Method for temperature monitoring of a battery pack

Abstract

The present invention relates to a method for temperature monitoring of a battery pack with at least two cell stacks. The method comprises the steps of determining a temperature using a temperature sensor at a defined location in the battery pack, determining a first power loss of a first cell stack, determining a second power loss of a second cell stack, and determining a temperature distribution in the battery pack based on a ratio of the first power loss, the second power loss, and the temperature at the defined location.

Description

Prior ArtThe present invention relates to a method for monitoring the temperature of a battery pack and to a battery pack.In order to operate battery cells in their intended temperature range, the temperature of the cells must be monitored. If a few temperature sensors are used, the warmest position is determined. This value is also used for derating functions and for shutting down the battery at excess temperature. Over the lifetime, a thermal hotspot within the battery pack can shift as a result of different aging of the battery cells, but also as a result of their electrical contacting. This can lead to thermal hotspots no longer being detectable by measurement technology, since temperature measuring elements cannot change their position over the lifetime of the battery. It can happen that not all cells are held in the provided temperature window.Disclosure of the InventionThe method according to the invention having the features of claim 1 and the battery pack having the features of claim 9 have the advantage that a possible hotspot shift can be detected or estimated without further temperature measurements. According to the invention, this is achieved in that a temperature is determined by a temperature sensor at a defined location in the battery pack. Furthermore, a first power loss is determined in a first cell stack and a second power loss is determined in a second cell stack. A cell stack has at least one battery cell. A cell stack preferably has a multiplicity of battery cells connected in parallel. A temperature distribution in the battery pack is then determined as a function of the first power loss, the second power loss and the temperature at the defined point. The temperature distribution is preferably determined via a ratio of the first power loss, the second power loss and the temperature at the defined point. In this case, it can be assumed approximately that no heat exchange takes place into the environment (thermal resistance approximately infinite) and each cell stack considered has the same thermal capacity. The thermal capacitance Cth can be estimated by the heat Q supplied to a cell stack divided by the associated temperature change DeltaT: Cth=Q / DeltaT In this case, the heat Q is calculated via the product of power loss P_loss and time: Q=P_loss*t= The power loss of the cell stack is directly related to the heat generation in the respective cell stack. By ascertaining the power losses of the individual cell stacks, the heating thereof can be ascertained in each cell stack of a battery pack, by means of which heating a temperature distribution within the battery pack can be determined. In this case, an increased difference in the first and second power losses can occur with increasing aging, as a result of which the temperature distribution in the battery pack changes. The temperature distribution can preferably be determined on the basis of an analytically, numerically or experimentally determined relationship between the first power loss, the second power loss and the temperature at the defined point. For example, the battery pack can be measured at different resistance distributions or voltage drops.The dependent claims show further preferred developments of the invention.Preferably, the determination of the power loss comprises the determination of a cell stack resistance and the determination of a current flow of the respective cell stack. The power loss is calculated according to the following formula: P=I 2 xR. Thus, by determining the cell stack resistance and the current flow, the power loss of the individual cell can be determined in a simple manner. The current flow of the cell stack can be determined, for example, with the aid of a battery management system.Particularly preferably, the cell stack resistance is determined by the ratio of a determined voltage drop at the cell stack resistance to the current flow of the respective cell stack.In this case, the voltage drop is preferably determined by the difference of a known open circuit voltage of the cell stack from a measured cell voltage in the cell stack. The open-circuit voltage is a known characteristic variable of the cell stack which is present when no load is connected. The cell voltage is understood to mean the voltage which is present at a cell stack. The cell voltage cell stacks is preferably determined for the respective current flow by means of single cell voltage monitoring of the battery management system.Alternatively, the cell stack resistance is preferably determined from the ratio of a cell voltage difference to a current flow difference of the respective cell stacks. As a result, the cell stack resistance can also be determined without knowledge of the open circuit voltage.The cell voltage difference preferably describes the difference of a first cell voltage at a first point in time to a second cell voltage at a second point in time. In addition, the current flow difference preferably describes the difference of a first current flow at the first point in time to a second current flow at the second point in time.The temperature distribution particularly preferably describes the position of a thermal hotspot at which a maximum temperature is present in the battery pack. In the initial state of the battery pack, the temperature sensor is preferably arranged in the region of the thermal hotspot. Aging processes can alter the position of the thermal hotspot within the battery pack. Thus, the temperature of the thermal hotspot can no longer be determined directly by the temperature sensor. The indirect determination of the thermal hotspot with the aid of the temperature distribution enables reliable temperature monitoring of the battery pack.Preferably, in a further step, the current flow is throttled if the determined maximum temperature exceeds a defined temperature value. The power loss can be limited by throttling the current flow. Thus, safe operation of the battery pack can be enabled.The invention further relates to a battery pack which comprises at least a first cell stack, a second cell stack, a temperature sensor and a control unit. The control unit is configured to