Method and apparatus for diagnosing isolating relays in a battery charger with an AC power supply connection

Abstract

The invention relates to a method for testing isolating relays (91-94) in a battery charger (1), in particular in an onboard battery charger for a motor vehicle, wherein the battery charger (1) has an AC / DC converter (2) which can be supplied and operated via phase connections (L1, L2, L3) and a neutral connection (N) to selectively supply an intermediate circuit voltage (U). z) to connect or disconnect from the phase terminals (L1, L2, L3) and the neutral terminal (N), wherein voltage measuring devices (101, 102, 103) are provided for each phase terminal (L1, L2, L3) to measure a voltage at the respective phase terminal (L1, L2, L3), wherein one of the isolating relays is provided between the terminals of the voltage measuring devices (101, 102, 103) and the AC / DC converter (3) as a phase terminal isolating relay (91 - 93) for the phase terminals (L1, L2, L3) or as a neutral terminal isolating relay (94) for the neutral terminal, wherein the following steps are carried out to diagnose the isolating relays: - Pre-charging (S2) of the intermediate circuit capacitor (5), in particular using a battery voltage (U) Bat ); - Select one of the phase connection disconnect relays (91 - 93) and the neutral conductor disconnect relay (94); - Operating the AC / DC converter (3) to supply the intermediate circuit voltage (U z) between the phase terminal of the selected phase terminal isolating relay (91 - 93) and the neutral terminal (N), - Detecting a stuck-at fault by alternately activating to open and close the selected isolating relays (91 - 93, 94) depending on a voltage that can be measured with the corresponding voltage measuring device (101, 102, 103).

Description

Technical field

[0001] The invention relates to battery chargers, in particular onboard battery chargers for motor vehicles, which can be connected to an AC power supply network for charging a vehicle battery. The invention further relates to a diagnostic method for detecting faults in isolating relays with which the battery charger can be disconnected, either completely or selectively, from individual connecting lines of the AC power supply network. Technical background

[0002] Between the AC power supply lines and a battery charger for charging a vehicle battery, isolation relays of a relay matrix are typically installed to galvanically isolate the battery charger from the AC power supply. The connection lines, which comprise the individual phase lines and the neutral conductor, can be disconnected by appropriately controlling the isolation relays.

[0003] In accordance with legal requirements, every functional component installed in the automotive sector must be checked for its function using a diagnostic procedure.

[0004] With isolation relays, so-called "stuck-at" errors can occur, which cause the isolation relays to be permanently open or closed regardless of their control signal.

[0005] Diagnostic procedures are therefore necessary to check the functionality of the isolation relays. To test the relays using a diagnostic procedure, additional voltage sensors are currently provided, which are arranged between the relay and an AC / DC converter circuit of the battery charger.

[0006] It is therefore an object of the present invention to provide an improved diagnostic method for the function of isolating relays in a battery charger for connection to an AC power supply network, with which the additional voltage sensors on the isolating relays are not required. Disclosure of the invention

[0007] According to the invention, a method for diagnosing isolating relays in a battery charger for providing a battery voltage from an AC power supply network according to claim 1 and a corresponding device according to the dependent claim are provided.

[0008] Further details are specified in the dependent claims.

[0009] According to a first aspect, a method for testing isolating relays in a battery charger, in particular in an onboard battery charger for a motor vehicle, is provided, wherein the battery charger has an AC / DC converter which can be supplied and operated via phase terminals and a neutral terminal to selectively apply or disconnect an intermediate circuit voltage to the phase terminals and the neutral terminal, wherein voltage measuring devices are provided for each phase terminal to measure a voltage at the respective phase terminal, wherein one of the isolating relays is provided between the terminals of the voltage measuring devices and the AC / DC converter as a phase terminal isolating relay for the phase terminals or as a neutral terminal isolating relay for the neutral terminal, wherein the following steps are carried out to diagnose the isolating relays: - Pre-charging the intermediate circuit capacitor, in particular using a battery voltage; - Select one of the phase connection disconnect relays and the neutral conductor disconnect relay; - Operating the AC / DC converter to apply the intermediate circuit voltage between the phase terminal of the selected phase-connection isolating relay and the neutral terminal, - Detecting a stuck-at fault by alternately activating to open and close the selected isolating relays (91 - 93) depending on a voltage that can be measured with the corresponding voltage measuring device (101, 102, 103).

