The present invention relates to an electrical contact element for a plug connector, a corresponding plug connector and to a method for monitoring the temperature at a plug connector during an electrical current flow. The present invention is advantageous in connection with a charging cable for connecting a battery of a motor vehicle to a voltage source, in particular applicable for charging an electric vehicle together with a charging control unit for controlling a charging operation of a battery in a motor vehicle. The principles of the present invention can, however, also be used

advantageously for any other type of temperature-monitored transmission of current.

Electric vehicles can be charged in various charging modes. At present, modes 1 to 5 are defined. These vary, inter alia, in relation to safety devices, communication with the vehicle and charging performance. Essentially a distinction is made between charging by means of alternating current (modes 1 to 3), which can also take place via a common household power supply, but takes a relatively long time, and substantially more rapid direct-current charging (mode 4 and mode 5). At present, so-called CCS plugs ("Combined Charging System") exist, for example, which enable both charging types and support a power of 350 kW. Electric vehicle batteries can thus be charged to 80 per cent within a few minutes at appropriate charging stations by means of direct current. For comparison: At a plug socket in one's own garage, the charging of a conventional electric vehicle battery takes seven to eight hours. Future direct current charging plugs must safely transmit even higher power levels of, for example, up to 500 kW.

In order to make sure that the contact elements of the plug apparatus cannot overheat, the temperatures of the connector, cable and the other power components must be monitored in all rapid DC charging systems. For operation with currents above 200 A, coupler and plug must be equipped with independent means for a permanent monitoring of temperature in all DC contacts. If one of the DC contacts reaches a temperature of 90°C or higher, at least one sensor at the plug and at the coupler must forward this to an evaluation unit within 30 seconds. In the case of a plug connector, the contact zone represents the hottest and thus the region most at risk. Flowever, there is no practicable possibility of detecting the temperature directly in the contact zone.

By way of example, it is known from DE 10 2016 107401 A1 to provide an electrically conductive contact element with a contact region for producing a contact to a complementary contact element and with a connection region for the connection of an electrical line and at least one temperature sensor, which detects the temperature in a measurement region of the contact element, which lies between the aforementioned connection region and the contact region.

It can be shown, however, that the heat which arises in the contact region is transmitted so slowly in the direction of the measurement region that the temperature detected in the measurement region does not reach critical values until the contact region has already overheated to dangerous levels. A separation of 20 mm between the temperature sensor and the contact region is already too large to still satisfy the requirements of the corresponding safety regulations.

The temperature in the contact region thus would have to be extrapolated from the temperature in the measurement region. However, in order to derive a reliable model for calculating an estimated value for the temperature in the contact region from the temperature in the measurement region, and thus to predict at which measured temperatures a reduction in, and/or interruption of, the power transmission must be undertaken, too many heat equations must be solved. In particular, a temperature sensor arrangement which is located in a single measurement region along the longitudinal axis of the contact element, cannot differentiate between the effects which come from the contact region, and consequently the heat source, and the effects which come from the connection region, i.e. the heat sink. The influences of the cable on the connection region can vary dramatically in the application environment, such that a prediction is plagued by dangerous uncertainties.

There is therefore a requirement for means which permit a safe and precise monitoring of the temperature in a contact region of a plug connector, and thus enhance and simplify the monitoring of an electrical current flow in particular when charging the battery of an electric vehicle.

This problem is solved by the subject-matter of the independent claims. Advantageous developments of the present invention are the subject-matter of the subclaims.

The present invention is based on the idea of placing two temperature sensors at the contact element along the course of the heat flow. As a result, the thermal resistance between the two temperature sensors (also referred to as temperature probes hereinbelow) can be determined via a calibration step and thus the number of thermal unknowns can be reduced. If the thermal resistance between the two temperature sensors is known, the output signals of the two temperature sensors can be evaluated in their temporal profile (for example in discrete time steps) and the size and direction of the heat flow and the temporal change thereof can be calculated therefrom. The thermal resistances for the heat conductivity at the transition between the contact element and a corresponding mating contact element and for the connection zones (for example a crimp connection) are dependent on the design and can likewise be ascertained by calibration. It can be shown that, for a reliable prediction of the temperature in the contact region, moreover only the influence of the thermal convection losses to the environment is still important, but this can be taken into account for example via the ascertaining of the ambient temperature.

