect of the present disclosure, the carbon nanotube sheet is positioned at an inner wall of a housing which contains the battery pack, the carbon nanotube sheet when energized by the current flow producing radiant heat energy acting to heat the battery pack.

According to several aspects, a battery cell thermal conditioning system includes a battery pack having multiple battery cells. A foam layer is positioned between adjacent ones of the battery cells. A carbon nanotube sheet positioned in direct contact with one of the battery cells with the carbon nanotube sheet positioned on a first side of the foam layer. A controller directs current flow to the carbon nanotube sheet when a temperature of the one of the battery cells contacted by the carbon nanotube sheet drops below a predetermined threshold temperature.

In another aspect of the present disclosure, the controller directs the current flow to the carbon nanotube sheet until a temperature of the one of the battery cells increases to a minimum optimum temperature of approximately 25 degrees Centigrade.

In another aspect of the present disclosure, a second carbon nanotube sheet is positioned in direct contact with a second one of the battery cells on a second side of the foam layer, the controller also directing the current flow to the second carbon nanotube sheet.

In another aspect of the present disclosure, a battery control unit receives sensor signals from the battery pack and monitors a voltage of the battery pack. The power source defines a direct current battery independent of the battery pack providing the current flow upon command from the controller.

In another aspect of the present disclosure, the controller defines a pulse width modulated controller allowing current to the carbon nanotube sheet to be directly controlled within predetermined ranges and power levels to provide different heat-up rates for different temperature ranges of the battery pack.

In another aspect of the present disclosure, the foam layer is a polymeric material which is elastically compressible to allow thermal expansion and contraction of the battery cells and vibrational motion of the battery pack and the battery cells while mitigating against direct contact between any of the battery cells

In another aspect of the present disclosure, a determination is performed if an immediate high battery demand follows a period of non-operation of the vehicle when an ambient temperature is below an optimum operating temperature of the battery pack but has not dropped to the predetermined threshold temperature, demanding current flow be initiated to the carbon nanotube sheet.

According to several aspects, a battery cell thermal conditioning system for a vehicle includes a battery pack having multiple battery cells and multiple sensors including a temperature sensor and a voltage sensor identifying a temperature and a voltage of the battery pack. Multiple foam layers are individually positioned between first successive ones of the battery cells. A first carbon nanotube sheet is positioned in direct contact with one of the battery cells on a first side of the foam layers and a second carbon nanotube sheet is positioned in direct contact with a different one of the battery cells on a second side of the foam layers. A cooling plate is positioned between second successive ones of the battery cells. A controller directs current flow to the first carbon nanotube sheet and the second carbon nanotube sheet when a temperature indicated by a temperature signal from the temperature sensor of the one of the battery cells contacted by the first or the second carbon nanotube sheet drops below a predetermined threshold temperature.

In another aspect of the present disclosure, a battery charging rate is determined to identify if a fast charging operation is present. A battery pack temperature condition is determined using the sensors to identify if a predetermined low temperature threshold is met at which current flow is initiated to the first and the second carbon nanotube sheets.

In another aspect of the present disclosure, a first face of the first one of the battery cells directly abuts the cooling plate to provide conductive cooling of the first one of the battery cells. A second face of the first one of the battery cells is oppositely directed with respect to the first face of the first one of the battery cells and directly abuts the first carbon nanotube sheet. A first face of the second one of the battery cells directly abuts the cooling plate opposite to the first face of the first one of the battery cells to provide conductive cooling of the second one of the battery cells. A second face of the second one of the battery cells is oppositely directed with respect to the first face of the second one of the battery cells and directly abuts the second carbon nanotube sheet.

DETAILED DESCRIPTION

Referring toFIG. 1, a battery cell thermal conditioning system10for a vehicle11includes a battery pack12having multiple battery cells14. Multiple metal plates16are positioned in the battery pack12, with individual ones of the metal plates16separating successive pairs of the battery cells such as for example a first pair of battery cells14′,14″. Individual ones of the metal plates16function as cooling plates and can be supplied with a flow of a cooling fluid from a coolant passage18. Sequential pairs of the battery cells such as the first pair of battery cells14′,14″ and a second pair of battery cells14′″,14″″ are separated by a foam layer20. The foam layer20is a polymeric material which is elastically compressible to allow thermal expansion and contraction of the battery cells and vibrational motion of the battery pack and battery cells while mitigating against direct contact between any of the battery cell pairs.

The battery cells14include a positive tab22, with multiple positive tabs22interconnected at a positive terminal such as positive terminals24,24′. The battery cells14also include a negative tab26, with multiple negative tabs26interconnected at a negative terminal such as negative terminals28,28′. Individual ones of the positive terminals24can be connected to individual ones of the negative terminals28using one or more connecting plates30.