[0010] Furthermore, a stuck-at fault can be detected by alternately activating the selected isolating relays to open and close them. - if a voltage other than 0 volts can be measured with the appropriate voltage measuring device when the selected isolation relays are closed, and / or - if, when one of the selected isolating relays is closed and another selected isolating relay is already closed, a voltage other than 0 volts can be measured with the corresponding voltage measuring device, and / or - if, with the selected isolation relays closed and one of the selected isolation relays opened, a voltage of 0 volts can be measured with the corresponding voltage measuring device.

[0011] Furthermore, the AC / DC converter can have a half-bridge circuit assigned to each phase terminal and the neutral terminal, across which the intermediate circuit voltage is applied and which provides a center node, wherein the center node is connected to the neutral terminal via switchable transmission gates and via a second neutral isolating relay; wherein, to check the second neutral isolating relay, the first neutral isolating relay is opened or remains open, wherein a stuck-at fault is detected by alternately activating to open and close the selected phase terminal isolating relay and the second neutral terminal. • if a voltage other than 0 volts can be measured with the appropriate voltage measuring device when closing the selected phase terminal disconnect relay and the second neutral terminal disconnect relay, and / or • if, when closing one of the selected phase-connection disconnect relays and the second neutral terminal, a voltage other than 0 volts can be measured with the appropriate voltage measuring device while another of the selected phase-connection disconnect relays and the second neutral terminal is already closed, and / or ◯ if, with the selected phase terminal isolating relay closed and the second neutral terminal isolating relay closed, and one of the selected isolating relays opened, a voltage of 0 volts can be measured with the corresponding voltage measuring device.

[0012] It may be provided that the precharging of the intermediate circuit capacitor is carried out by an inverse operation of a DC / DC converter connected to the intermediate circuit, which is powered by a battery.

[0013] Furthermore, a battery-powered AC supply can be provided, which can be supplied from the intermediate circuit voltage by means of appropriate control of the AC / DC converter, wherein the AC / DC converter is operated to apply a supply AC voltage to the terminals of the battery-powered AC supply, wherein a further voltage measuring device is provided to measure a voltage between the terminals of the AC supply, wherein a V2L phase-connection disconnect relay and a V2L neutral-connection disconnect relay are provided, which are arranged between the terminals of the further voltage measuring device and the AC / DC converter, wherein a stuck-at fault is detected by alternately controlling the V2L phase-connection disconnect relay and the V2L neutral-connection disconnect relay to open and close. • if, when closing one of the V2L phase-connection disconnect relays and the V2L neutral-connection disconnect relays, while another of the V2L phase-connection disconnect relays and the V2L neutral-connection disconnect relays is already closed, a voltage other than 0 volts can be measured with the corresponding voltage measuring device, and / or ◯ if, with the V2L phase connection isolating relay and the V2L neutral connection isolating relay closed and one of the V2L phase connection isolating relays and the V2L neutral connection isolating relay open, a voltage of 0 volts can be measured with the appropriate voltage measuring device.

[0014] A battery charger that draws battery voltage from an AC power supply to charge a vehicle battery typically includes an AC / DC converter. This converter first provides a DC link voltage. The AC / DC converter can be a power factor correction (PFC) converter or similar device and usually incorporates a number of half-bridge circuits, corresponding to the number of phase connections in the AC power supply. By appropriately controlling the AC / DC converter, portions of the voltage half-waves are selectively switched to the DC link potential lines, creating a DC link voltage that is buffered in a DC link capacitor. A subsequent DC / DC converter then converts this DC link voltage into the desired battery voltage.Furthermore, the DC / DC converter must be bidirectionally operable so that the intermediate circuit voltage can be generated from the battery voltage, even if no AC power supply network is connected.

[0015] Each connection line to the AC power supply network is equipped with a controllable isolating relay to disconnect or connect the respective line to the AC / DC converter. To verify the presence of phase voltages, each connection line is also equipped with voltage measuring devices that check whether a phase line of the AC power supply network is live or not. The voltage measuring devices are located between the AC power supply network and the respective isolating relay.