execute a method described above. The control unit is preferably configured to determine the cell voltages of the individual cell stacks.Further preferably, the temperature sensor is arranged centrally between the first cell stack and the second cell stack. The temperature can be dissipated more poorly between the first cell stack and the second cell stack, so that a thermal hotspot is formed there with increased probability. The arrangement of the temperature sensor in the vicinity of the thermal hotspot improves the accuracy of the determination of a maximum temperature.The temperature sensor is particularly preferably arranged at an initial thermal hotspot. The position of the initial thermal hotspot can be determined numerically, for example, and relates to a battery pack in the new state. Even in the case of an aging-dependent displacement of the thermal hotspot, the temperature sensor is still arranged in the vicinity of the thermal hotspot.Further preferably, the battery pack comprises at least four cell stacks which are arranged in at least two columns and two rows. Cell stacks in a column are connected in parallel and cell stacks in a row are connected in series.Brief Description of the DrawingsHereinafter, a preferred embodiment of the invention will be described in detail with reference to the accompanying drawings. In the drawing, the following is: FIG. 1 shows a schematic flow diagram of a method according to a first exemplary embodiment of the invention, and FIG. 2 shows a schematic illustration of a battery pack which is configured to carry out the method according to the first exemplary embodiment of the invention.Preferred Embodiment of the Invention.In the following, with reference to FIGS. 1 and 2, a method for temperature monitoring of a battery pack 1 and a battery pack 1 according to a preferred exemplary embodiment of the invention will be described in detail.In a step S 1, a temperature is determined by a temperature sensor 2 at a defined location in the battery pack 1. Alternatively, a plurality of temperatures may be detected by a plurality of temperature sensors 2 in the battery pack 1.In a step S 2, a first power loss P 1 of a first cell stack 11 is determined and in a step S 3, a second power loss P 2 of a second cell stack 12 is determined. The first power loss P 1 and the second power loss P 2 can be determined, for example, by ascertaining a voltage drop U 1 at the cell resistor to a current flow I. The power losses P are preferably determined in all cell stacks of the battery pack 1.Steps S 1, S 2, and S 3 may be performed in parallel or sequentially.Subsequently, in a step S 4, a temperature distribution on the battery pack is determined. The determination of the temperature distribution is based on a ratio of the first power loss P 1, the second power loss P 2 and the temperature at the defined point. The ratio of the first power loss P 1, the second power loss P 2 and the temperature at the defined point to the temperature distribution in the battery pack 1 can be determined numerically, analytically or experimentally, for example. The temperature distribution provides information about a thermal hotspot 4 in which a maximum temperature is present in the battery pack 1.If the determined maximum temperature exceeds a defined temperature value, the current flow I can be throttled in a step S 5. The current flow I of the entire battery pack 1 can be throttled. In this case, the current flow I in the cell stack in which a thermal hotspot 4 is determined is preferably throttled. The throttling of the current flow I reduces the power loss in the cell stack, as a result of which the temperature development in the cell stack is reduced.FIG. 2 shows a battery pack 1 having a plurality of cell stacks, the temperature sensor 2 and a control unit 3.The cell stacks are arranged in two columns and three rows in FIG. 2. In the second row, a first cell stack 11 and a second cell stack 12 are shown in a column in each case. A third cell stack 13 and a fourth cell stack 14 are arranged in the first row, and a fifth cell stack 15 and a sixth cell stack 16 are arranged in the third row. The first cell stack 11 and the second cell stack 12, the third cell stack 13 and the fourth cell stack 14 and the fifth cell stack 15 and the sixth cell stack 16 are connected in parallel with one another.A temperature sensor 2 is arranged centrally in the battery pack 1 between the first cell stack 11 and the second cell stack 12. In this case, the battery pack 1 has a thermal hotspot 4 between the first cell stack 11 and the second cell stack 12 in the region of the temperature sensor 2. The thermal hotspot can shift over time as a result of the aging of the cell stacks of the battery pack 1.The temperature sensor 2 is connected to the control unit 3 via an electrical line. The control unit 3 is configured to determine the cell voltages U 3 of the individual cell stacks. The temperature sensor 2 is preferably an NTC resistor.As is schematically shown in the first cell stack 11, the cell stacks have a cell resistor R in which a voltage U 1 drops. The voltage drop U 1 can be determined by the difference of a known open circuit voltage U 2 of the first cell stack 11 from a measured cell voltage U 3 to the current flow I at the first cell stack 11. The cell voltage U 3 is determined at the same time as the current flow I and preferably measured at the contacts of the first cell stack 11.Due to aging phenomena, for example, the electrical contacting of the first battery cell 11 may deteriorate, so that the cell resistance R of the first battery cell 11 increases. The cell resistance R can be determined by the ratio of a determined voltage drop U 1 at the cell resistance R to the current flow I of the first battery cell 11.As a result of the increase in the cell resistance R on account of the alternating electrical contacting or cell chemistry in the first battery cell 11 and the thus increased cell resistance R in the first battery cell 11, the first power loss P 1 of the first battery cell 11 increases at a constant current flow I. The power loss is emitted in the form of heat to the battery pack 1. As a result of the first power loss P 1 in the first battery cell 11 that has ceased in comparison with the second power loss P 2 in the second battery cell 12, the first battery cell 11 heats up to a greater extent, with the result that a thermal hotspot migrates in the direction of the first battery cell 11.If the maximum temperature in the thermal hotspot 4 in the region of the first battery cell 11 exceeds a defined temperature value, the current flow I through the first battery cell 11 can be limited. Thus, overheating of the first battery cell 11 in the battery pack 1 is prevented.