In particular, an inventive contact element for a plug connector has an electrically conductive base body, which can be connected in an electrically conductive manner to an associated mating contact element, the base body having a longitudinal axis which runs along a plugging direction between the contact element and the mating contact element. According to the present invention, at least one first temperature probe and at least one second temperature probe are provided which, for detecting the temperature, are arranged at two different measurement regions of the contact element spaced apart from each other along the longitudinal axis of the contact element.

Such a contact element makes it possible, in a particularly simple and reliable manner, to predict the temperature in a region of contact to the mating contact element, which is situated away from the two measurement regions, and thereby to recognise dangerous overheating in a timely manner.

The arrangement according to the invention can be implemented mechanically (and computationally for the model formation) in a particularly simple way, when the contact element has a contact pin and the mating contact element has a contact bushing. Naturally, however, the principles according to the invention can also be used when the contact element has a contact bushing. Furthermore, more than just one first and one second temperature probe can also be provided, which are arranged on the same or further offset measurement regions, in respect of the longitudinal axis. In a particularly advantageous manner, the present invention can be used for contact pins, which are used in a charging socket of an electric vehicle.

For ascertaining an estimated value for predicting the temperature in the contact region, a value for the temperature difference between the temperature probes is also necessary in addition to the absolute temperatures of the temperature probes. In a particularly advantageous manner, the principles according to the invention can be implemented if at least one of the temperature probes has a thermocouple. Thermocouples are based on the so- called thermoelectric effect: If two wires of different materials are connected, a voltage can be measured at their free ends if the connection point is at a different temperature than these free ends. The temperature difference is always measured between the temperature at the connection point and the temperature at the terminals (clamps) of a measuring appliance. The thermoelectric effect is based on a material-specific property of electrically conductive materials. In the interior of a conductor, the temperature effect causes a shift in the electron density (volume diffusion effect), when a temperature change (rise or fall) occurs via the conductor. In mathematical terms, this change is referred to as a temperature gradient. At the hot end, owing to the higher kinetic energy, an impoverishment occurs and at the cold end an enrichment of the charge carrier occurs. Each conductor piece is a voltage source for itself alone. The arrangement of two connected wires is called a thermocouple or thermopair. Only the difference in voltage sums in the wires of different materials produces a measurable voltage, which is a measure for the temperature difference between the connection point and the clamps of the measuring appliance.

Depending on the material combination, there arises a reproducible dependency of the

thermoelectric voltage on the temperature at the connection point. From the multiplicity of possible wire pairings (over 300 material pairings for the temperature measurement are known), some are used for industrial temperature measurement. The thermoelectric voltages here lie in the range from 5 mV/°C to approximately 100 mV/°C. The materials suitable for industrial temperature

measurement are standardised in DIN EN 60584-1 (DIN EN 60584-1:2014-07, title (German):

Thermoelemente - Teil 1: Thermospannungen und Grenzabweichungen [Thermocouples - part 1: thermoelectric voltages and limit deviations] (IEC 60584-1:2013); German version EN 60584-1:2013. The thermoelectric voltages specified in this standard always relate to a reference point temperature of 0°C.

The materials suitable for temperature measurement can be depicted mostly in relation to platinum as reference metal, ordered according to thermoelectric voltage in a thermoelectric voltage series, as is commonly known to a person skilled in the art. Examples of thermocouples are platinum/rhodium- platinum (Pt/RhPt) type S, type R, nickel-chromium/nickel (NiCr/Ni) type K or copper/constantan (Cu/CuNi) type T. The present description of the advantageous embodiments is limited to the exemplary use of thermocouples as temperature probes. For a person skilled in the art, however, it is clear that every other type of temperature sensors can also be used as a temperature probe according to the present invention, for example resistance sensors such as PtlOO or NilOO elements, and also resistance elements with negative or positive temperature coefficients (NTC or PTC elements).