[0016] The above method now makes it possible to diagnose the isolation relays on the input side of the AC / DC converter using the voltage measuring devices provided between the connecting lines of the AC power supply network and the relays, without having to provide additional voltage measuring devices for the diagnosis.

[0017] The diagnosis is performed according to the above procedure with the AC power supply disconnected, utilizing bidirectional operation of the DC / DC converter. This takes advantage of the fact that the voltage measuring devices, used to detect the voltage at each phase terminal, can also be used to measure a voltage supplied by the AC / DC converter operating in reverse, provided the corresponding isolation relay is closed. The AC / DC converter operating in reverse, through appropriate control, provides a voltage generated from the intermediate circuit voltage at one of the phase terminals, so that, with a corresponding isolation relay closed, the voltage can be measured by the voltage measuring device. The AC / DC converter operating in reverse can, for example,By switching a high-side semiconductor switch of the half-bridge circuit connected to the corresponding phase terminal, the intermediate circuit voltage is applied as a voltage to the relevant phase terminal, so that it can be determined whether a corresponding relay for the phase terminal is closed or open.

[0018] Depending on whether the control signal of the isolating relay, which specifies a closing or an opening of the corresponding isolating relay, corresponds to the voltage measured at a voltage measuring device for the relevant phase connection, a fault or proper operation of the corresponding isolating relay can be determined.

[0019] A stuck-closed fault can be identified if, after a control signal to open the isolating relay, the converter voltage is still present at the corresponding phase terminal associated with the isolating relay. Similarly, a stuck-open fault of an isolating relay can be identified if, after a control signal to close the relay and after applying voltage to the relay, no voltage can be measured with the corresponding phase voltage measuring device associated with the relay.

[0020] Before performing the diagnosis, it must be checked that none of the AC power supply lines are live, i.e., that the AC power supply is disconnected. This is the case, for example, with a vehicle if the charging port cover is closed or the vehicle is in operation.

[0021] The diagnostic procedure described above eliminates the need for a redundant setup with multiple voltage measuring devices for each connection, i.e., phase and neutral connections, for connection detection and diagnosis, thereby reducing the complexity of the battery charger's design. Furthermore, it eliminates the need for special diagnostic relays with a corresponding feedback line that transmits the relays' actual switching state.

[0022] According to another aspect, a battery charger, in particular an onboard battery charger for a motor vehicle, is provided, comprising: - Phase connections and a neutral connection; - an AC / DC converter that can be supplied via the phase terminals and the neutral terminal and can be operated in reverse to selectively apply or disconnect an intermediate circuit voltage to the phase terminals and the neutral terminal, - Voltage measuring devices for each phase terminal, designed to measure a voltage at the respective phase terminal, in particular against the neutral terminal, - Isolation relays as phase connection isolation relays for the phase connections and as neutral conductor isolation relays for the neutral conductor connection, which are connected between the terminals of the voltage measuring devices and the AC / DC converter; - A control unit that is trained to: ◯ Pre-charging the intermediate circuit capacitor, in particular using a battery voltage; ◯ Select one of the phase connection disconnect relays and the neutral conductor disconnect relay; ◯ Operating the AC / DC converter to apply the intermediate circuit voltage between the phase terminal of the selected phase-connect relay and the neutral terminal, ◯ Detecting a stuck-at fault by alternately activating the selected isolating relays to open and close them, ▪ if a voltage other than 0 volts can be measured with the appropriate voltage measuring device when the selected isolation relays are closed, and / or ▪ if, when one of the selected isolating relays is closed and another selected isolating relay is already closed, a voltage other than 0 volts can be measured with the corresponding voltage measuring device, and / or ▪ if, with the selected isolation relays closed and one of the selected isolation relays opened, a voltage of 0 volts can be measured with the corresponding voltage measuring device. Brief description of the drawings

[0023] The embodiments are explained in more detail below with reference to the accompanying drawings. These show: Fig. 1 a schematic representation of the construction of a battery charger for providing battery voltage from an AC power supply network; Fig. 2. A flowchart illustrating a procedure for diagnosing isolating relays in the battery charger of the Fig. 1. Description of embodiments

[0024] Fig. Figure 1 schematically shows a battery charger 1, for example, for use as an onboard charger for a vehicle. The battery charger 1 serves to rectify a single-phase or multi-phase mains supply voltage and provide an output voltage U. Bat to provide for charging a vehicle battery 2.