Multiple sensors32such as temperature sensors, pressure sensors, strain gages, and the like are provided at multiple locations within the battery pack12. Signals from the multiple sensors32are communicated to a battery control unit34which monitors conditions such as voltage, temperature, charging rate and the like within the individual battery cells14and for the battery pack12in its entirety. When the battery control unit34identifies that a low temperature condition is present and/or a high charging rate condition is present the battery control unit34initiates a thermal conditioning stage. The low temperature condition is defined as a battery pack temperature or individual batter cell temperatures below a predetermined threshold temperature, for example below approximately 25 degrees Centigrade. The high charging rate condition is defined as a battery pack charging rate above a predetermined current or above a predetermined power rate (Amp-hr) for a predetermined period of time. A high charging rate is commonly present when a full charge or at least approximately an 80% battery charge is achieved in approximately 30 minutes.

When the battery control unit34identifies that the low temperature condition is present and/or the high charging rate condition is present and initiates the thermal conditioning stage, an electrical current is connected to one or more flexible and configurable carbon nanotube (CNT) sheets36. According to several aspects, individual ones of the CNT sheets36are positioned on opposite faces of the foam layers20. For example, a first CNT sheet36′ is positioned between the foam layer20and a battery cell14″″ facing side of the foam layer20with the first CNT sheet36′ directly contacting the battery cell14″″. A second CNT sheet36″ is positioned between the foam layer20and a battery cell14′ facing side of the foam layer20with the second CNT sheet36″ directly contacting the battery cell14′. Because only a low voltage source can provide the electrical current, the electrical current is supplied by a power source38such as a 12 volt DC (direct current) battery or a 24 VDC (volt direct current) battery independently of the battery pack12. A positive terminal40of the CNT sheets36is connected to a positive terminal of the power source38, directing current flow through a CNT sheet positive busbar42, and a negative terminal44of the CNT sheets36is connected to a negative terminal of the power source38, directing current flow through a CNT sheet negative busbar46.

According to several aspects the CNT sheets36are available from Nanocomp Technologies, Inc. located in Merrimack, N.H., United States. The CNT sheets36can be made for example as sheet material of pure carbon nanotube non-woven material. The CNT sheets36can have a thickness of approximately 100 microns and a length in millimeters or greater.

Current flow through the CNT sheets36rapidly heats, for example within approximately one minute or less, to transfer heat energy to the adjoining battery cell. The various sensors32of the battery pack12such as temperature sensors identify when the battery cell temperature exceeds a predetermined minimum operating temperature, for example approximately 25 degrees Centigrade. The battery control unit34then opens the circuit between the power source38and the CNT sheets36to stop battery cell heat-up. CNT sheets36of the present disclosure are capable of increasing from approximately 7 degrees Centigrade (room temperature) to approximately 130 degrees Centigrade in approximately one minute. CNT sheets36of the present disclosure therefore provide a rapid temperature increase of approximately 10 to 20 degrees Centigrade within several seconds (approximately 2 to 10 seconds) of being energized, thereby providing rapid heat-up of the battery cells14of the battery pack12. This temperature increase of the battery cells14helps mitigate against lithium plating during rapid charging events and rapid battery power draw events when low temperature conditions are present.

According to an exemplary configuration of a battery cell thermal conditioning system10for a vehicle shown, the battery pack12has multiple battery cells14, for example also sequentially numbered 01, 02, 03, 04, and the like. Foam layers20are individually positioned between first successive ones of the battery cells, such as for example between battery cells01and02and between first successive cells09and10. A first carbon nanotube sheet such as carbon nanotube sheet36′ is positioned in direct contact with one of the battery cells, such as battery cell09on a first side of the foam layer20and a second carbon nanotube sheet such as the carbon nanotube sheet36″ positioned in direct contact with a different one of the battery cells, such as battery cell10on a second side of the foam layer20. A first cooling plate16′ is positioned between second successive ones of the battery cells such as between battery cells08and09and a second cooling plate16″ is also positioned between second successive ones of the battery cells such as between battery cells10and11. A controller110directs current flow to the first carbon nanotube sheet36′ and the second carbon nanotube sheet36″ when a temperature of the one of the battery cells such as the battery cell09or the battery cell10contacted by the carbon nanotube sheet36drops below the predetermined threshold temperature.

Referring toFIG. 2and again toFIG. 1, a graph48identifies exemplary pulse width modulation (PWM) voltage signals for a 0% current flow50over time, a 25% current flow52and a 100% current flow54to individual or all CNT sheets36. A common period56can be induced for the PWM voltage signals by the battery control unit32to maintain consistent battery cell heat-up rates. The different voltage signals inducing different current delivery to the CNT sheets36enables different heat-up rates depending for example on different predetermined ambient or battery pack temperature thresholds. For example, a higher current flow such as the 100% current flow54can be generated when battery pack temperatures are below approximately zero degrees Centigrade and a lower current flow such as the 25% current flow52can be generated when battery pack temperatures are originally above or rise above 10 or 15 degrees Centigrade.

Referring toFIG. 3and again toFIG. 2, an exemplary battery cell pair58of the battery pack12includes a first metal plate16adefining a cooling plate and a second metal plate16bdefining a cooling plate. A first face60of a first battery cell14adirectly abuts the first metal plate16ato provide conductive cooling of the first battery cell14a.The first battery cell14aincludes a positive tab22aand a negative tab26a.The positive tab22aand the negative tab26aare indicated to extend from a common side of the first battery cell14a,however according to other aspects, the positive tab22aand the negative tab26acan extend from either opposite or successive ones of the sides of the first battery cell14a.A second face62of the first battery cell14ais oppositely directed with respect to the first face60and directly abuts a first CNT sheet36a.A first surface64of a foam layer20ais positioned to abut directly against the first CNT sheet36a.A second surface66of the foam layer20aoppositely directed with respect to the first surface64is positioned to abut directly against a second CNT sheet36b.