A particularly precise prediction of the temperature in the contact region can be ensured if the two temperature probes are placed in such a way that the base body has a smaller thermally conductive cross-sectional area in the two measurement regions than in the other regions of the contact element. In this way, the sensor arrangement is able to detect the temperature gradient in the place where it is highest.

The present invention moreover relates to a plug connector for a charging cable, the plug connector having at least one contact element according to any one of the preceding claims. In particular, the plug connector according to the invention can be part of a charging socket of an electric vehicle.

According to an advantageous development of the present invention, arranged in the plug connector is an electronic evaluation and control unit, which is connected to at least one first and second temperature probe, and which, in a monitoring region of the contact element different from the measurement regions, serves to calculate an estimated value for the temperature prevailing there. Alternatively, provision can also be made for the electronic evaluation and control unit to be part of an "In-Cable Control- and Protecting Device" (IC-CPD) or a part of the vehicle electronics. Moreover, provision can also be made for both the plug connector and the mating plug connector to be equipped with temperature probes according to the present invention and also for the charging station to have a corresponding evaluation and control unit.

The plug connector according to the present invention comprises, according to an advantageous embodiment, a housing and a contact locking apparatus (not depicted in the figures), which fixes the contact to the housing, the base body having a circumferential groove between the two

measurement regions, into which groove the contact locking apparatus engages. Thus the temperature probes are arranged at a functional unit of the contact element, which unit normally does not change even in the case of different miscellaneous configurations of the contact element, and always represents the constriction with the strongest temperature gradient in the longitudinal direction. Finally, the present invention relates to a method for monitoring the temperature at a plug connector during an electrical current flow, which takes place through the plug connector, the method comprising the following steps:

(a) applying an electric current to a contact element, wherein the contact element is connected to an associated mating contact element,

(b) at a first time, ascertaining first temperature values at a first measurement region and at a second, spaced apart from the first measurement region, along the longitudinal axis of the contact element,

(c) calculating a first local temperature gradient from the first temperature values,

(d) at a second time, ascertaining second temperature values at the first and at the second measurement region,

(e) calculating a second local temperature gradient from the second temperature values,

(f) calculating a temporal change in the local temperature gradient,

(g) calculating an estimated value for a temperature in a monitoring region of the contact element different from the measurement regions on the basis of the temporal change in the local temperature gradient,

(h) outputting the estimated value to an evaluation unit.

For the calculation of the estimated value, the difference between the two first and the two second temperature values and also an absolute value for at least one each of the two first and the two second temperature values is necessary.

The estimated value can be compared to a target value, for example, the electrical current flow being regulated in such a way that the estimated value does not exceed a predetermined deviation from the target value. Alternatively, a comparison of the estimated value with a threshold value can also be provided. Such an estimated value can, for example, predict the temperature occurring at the contact region between the contact element and the mating contact element. Thus, by measuring the two temperature values in the first and second measurement regions, it can be ensured that an imminent overheating in the contact region, for example, is prevented. According to the invention, a warning signal for example is generated for interrupting or reducing the current flow, when the estimated value exceeds the threshold value, and/or the extrapolated contact temperature is made available to the control appliance.

For example, a particularly simple excess temperature shut-down can be implemented, by using two threshold values. If, on the one hand, a first threshold value is set for the temperature difference and, on the other hand, a second threshold value is specified for the absolute temperature in the first measurement region, it is possible, by appropriate selection of the threshold values, to reach conclusions on the attainment of a certain critical temperature within a predetermined time from the attainment of the threshold values. For example, the contact element can be made such that, when a difference T1-T2 is greater than or equal to 14°C and an absolute temperature of T1 is greater than or equal to 50°C, it can always be predicted that a contact temperature Tcontact of more than 90°C prevails at the contact zone.

A further advantageous embodiment includes the temporal change in the temperature and the ambient temperature. This is necessary in particular in the case of a temperature-controlled charging operation in mode 5.