[0025] The battery charger 1 comprises an AC / DC converter 3, which can be configured, for example, as a PFC, e.g., of the Vienna or TNPC type, and a DC / DC converter 4. The AC / DC converter 3 can be operated single-phase or multi-phase (in particular three-phase) and provides an intermediate circuit voltage U. z in an intermediate circuit 5 of the battery charger 1.

[0026] The intermediate circuit voltage U z is referred to as intermediate circuit potentials V zh ,V zl provided via intermediate circuit lines 51,52, which connect the AC / DC converter 3 and the DC / DC converter 4.

[0027] An AC filter 6 is provided on the input side of the AC / DC converter 3 to filter out interference on the connecting lines 7 of the AC power supply network 8. Furthermore, isolating relays 91-94 of a relay arrangement are provided between the AC filter 6 (i.e., the connecting line) and the AC / DC converter 3 to allow each connecting line 7 to be galvanically isolated separately from the AC / DC converter 3. That is, an isolating relay 91-94 is provided for each of the phase lines L1, L2, L3 of the AC power supply network 8 and the neutral line N. The isolating relays for the phase connections correspond to a first 91, a second 92, and a third phase connection isolating relay 93, and the isolating relay for the neutral line N corresponds to a first neutral line isolating relay 94.

[0028] Voltage measuring devices 101, 102, 103 are provided for each of the phase connections L1, L2, L3, which detect and provide a voltage between the connecting lines 7 of the phase connections L1, L2, L3 and the respective phase connection isolating relays 91 - 93.

[0029] The AC / DC converter 3 has a half-bridge circuit 31 for each of the phase terminals L1, L2, L3 and the neutral conductor N, each of which is configured with a high-side semiconductor switch 32 and a low-side semiconductor switch 33 connected in series. The respective center nodes between the high-side semiconductor switch 32 and the low-side semiconductor switch 33 are connected via transmission gates 36 to a neutral node NK and via a second neutral isolating relay 95 to the neutral conductor N. The neutral node NK is connected to the intermediate circuit potentials V via intermediate circuit capacitors 34, 35. zh , V zl coupled.

[0030] The operation of the battery charger 1 is controlled by a control unit 15, which operates both the AC / DC converter 3 and the DC / DC converter 4. Furthermore, the control unit 15 controls the isolating relays 91-95 to disconnect or connect phase lines L1, L2, L3 and / or the neutral conductor N to the AC / DC converter 3. The control unit 15 is also connected to the voltage measuring devices 101, 102, 103 to obtain a reading of the voltage on the respective phase line L1, L2, L3.

[0031] To ensure the proper functioning of battery charger 1, its components must be diagnosed. The operation of the phase-connection disconnect relays 91-93 and the neutral-connection disconnect relays 94, 95 must be checked regularly, especially for stuck-at faults. Stuck-at faults correspond to malfunctions of a relay where the relay no longer responds to an opening or closing signal. The relay then remains permanently closed (stuck-closed fault) or permanently open (stuck-open fault).

[0032] The verification is carried out using a procedure, which is illustrated below by the flowchart of the Fig. 2 is described in more detail. The procedure is carried out in control unit 15.

[0033] In step S1, the system first checks whether the AC power supply network 8 is connected. If so (alternative: Yes), the procedure is terminated; otherwise (alternative: No), the diagnosis of the isolation relays 91–95 can be performed. Determining whether the AC power supply network 8 is connected can be done directly or indirectly, for example, by checking whether the vehicle's battery charging flap is closed or whether the vehicle is in motion.

[0034] Now, for each of the selected specific phase connection disconnect relays 91, 92, 93 and one of the neutral conductor disconnect relays 94, 95, the following checks for stuck-at faults are carried out.

[0035] In step S2, a predetermined intermediate circuit voltage is set using the bidirectional DC / DC converter 4. This precharges the intermediate circuit capacitor 5.

[0036] In step S3, the selected phase-connection disconnect relay and the selected neutral-conductor disconnect relay are closed, and all other disconnect relays 91-95 are either opened or remain open. For all subsequently described tests, the low-side holder switch 33 of the half-bridge circuit 31 assigned to the neutral conductor can be closed simultaneously. If the second neutral-conductor disconnect relay 95 is to be closed or is selected, the transmission gate 36 of the half-bridge circuit 31 assigned to the neutral conductor is also closed.