Claims

Method for temperature monitoring of a battery pack (1) having at least one first cell stack (11) and one second cell stack (12), comprising the steps of - determining (S1) a temperature by a temperature sensor (2) at a defined location in the battery pack, - determining (S2) a first power loss (P1) of the first battery cell (11), - determining (S3) a second power loss (P2) of the second battery cell (12), and - determining (S4) a temperature distribution in the battery pack (1) as a function of the first power loss (P1), the second power loss (P2) and the temperature at the defined location.Method according to Claim 1, wherein the ascertainment of the power loss (P) comprises the ascertainment of a cell stack resistance (R) and the ascertainment of a current flow (I) of the respective cell stack, wherein the power loss (P) is calculated according to the following formula: P=I 2 R.Method according to Claim 2, wherein the cell resistance (R) is determined by the ratio of a determined voltage drop (U1) at the cell stack resistance (R) to the current flow (I) of the respective cell stack.Method according to Claim 3, wherein the voltage drop (U1) is determined by the difference of a known open-circuit voltage (U2) of the cell stack from a measured cell voltage (U3) at the cell stack.Method according to Claim 2, wherein the cell stack resistance (R) is determined from the ratio of a cell voltage difference (U4) to a current flow difference (I4) of the respective cell stack.Method according to claim 5, wherein the cell voltage difference (U4) describes the difference of a first cell voltage (U4.1) at a first point in time (t1) from a second cell voltage (U4.2) at a second point in time (t2), and wherein the current flow difference (I4) describes the difference of a first current flow (14.1) at the first point in time (t1) from a second current flow (I4.2) at the second point in time (t2).Method according to one of the preceding claims, wherein the temperature distribution determines the position of a thermal hotspot (4) at which a maximum temperature is present in the battery pack (1).Method according to claim 7, comprising the step (S5): restricting the current flow (I) when the determined maximum temperature exceeds a defined temperature value.Battery pack comprising a first cell stack (11), a second cell stack (12), a temperature sensor (2) and a control unit (3), wherein the control unit (3) is configured to carry out a method according to one of the preceding claims.The battery pack of claim 9, wherein the temperature sensor (2) is disposed centrally between the first cell stack (11) and the second cell stack (12).The battery pack according to any one of claims 9 or 10, wherein the temperature sensor (2) is disposed at an initial thermal hotspot (4).The battery pack according to any one of claims 9 to 11, wherein the battery pack (1) comprises at least four cell stacks arranged in at least two columns and at least two rows.

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