In an advantageous way, the above steps (b) to (h) are repeated until the estimated value exceeds the threshold value, or the connection between the contact element and the mating contact element is cut. In this way, monitoring can take place during the entire current transmission process.

In order to take into account the influence of the thermal losses to the environment, and to thereby make the calculation of the estimated value even more accurate, provision can be made, moreover, for the ambient temperature, remote from the at least one contact element, to be detected and to be entered into the calculation of the estimated value. The detection of the ambient temperature can take place, for example, by means of a temperature probe, which is an integral component of an electronic module, which contains the evaluation and control unit, or else can be made available by the vehicle via a data bus system. To better understand the present invention, it is explained in greater detail using the exemplary embodiments depicted in the following figures. Identical parts here are provided with identical reference numbers and identical component names. Furthermore, individual features or combinations of features from the shown and described embodiments taken separately may represent independent inventive solutions or solutions according to the invention.

In the drawings:

Figure 1 shows a schematic representation of a contact pin in accordance with the present invention and its thermal equivalent circuit diagram;

Figure 2 shows a schematic representation of a measurement circuit for detecting the

temperature at the first and second measurement regions of the contact pin;

Figure 3 shows a simplified flowchart of a method in accordance with the present invention for monitoring a charging operation.

The present invention is explained in greater detail hereinbelow with reference to the figures, and firstly with reference to Figure 1.

Figure 1 shows a contact pin 100, as used in a charging cable plug connector for charging the battery of an electric vehicle, and the thermal equivalent circuit diagram 102 of this contact pin 100. In order to transmit an electric current, the contact pin 100 (hereinbelow also referred to as a pin) is connected to a corresponding mating contact element, i.e. to a contact bushing (not shown in the figures). The electrical transition between the contact pin 100 and the contact bushing takes place in the contact region 104.

The contact pin 100 has a base body 105 and furthermore a connection region 106, which in the present case is formed as a crimp connection for the connection of a cable. According to the invention, the contact pin 100 has a first measurement region 108 and a second measurement region 110, which have a known spacing d from each other in respect of a longitudinal axis 112 of the base body 105. The longitudinal axis 112 runs along a plug-in direction of the contact pin 100 into the associated bushing (not shown). A first temperature probe is placed in the first measurement region 108, and a second temperature probe is placed in the second measurement region 110 (the temperature probes are not shown in Figure 1). Advantageously, the two measurement regions 108, 110 are placed in a region with the smallest cross-section of the contact pin 100.

Figure 1, moreover, shows the thermal equivalent circuit diagram of the contact pin 100. In the case of such a thermal equivalent circuit diagram, the heat flow Q corresponds to the electric current of an electrical circuit diagram, while a temperature difference should be equated to the electric voltage in an electrical circuit diagram. As can be seen in the thermal equivalent circuit diagram 102 shown in Figure 1, the section between the two measurement regions 108, 110 has a thermal resistance R _k n _o wn, which can be ascertained by means of appropriate calibration measurements. This known thermal resistance R _k n _o wn can therefore be used for the thermal modelling of the pin 100.

In the equivalent circuit diagram 102, the transition to the mating contact element forms a heat source 114 with the thermal resistance R _{cond co} n _tact , which corresponds to the heat conduction through the mating contact element, and the heat capacity C _contact of the mating contact element, while the thermal resistance R _cabie and the heat capacity C _cabie of the cable represent a heat sink 116. The thermal resistance R _{cond P} m, which corresponds to the heat conduction through the pin, and the heat capacity C _P m of the pin reflect the material properties and the design of the base body 105. In a corresponding manner, the thermal resistance R _Co n _{d c} rim _P , which corresponds to the heat conduction through the crimp connection, and the heat capacity C _C rim _P of the crimp connection reflect the material properties and geometry of the crimp connection. These material and design variables are identical for all contact pins in a series, and can be ascertained by means of suitable calibration methods and integrated into the model.