[0037] In step S4, the voltage at the phase terminal of the selected phase terminal isolating relay is measured using the voltage measuring device.

[0038] In step S5, it is checked whether a voltage other than 0 V can be measured at the phase terminal. If this is the case (alternative: Yes), then the selected phase terminal disconnect relay and the selected neutral conductor disconnect relay have been successfully closed, and there are no stuck-open errors of the selected phase terminal disconnect relay and the selected neutral conductor disconnect relay. The procedure continues with step S6.

[0039] If step S5 detects that a voltage of 0 volts or approximately 0 volts is present at the phase terminal (alternative: No), then at least one of the selected phase terminal isolating relays and the selected neutral terminal isolating relay could not be closed and there is a stuck-open fault of at least one of the two selected isolating relays.

[0040] In a subsequent step S15, the occurrence of an error is signaled and the procedure is terminated if necessary. In particular, battery charger 1 may be deactivated.

[0041] In step S6, all phase connection disconnect relays are closed and the selected neutral conductor disconnect relay is closed, while the unselected neutral conductor disconnect relay remains or becomes open.

[0042] Now the selected phase connection disconnect relay is opened.

[0043] In step S7, the voltage at the phase terminal of the selected phase terminal isolating relay is measured using the voltage measuring device.

[0044] In step S8, it is checked whether a voltage of 0 volts can be measured at the phase terminal. If this is the case (alternative: Yes), the selected phase terminal disconnect relay was successfully opened and there are no stuck-open errors of the selected phase terminal disconnect relay. The procedure continues with step S9.

[0045] If a voltage other than 0 volts is measured at the phase terminal, the selected phase terminal disconnect relay could not be opened successfully, resulting in a stuck-closed fault. In a subsequent step S15, the occurrence of this fault is signaled, and the procedure may be terminated.

[0046] In step S9, all phase connection disconnect relays are closed and the selected neutral disconnect relay remains or becomes closed, while the unselected neutral disconnect relay remains or becomes open.

[0047] Now the selected neutral conductor isolating relay 94 is opened.

[0048] In step S10, the voltage at the phase terminal of the selected phase terminal isolating relay is measured using the voltage measuring device.

[0049] In step S11, it is checked whether a voltage of 0 volts can be measured at the phase terminal. If this is the case (alternative: Yes), the selected neutral disconnect relay was successfully opened and there are no stuck-open errors of the selected neutral disconnect relay 94. The procedure continues with step S12.

[0050] If a voltage other than 0 volts is measured at the phase terminal (alternative: No), the selected neutral disconnect relay could not be opened successfully, resulting in a stuck-closed fault. In a subsequent step S15, the occurrence of this fault is signaled, and the procedure may be terminated.

[0051] Step S12 checks whether the above procedure has been carried out for all combinations of selected phase-connection disconnect relays and selected neutral-connection disconnect relays. If so (alternative: Yes), the procedure continues with step S13. Otherwise (alternative: No), the process returns to step S2.

[0052] Step S13 signals that the isolation relays are error-free.

[0053] By performing the test with all combinations of selected phase-connection disconnect relays 91-93 and selected neutral-connection disconnect relays 94, pinpointing for the stuck-closed fault can be carried out. If the stuck-open fault is found to be present for at least two selected phase-connection disconnect relays 91-93 and one selected neutral-connection disconnect relay 94, a stuck-open fault of the selected neutral-connection disconnect relay can be concluded. Otherwise, a stuck-open fault of the selected phase-connection disconnect relay can be concluded.

[0054] In Fig.Figure 1 shows a further embodiment in which the battery charger 1 can be operated with a battery-powered AC supply in the sense of V2L (Vehicle-to-Load) operation. The circuit section is designated by reference numeral 20. The circuit section 20 comprises two isolating relays 96, 97 and a further voltage measuring device 104.

[0055] For this purpose, the semiconductor bridges 31 for the phase connections are switched off except for one, and the active semiconductor bridge is controlled by the control unit 15 so that an AC voltage is output via the neutral node NK. The semiconductor bridge for the neutral conductor is operated inverted for this purpose. The intermediate node of the semiconductor bridge for the neutral conductor then represents the neutral conductor of the circuit section, and the intermediate node of the semiconductor bridge for the phase connection represents the phase connection of the circuit section for a battery-powered AC supply. To extract the AC supply, the V2L phase connection is equipped with a V2L phase connection isolating relay 96, and the V2L neutral connection with a V2L neutral connection isolating relay 97, in order to selectively achieve galvanic isolation. A further voltage measuring device 104 is provided on the output side.The AC power supply is made possible by coupling the center nodes to a single-phase AC connection of the V2L interface, whereby one of the phases can be selectively chosen by a respective selection element.

[0056] The procedure for checking the functionality of the isolating relays can additionally include opening isolating relays 91-95 for the AC power supply connection and operating the half-bridge circuits 31 in such a way that a V2L AC supply voltage is provided. The AC / DC converter 3 is then operated in reverse and generates a corresponding V2L AC supply voltage from the battery-supplied intermediate circuit voltage.

[0057] A corresponding additional current measuring device is provided between the V2L interface and the isolating relays for the neutral conductor and the phase connection that has been selected by the selection unit.

[0058] The operating functions of the corresponding V2L phase-connection disconnect relays 96 and 97 can be diagnosed by alternately switching between a closed and an open state, and by measuring the voltage present at the V2L interface. A stuck-open fault can be detected if, with one disconnect relay 96 or 97 controlled as closed, the other of the two disconnect relays intended for the V2L supply closes and no transition from a voltage of 0 volts to a converter voltage (not equal to 0 volts) is detected using the additional voltage measuring device 104.

[0059] Accordingly, a stuck-closed fault can be detected if, with the isolation relays closed, opening one of the isolation relays 96, 97 does not disconnect the V2L interface from the supply voltage. This can also be detected by measuring the voltage using the additional voltage measuring device 104.