A first face68of a second battery cell14bdirectly abuts the second CNT sheet36bopposite to the foam layer20a.The second battery cell14bincludes a positive tab22band a negative tab26b.The positive tab22band the negative tab26bare indicated to extend from a common side of the second battery cell14b,however similar to the first battery cell14a,according to other aspects, the positive tab22band the negative tab26bcan extend from either opposite or successive ones of the sides of the second battery cell14b.A second face70of the second battery cell14bdirectly abuts the second metal plate16bto provide conductive cooling of the second battery cell14b.According to further aspects either the first CNT sheet36aor the second CNT sheet36bcan be omitted if reduced battery pack heating provided by a single one of the CNT sheets positioned about the foam layer20ameets the desired temperature threshold.

Referring toFIG. 4and again toFIGS. 2 and 3, a diagram of a flow control program72identifies control steps for operation of the battery cell thermal conditioning system10. In a first step74a battery charging rate is determined to identify if a fast charging operation is being performed. In a parallel second step76battery pack and/or ambient temperature conditions are determined using vehicle sensors (not shown) and battery pack temperature sensors such as sensors32to identify if a predetermined low temperature threshold is met at which current flow can be initiated to the CNT sheets36. Also, in parallel with the first step74and the second step76, in a third step78an operating schedule is determined. The operating schedule includes determination of an immediate high battery demand which follows a period of non-operation of the vehicle, for example when the vehicle is not operated overnight and when the ambient temperature has dropped below an optimum operating temperature of the battery pack12but has not dropped to the predetermined low temperature threshold, however the vehicle operator demands rapid vehicle acceleration upon startup. This operating schedule also demands current flow be initiated to the CNT sheets36due to the sudden high-power demand on the battery pack12which can also result in Lithium plating.

Following the first step74, the second step76and the third step78, in a fourth step80a battery pack fuel cell stack temperature is determined. The battery pack fuel cell stack temperature is calculated using output signals received from the various temperature sensors32in a fifth step82. In a subsequent sixth step84a determination is made if the battery pack or fuel cell stack is at a safe operating temperature, defined as being above the predetermined low temperature threshold. If a response86to the determination performed in the sixth step84is YES, in a seventh step88the flow control program72stops. If a response90to the determination performed in the sixth step84is NO, a command step92is performed to activate current flow to the CNT sheets36. In a following seventh step94a determination is made if a predetermined setpoint temperature of the battery pack or fuel cell stack has been achieved at which current flow to the CNT sheets36is stopped. If a response96to the determination performed in the seventh step94is NO, the predetermined setpoint temperature has not yet been reached and the program returns to the command step92. If a response98to the determination performed in the seventh step94is YES, the predetermined setpoint temperature has been reached and in a eighth step100the flow control program72stops.

Referring toFIG. 5and again toFIGS. 1 through 4, according to several aspects, in lieu of positioning one or more of the CNT sheets36directly against one or more of the foam layers20, one or more CNT sheets104which operate similar to the CNT sheets36are positioned at an inner wall of a housing106which contains the battery pack12. Similar program steps are performed as previously described in reference to the flow control program72. If the battery pack or fuel cell stack is at or below the safe operating temperature, current flow to the one or more CNT sheets104is initiated which provide a source of radiant heat within the housing106. The CNT sheets104radiate energy which heats an inner atmosphere108of the housing106and radiantly heats the battery pack12. Additional CNT sheets104can be positioned about any of the accessible inner walls of the housing106.

Referring again toFIGS. 1-5, thin, flexible and configurable carbon nanotube (CNT) based non-woven heating sheets are positioned in a battery pack between battery cells or against inner walls of the battery pack enclosure and are energized to heat up the battery cells through either direct contact or indirectly by radiant heating. The CNT sheets36,104are connected to a power source such as either a 12 VDC or 24 VDC battery. A current is applied to the CNT sheets and is controlled using a pulse width modulation (PWM) controller110such as described in reference toFIG. 2, with the PWM controller110provided with the battery control unit34or provided separately. The PWM controller110allows current to the CNT sheets36,104to be directly controlled within predetermined ranges and power levels to provide different heat-up rates for different temperature ranges of the battery pack.

A battery cell thermal conditioning system10of the present disclosure offers several advantages. These include thermal conditioning of a vehicle battery pack which allows for fast charging at low temperatures. Thermal conditioning helps mitigate lithium plating in lithium-ion battery cells at low temperature conditions, below a predetermined temperature threshold which can result in lithium plating. The present system operates with low energy consumption for the CNT heating sheet or pad. The present system also does not influence the cooling characteristics of the battery pack12. CNT sheets of the present disclosure are also provided of a lightweight material with substantially no additional packaging space required.