If the absolute temperature at the first node point 113 and the temperature difference T1-T2, which drops over the resistance R _k n _o wn, are known, the heat flow which flows through the resistance R _k n _o wn can be calculated. Assuming that the heat capacity C _P m of the contact pin is completely saturated, the heat flow through the resistance R _¥ n _{d Pi} n is identical to the heat flow through R _k n _o wn. This is the sought heat flow Q at the second node point 115, provided that the influence of the convection losses to the environment is disregarded.

The thermal resistance R _CO m _{/ ioss} symbolises the convection losses to the environment. If the ambient temperature Ta is known, then this value also can be modelled.

In this way, the value of the contact temperature can be predicted from the values of the absolute temperature at the first node point 113 and the temperature difference T1-T2.

According to the invention, a first thermocouple which ascertains the first temperature value T1 is placed at the first measurement region 108. A second thermocouple which ascertains a second temperature value T2 is arranged at the second measurement region 110. The thermocouples are welded onto the base body 105, for example. In an advantageous manner, such thermocouples can be produced with a very low parameter variance, such that the measurement of the two values T1 and T2 can take place with largely matching accuracy. As a result of the fact that the two

thermocouples are attached directly onto the base body 105 and have a low heat capacitance themselves, low thermal coupling can be ensured between the two measured temperature values T1 and T2.

Fig. 2 shows an example of an evaluation circuit 118, which can be connected to a first and a second thermocouple 120, 122, in order to ascertain the heat flow sought.

The two thermocouples are connected to the analogous input terminals 126 of the evaluation circuit 118 via a filtering and stabilising circuit 124, for example. An input multiplexer 128 makes it possible to measure either four individual or two differential signals. The respectively selected signals are fed to an amplifier 130. The amplified signal is applied to the input of an analogue/digital converter 132, e.g. a delta-sigma analogue/digital converter. A serial interface 134 produces the connection to the digital terminals 136. A voltage reference 138 and an oscillator 140 are likewise provided.

Furthermore, the evaluation circuit 118 has an internal third temperature sensor 142, which measures the temperature of the evaluation circuit 118 and thus an ambient temperature Ta.

The evaluation circuit 118 can be formed, for example, by a delta-sigma analogue/digital converter of the ADS1118 type from the manufacturer Texas Instruments.

According to the present invention, with the aid of this temperature detection by means of the temperature sensor 142, on the one hand a cold junction compensation (CJC) can be carried out, and on the other hand a value for the ambient temperature Ta can be made available, which is required for the modelling of the pin 100.

In an advantageous manner, the evaluation circuit 118 according to the invention makes it possible to carry out the A/D conversion of the values for the temperatures Tl, T2 and Ta on the high-voltage side, so that only digital signals from the high-voltage side have to be transmitted securely for low- voltage signal processing. Digital information can be transmitted substantially more cost-effectively than analogue signals in a galvanically separated way via optical or magnetic isolation. Moreover, a diagnosis unit (not shown in the figures) integrated in the evaluation circuit makes it possible to make safety-relevant diagnosis data available such as, for example, the determination of a wire breakage or a short-circuit at the temperature probes and thus makes self-diagnosis of the plug connector possible.

If, in the case of a plug connector which is part of a charging cable, it is envisaged to use two evaluation circuits 118, namely an evaluation circuit for the DC+ pin and an evaluation circuit for the DC- pin respectively, an exceptionally high safety level (for example Automotive Safety Integrity Level C (ASIL C) according to ISO 26262-1:2011 "Road vehicles -- Functional safety") can be achieved for the temperature monitoring.

Fig. 3 illustrates, in the form of a flowchart, the individual steps which are carried out in the case of the temperature monitoring according to the invention, during a charging operation, for example of a battery of an electric vehicle.

After the charging operation has been started, and a current flow is taking place (step SI), the temperatures Tl, T2 and Ta are ascertained at a first time tl (step S2). A local temperature gradient can be calculated (step S3) from the amount and the sign of the difference between Tl and T2.