Claims

[1] Method for testing isolating relays (91 - 94) in a battery charger (1), in particular in an onboard battery charger for a motor vehicle, wherein the battery charger (1) has an AC / DC converter (3) which can be supplied and operated via phase terminals (L1, L2, L3) and a neutral terminal (N) to selectively supply an intermediate circuit voltage (U) z) to connect or disconnect from the phase terminals (L1, L2, L3) and the neutral terminal (N), wherein voltage measuring devices (101, 102, 103) are provided for each phase terminal (L1, L2, L3) to measure a voltage at the respective phase terminal (L1, L2, L3), wherein one of the isolating relays is provided between the terminals of the voltage measuring devices (101, 102, 103) and the AC / DC converter (3) as a phase terminal isolating relay (91 - 93) for the phase terminals (L1, L2, L3) or as a neutral terminal isolating relay (94) for the neutral terminal, wherein the following steps are carried out to diagnose the isolating relays: - Pre-charging (S2) of the intermediate circuit capacitor (5), in particular using a battery voltage (U) Bat ); - Select one of the phase connection disconnect relays (91 - 93) and the neutral conductor disconnect relay (94); - Operating the AC / DC converter (3) to supply the intermediate circuit voltage (U z) between the phase terminal of the selected phase terminal isolating relay (91 - 93) and the neutral terminal (N), - Detecting a stuck-at fault by alternately activating to open and close the selected isolating relays (91 - 93, 94) depending on a voltage that can be measured with the corresponding voltage measuring device (101, 102, 103). [2] Method according to claim 1, wherein a stuck-at fault is detected by alternately activating to open and close the selected isolating relays (91 - 93, 94), - if a voltage other than 0 volts can be measured with the corresponding voltage measuring device (101, 102, 103) when the selected isolating relays (91 - 93) are closed, and / or - if, when one of the selected isolating relays (91 - 93) is closed, a voltage other than 0 volts can be measured with the corresponding voltage measuring device (101, 102, 103) while another selected isolating relay is already closed, and / or - if, with the selected isolation relays (91 - 93) closed and one of the selected isolation relays opened, a voltage of 0 volts can be measured with the corresponding voltage measuring device (101, 102, 103). [3] Method according to claim 1 or 2, wherein the AC / DC converter (3) has a half-bridge circuit associated with each phase terminal and the neutral terminal, across which the intermediate circuit voltage is applied and which provides a center node, wherein the center node is connected to the neutral terminal via switchable transmission gates and via a second neutral isolating relay; wherein, to check the second neutral isolating relay (95), the first neutral isolating relay is opened or remains open, wherein a stuck-at fault is detected by alternately activating to open and close the selected phase terminal isolating relay and the second neutral terminal depending on a voltage that can be measured with the corresponding voltage measuring device (101, 102, 103). [4] Method according to claim 3, wherein the stuck-at fault is detected by alternately activating to open and close the selected phase terminal disconnect relay (91 - 93) and the neutral terminal (N) depending on a voltage that can be measured with the corresponding voltage measuring device (101, 102, 103), - if a voltage other than 0 volts can be measured with the appropriate voltage measuring device when closing the selected phase terminal disconnect relay and the second neutral terminal disconnect relay, and / or - if, when closing one of the selected phase-connection isolating relays and the second neutral terminal, a voltage other than 0 volts can be measured with the appropriate voltage measuring device while another of the selected phase-connection isolating relays and the second neutral terminal is already closed, and / or - if, with the selected phase connection isolating relay closed and the second neutral connection isolating relay closed, and one of the selected isolating relays opened, a voltage of 0 volts can be measured with the corresponding voltage measuring device. [5] Method according to any one of claims 1 to 4, wherein the precharging of the intermediate circuit capacitor (5) is carried out by an inverse operation of a DC / DC converter (4) connected to the intermediate circuit, which is supplied by a battery (2). [6] Method according to any one of claims 1 to 5, wherein a battery-operated AC power supply (20) is provided which is supplied by a corresponding control of the AC / DC converter (3) from the intermediate circuit voltage (U z) is available, wherein the AC / DC converter (3) is operated to apply an AC supply voltage to terminals of the battery-powered AC supply, wherein a further voltage measuring device (104) is provided to measure a voltage between the terminals of the AC supply, wherein a V2L phase-connection disconnect relay (96) and a V2L neutral-connection disconnect relay (97) are provided, which are arranged between the terminals of the further voltage measuring device (104) and the AC / DC converter (3), wherein a stuck-at fault is detected by alternately activating to open and close the V2L phase-connection disconnect relay (96) and the V2L neutral-connection disconnect relay (97), - if, when one of the V2L phase connection isolating relays (96) and the V2L neutral connection isolating relays (97) is closed, and another of the V2L phase connection isolating relays (96) and the V2L neutral connection isolating relays (97) is already closed, a voltage other than 0 volts can be measured with the additional voltage measuring device (104), and / or - when the V2L phase connection isolating relay (96) and the V2L neutral connection isolating relay (97) are closed and one of the V2L phase connection isolating relays (96) and the V2L neutral connection isolating relay (97) is opened, a voltage of 0 volts can be measured with the additional voltage measuring device (104). [7] Battery charger (1), in particular an onboard battery charger for a motor vehicle, comprising: - Phase connections (L1, L2, L3) and a neutral connection (N); - an AC / DC converter (3) which can be supplied via the phase terminals (L1, L2, L3) and the neutral terminal (N) and can be operated in reverse to selectively generate an intermediate circuit voltage (U) z ) to connect to or disconnect from the phase terminals (L1, L2, L3) and the neutral terminal (N), - Voltage measuring devices (101, 102, 103) for each phase terminal (L1, L2, L3) which are designed to measure a voltage at the respective phase terminal (L1, L2, L3) in particular against the neutral terminal (N), - Isolation relays as phase connection isolation relays (91 - 93) for the phase connections and as neutral conductor isolation relays (94) for the neutral conductor connection (N), which are connected between the terminals of the voltage measuring devices (101, 102, 103) and the AC / DC converter (3); - a control unit (15) that is trained to: ◯ Pre-charging the intermediate circuit capacitor (5), in particular using a battery voltage (U) Bat ); ◯ Select one of the phase connection disconnect relays (91 - 93) and the neutral conductor disconnect relay (94); ◯ Operating the AC / DC converter (3) to supply the intermediate circuit voltage (U z ) between the phase terminal (L1, L2, L3) of the selected phase terminal isolating relay (91 - 93) and the neutral terminal (N), ◯ Detecting a stuck-at fault by alternately activating to open and close the selected isolating relays (91 - 93, 94) depending on a voltage that can be measured with the corresponding voltage measuring device (101, 102, 103).

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