Subsequently, the temperatures Tl, T2 and Ta are ascertained (step S4) at a second time t2 and the temporal change in the temperature gradient is ascertained (step S5) from the comparison of the two local temperature gradients. The heat flow through the contact pin 100 can be calculated (step

56) from this temporal change in the temperature gradient according to amount and sign.

Lastly, this determination of the heat flow Q corresponds to a calorimetry, in which the fed-in temperature difference is ascertained from the heat flow.

With the aid of the heat flow Q and the thermal resistances and thermal capacitances known from the equivalent circuit diagram 102, a predicted temperature value Tcontact can be calculated (step

57) for the temperature of the contact zone.

The predicted temperature value Tcontact is outputted to an evaluation unit in step 8 and can now be compared with a stored limit value Tgrenz for the temperature at the contact zone. If the predicted temperature value Tcontact has reached or exceeded the threshold value Tgrenz, the charging operation can be aborted, for example, and/or further safety-relevant measures can be initiated. Provision can also be made for regulation of the charging current density to be carried out, which ensures that the calculated temperature in the contact zone remains around a desired target temperature within a predetermined tolerance band.

If no criteria for ending the charging operation are recognised as fulfilled, the monitoring method returns to step S2 and the method is run through again. Otherwise, the charging operation is ended. Criteria for ending the charging operation, besides the emergency shut-down signals generated by the temperature monitoring, are also criteria for a regular shut-down, for example when the battery is sufficiently charged.

Through the use of the method according to the invention, in particular in the case of ultra-rapid charging operations for electric vehicles, it can be ensured that the temperature in the contact region not accessible for direct measurement can be monitored sufficiently safely and accurately and, if required, can even be regulated.

The principles of the present invention can be used in further advantageous variations. By way of example, the contact element itself can form part of the thermocouple. For this purpose, the thermocouple copper/constantan (Cu/CuNi) type T is particularly suitable, because the contact elements are frequently produced from copper. For this configuration, it is necessary to provide sufficient electrical insulation between the high-voltage side and the evaluation electronics.

In an advantageous manner, the first temperature probe has a first thermocouple, which comprises a first conductor and a second conductor, the first and the second conductor being produced from different materials and being connected to each other at a first measurement point, and the second temperature probe has a second thermocouple, which comprises a third conductor and a fourth conductor, the third and the fourth conductor being produced from different materials and being connected to each other at a second measurement point, the first measurement point and the second measurement point being in abutment with the base body of the contact element. In this way, a measuring arrangement which is reliable and robust can be implemented in a particularly simple and space-saving manner. In particular, the first and the fourth conductor can be produced from the same material. In this way, the series connection of the two thermocouples can be wired firmly, so that a line can be saved at a common terminal of the two thermocouples.

If a thermocouple is chosen, the one material of which matches the material of the contact element, the construction can be simplified even further, by forming at least one of the first to fourth conductors through the base body of the contact element. The abovementioned thermopair copper/constantan (type T) is suitable in this context, if the contact element is produced from copper (at least in the measurement region).

By way of example, two sensors are used which have opposing characteristic curves of their output signals as a function of the temperature. Such sensors can be connected in series, in order to ascertain a differential temperature. By way of example, two thermocouples of equal pairing can be used, which are connected to each other in a diametrically opposed manner in order to measure the total voltage. If this series circuit is wired firmly, a line can be saved at the common terminal of the two thermocouples. However, it must be ensured that, moreover, a measurement value for the absolute temperature is also ascertained at one of the two temperature probes.

In the illustration in Figure 1, the material of the base body 105 can be included in the temperature measurement, by it being part of the first and/or second thermopair. By way of example, the base body 105 of the contact pin 100 can be composed (at least partly) of copper at least in the measurement regions 108, 110, such that a first constantan (CuNi) wire placed in the first measurement region 108 forms a first thermocouple with the base body 105 and a second constantan wire placed in the second measurement region 110 forms a second thermocouple. The voltage which can be tapped off between the first and the second constantan wire forms the temperature difference between the first and the second measurement regions 108, 110.

List of reference symbols: