GB2534314A

GB2534314A - Battery system and method

Info

Publication number: GB2534314A
Application number: GB1604657.5A
Authority: GB
Inventors: John Miler Peter
Original assignee: Johnson Matthey PLC
Current assignee: Johnson Matthey PLC
Priority date: 2016-03-18
Filing date: 2016-03-18
Publication date: 2016-07-20
Anticipated expiration: 2036-03-18
Also published as: WO2017158375A1; GB2534314B; GB201604657D0

Abstract

A battery system comprising a plurality of cells 104 arranged in series in a battery pack 103, a first contactor relay switch (107,108) connected to a positive end 110 of battery pack 103 and a second contactor relay switch 106 connected to a negative end 111 of battery pack 103. A controller 114 is configured to measure a first voltage between the positive terminal 101 and a reference point, a second voltage between the negative terminal 102 and the reference point and a third voltage between a location 105 part way along the plurality of cells 104 and the reference point. A failure state of the battery is determined from the measured voltages. The failure state may be a failure of one or both of the contactor relay switches (106, 107, 108) or leakage of current from the battery system to the reference point. Embodiments are included in which the battery system is used to detect symmetric current leakage from the high voltage terminals of a battery pack used in an electric vehicle. The reference point may be ground or the chassis of the vehicle.

Description

BATTERY SYSTEM AND METHOD

Field of the Invention

The present invention relates to battery systems and methods of detecting a failure state in battery 5 systems.

Background

Battery packs are used in a wide variety of applications including portable electronics and electric vehicles. For many applications, particularly vehicle applications, the battery pack may be required to provide a high voltage output, for example in excess of 60V. A typical battery pack includes a plurality of individual cells connected in series or parallel and a battery management system (BMS) which can monitor parameters of the cells and pack and control the charging and discharging of the cells. The total voltage of the connected individual cells may be referred to as the pack voltage. In order to provide a high voltage output, a pack voltage greater than the voltage of the individual cells is likely to be required and at least some series arrangement of the cells is therefore required. The pack has positive and negative terminals to which external loads, such as a vehicle motor, can be connected. If no internal loads are present, the voltage available between the terminals will be the pack voltage. The pack may also include interfaces for communication systems, such as a CAN bus, to interface with the pack.

A typical battery pack that provides in excess of 60V normally has contactors (or relays) on both the positive and negative side of its power connections. The contactors serve to isolate the cells from the positive and negative terminals to reduce the risk of electric shock when the pack is not operating. The pack will also typically have a precharge system to charge up any external capacitance between the positive terminal and the negative terminal. The precharge system uses an additional contactor and a resistor connected in series across the primary contactor on the positive terminal side. The additional contactor is closed first so that the resistor limits the inrush current into the external capacitance. Once the external capacitance approaches a charged state, the primary contactor is closed so that the resistor does not affect the power output of the pack.

For safety reasons, the high voltage system is not normally electrically connected to ground or other local reference points such as a vehicle chassis and instead is left floating. This means, for example, a person who manages to touch either the positive or negative terminals in an electric vehicle will not receive an electric shock if they are also touching the vehicle chassis.

When the pack is not operating all the contactors are typically opened, this is partly for safety reasons in that it completely disconnects the high voltage terminals on the outside of the battery pack from the cells within the pack, but it also minimises the risk that the battery pack will be discharged by any residual current flowing in devices connected to the terminals.

The BMS is responsible for measuring any leakage of current from the pack to ground, or to the vehicle chassis or other local reference point, and also for testing the contactors to make sure none are stuck closed or open.

In a prior art system for an electric vehicle the battery management system measures the voltage at the negative terminal (HV-) with respect to the chassis and the voltage at the positive terminal (HV+) with respect to the chassis. With the contactors closed, the circuitry is symmetrical. As a result HV-is '% pack voltage negative with respect to the chassis and HV+ is % pack voltage positive with respect to the chassis. Any asymmetric leakage current to the chassis (e.g. from the positive terminal to the chassis) will unbalance the pack voltages and can be easily detected by the BMS. This circuitry can also detect whether contactors are open or closed by detecting the presence or absence of voltage at the measurement points. Combined with information about the expected state of the contactors, the BMS can therefore detect failures of the contactors to open or close as instructed.

The prior art systems cannot detect symmetric leakage current (e.g. leakage current from the middle of the pack to chassis) as this does not alter the measured voltages. In addition, to detect if a contactor is open or closed, the prior art system operates by closing the opposite contactor and measuring the voltages. Thus the state of the positive contactor would be checked by closing the negative contactor and checking whether a voltage exists between the measuring points. However, if a contactor is failed in the closed position, closing the other contactor will complete the circuit and apply a voltage to the output terminals, which may result in a safety hazard Also, if the primary positive contactor is stuck closed, closing the negative contactor in order to check the contactor state will cause the circuit to be complete without the precharge function being applied. In that case a high current may occur as the negative contactor is closed, which may cause damage to the negative contactor, for example as a result of arcing. The prior art systems also cannot distinguish between leakages external to the battery pack and leakages internal to the battery pack. That can be an issue as some devices will have their own systems for detecting leakages external to the battery pack and may test those systems by deliberately introducing a small external leakage. In prior art systems that leakage is detected as a failure of the battery pack by the battery management system.

There is demand for multiple battery packs to be deployed in parallel to power devices. The packs would typically be connected in parallel between a common set of HV+ and HV-terminals that provide the interface between the parallel packs and the device to be powered. That may be advantageous in that the battery packs can be mass-produced in a single size and configuration and then different devices can use different numbers of the battery packs to provide the necessary power for that particular device. Prior art systems can check for contactor failures in such parallel systems, but only by individually testing each connector in each of the parallel battery packs one after the other. That can result in a significant delay in switching on the battery pack.

Preferred embodiments of the present invention seek to overcome one or more of the above disadvantages of the prior art. In particular, preferred embodiments of the present invention seek to provide an improved system for monitoring leakage currents or contactor failures.

Summary of Invention

According to a first aspect of the invention, there is provided a battery system comprising: a battery pack comprising a plurality of cells arranged in series; a positive terminal electrically connected via a first contactor to a positive end of the battery pack; a negative terminal electrically connected via a second contactor to a negative end of the battery pack; and a controller configured to measure a first voltage between the positive terminal and a reference point, a second voltage between the negative terminal and the reference point, and a third voltage between a location part way along the plurality of cells in the battery pack and the reference point and, based on the measured voltages, to detect a failure state of the battery system.

By providing a controller configured to measure the first, second and third voltages the problem of detecting symmetric current leakages to the reference point is solved. That is because a leakage that is symmetric with respect to the first and second voltages will be asymmetric with respect to the first and third, or second and third, voltages and can therefore be detected by a relative change between the first and third, or second and third, voltages. Moreover, the provision of a controller configured to measure the first, second and third voltages also permits the detection of contactors that have failed to a closed state without requiring any other contactors to be closed as part of the detection. That is because any contactor failing to a closed state will result in the completion of a measurement circuit resulting in non-zero voltages for the third voltage and for either the first voltage or the second voltage depending on which contactor has failed. The measurement of the three voltages thus permits the controller to detect a failure of the first or second contactor without having to close any of the other contactors, thus removing the risk of a potentially hazardous unplanned completion of the circuit between the positive and negative terminals. The measurement of three voltages also allows the system to distinguish between internal current leakages and external current leakages. That is because the system can detect leakages without requiring both the first and second contactors to be closed and internal and external current leakages will affect the first, second and third voltages differently depending on whether the first or second contactors are open or closed. It may be that the system considers internal current leakages, that is current leakages happening on the cell side of the first and second contactors, as a failure state of the battery system but does not consider external current leakages, that is current leakages on the terminal side of the contactors, to be a failure state of the battery system. It may be that a device in which the battery system is used has its own system for detecting external current leakages and that it is desirable that the battery system does not treat them as a failure state of the battery system. However, in some cases it may be desirable for the battery system to include external current leakages as a failure state. Preferably the failure state is selected from leakage of current from the battery system to the reference point (that is, internal current leakages), failure of the first contactor, or failure of the second contactor.

The third voltage is measured between a location partway along the plurality of cells in the battery pack and the reference point. Preferably the location is in the central third of the plurality of cells.

Most preferably the location is the mid-point of the plurality of cells. It will be appreciated that the location will be at a point between two adjacent cells within the plurality of cells. For an even number of cells the mid-point may therefore be the point at which there are an equal number of cells either side of the location, but for an odd number of cells the mid-point may be adjacent on either side to the cell which has an equal number of cells on either side of it. The location is preferably in the central third and most preferably at the mid-point because the more central the location the greater the sensitivity of the detection of leakages that are symmetrical relative to the battery pack. Leakages that are highly asymmetric relative to the battery pack can be detected by using the first and second voltages in a similar manner to the prior art systems. The measurement of the third voltage between the location part way along the plurality of cells and the reference point effectively creates two additional measurement circuits, one either side of the location. Each of those circuits can detect leakages that are asymmetric relative to the circuit in question. If the location is in the central third of the plurality of cells, and preferably at the mid-point, leakages that are symmetric relative to the battery pack (and hence relative to the measurement circuit including the first and second voltages) will be highly asymmetric, and therefore detectable with high sensitivity, relative to the additional measurement circuits.

It may be that the failure state is a symmetric leakage of current from the battery pack to the reference point. As explained above, an advantage of the configuration of the present invention is that symmetric leakages can be detected by the controller. A symmetric leakage may result from a single leakage at a central point or may be the result of two or more leakages at different locations which result in a combined leakage of current that is symmetric.

Preferably the controller is configured to detect a failure state in which one of the first and second contactors has failed in a closed position, wherein the failure state is detected whilst the remaining contactor remains open. For example, the controller may be configured to detect a non-zero measurement for the third voltage and for one of the first or second voltages while all the contactors are supposed to be open and to deduce that a contactor has failed in a closed position. Because the battery system of the invention includes a measurement circuit between the location part way along the plurality of cells and the reference point, that measurement circuit permits a closed-loop to be created by failure of a contactor into a closed position, without a circuit including both the positive and negative terminals having to be closed. Thus the controller can monitor for a contactor failing in a closed position, without having to close a contactor as pad of the monitoring operation.

Preferably the battery system includes a third contactor, for example a precharge contactor in series with a resistor, in parallel with either the first contactor between the positive end of the battery pack and the positive terminal or the second contactor between the negative end of the battery pack and the negative terminal. Preferably the third contactor is in parallel with the first contactor between the positive end of the battery pack and the positive terminal. The controller may be configured, based on the measured voltages, to detect a failure state involving at least one of the first, second and third contactors. For example, the controller may be configured to detect, without closing any of the other contactors, that one of the two parallel contactors (preferably the first and third contactors) has failed in a closed position and to then determine which of the two contactors has failed in a closed position by closing one of the two parallel contactors and comparing the measurements of the first, second and third voltages before and after closing the contactor. It will be appreciated that such a configuration, even though it involves the closing of a contactor, does not suffer from the disadvantages described in relation to the prior art systems because the contactor is only closed after it has been determined that one of the two parallel contactors is failed in a closed position. Thus trying to close one of the two parallel contactors as part of the measurement process cannot result in a terminal being connected that was not already connected. Moreover, the controller may be configured to only carry out such a closure after it has determined that the contactor to the negative terminal is operating correctly in an open position so that no closed circuit is created including both the positive and negative terminals.

It may be that the reference point is ground. It may be that the reference point is a housing. However, most preferably the battery system is for use in a vehicle having a chassis and the reference point is the chassis. When performing maintenance on a vehicle, it is for example possible that a technician could inadvertently touch a terminal of the battery pack whilst also being in contact with the chassis of the vehicle. Thus it may be important for improving the safety of the vehicle that current leakages from the battery pack to the chassis are detected. While prior art systems may be able to detect asymmetric leakages, symmetric leakages could still involve voltages of up to half the cell pack voltage and may not be detectable with prior art systems. The present invention advantageously provides a method of detecting symmetric leakages.

Preferably the controller is configured to compare a sum of the first, second and third voltages with a sum of the voltages of the plurality of cells so as to detect a failure state of the battery system. For example, the controller may be configured to measure the first and third voltages with the first contactor closed and the remaining contactors open, to measure the second and third voltages with the second contactor closed and the remaining contactors open and to compare the sum of the measured voltages with the sum of the voltages of the plurality of cells. Advantageously such a comparison allows a self-test. In other words, the controller may be configured to perform such a comparison and, if the sums are not equal, to enter a failure mode and preferably issue an alert, such as an audible, visual or electronically communicated alert.

The battery system may include multiple battery packs, each battery pack comprising a plurality of cells arranged in series; a positive terminal electrically connected via respective first contactors to positive ends of each of the battery packs; a negative terminal electrically connected via respective second contactors to negative ends of the battery packs; and a controller configured to measure a first voltage between the positive terminal and a reference point, a second voltage between the negative terminal and the reference point, and respective third voltages between a location part way along each of the plurality of cells in the battery packs and the reference point and, based on the measured voltages, to detect a failure state of the battery system.

Thus the battery system may include multiple battery packs connected in parallel between the positive and negative terminals, each battery pack having its own contactors and its own measurement circuit for measuring the voltage between the location part way along the plurality of cells in that battery pack and the reference point. That may be advantageous in that the multiple battery packs can be mass-produced and then combined in any desirable number to satisfy a particular power requirement. Because the invention involves measuring the third voltages, which are measured separately for each battery pack, the invention can check for failures in the multiple battery packs and their associated contactors simultaneously. The system advantageously comprises multiple parts, each part having a battery pack, it's respective contactors and measurement circuits by which the controller can measure the first, second and third voltages for that part. The pads may advantageously be mass-produced and combined to form systems of differing sizes. Thus the controller may be configured to measure the first, second and third voltages for each part. It will be appreciated that the first and second voltages measured for each part may be the same since the parts are connected to the same positive and negative terminals. However, the third voltage is unique to each part. It will be appreciated that features described above in relation to a system with a single battery pack can also be applied to a battery system with multiple battery packs.

It will be appreciated that the invention may find particular utility in a vehicle and a second aspect of the invention therefore provides a vehicle comprising a battery system as described herein.

According to a third aspect of the invention there is provided a method of detecting a failure state in a battery system comprising: a battery pack comprising a plurality of cells arranged in series; a positive terminal electrically connected via a first contactor to a positive end of the battery pack; and a negative terminal electrically connected via a second contactor to a negative end of the battery pack, the method comprising measuring a first voltage between the positive terminal and a reference point, measuring a second voltage between the negative terminal and the reference point, measuring a third voltage between a location part way along the plurality of cells in the battery pack and the reference point and, based on the measured voltages, detecting the failure state.

Preferably the failure state is selected from leakage of current from the battery system to the reference point, failure of the first contactor or failure of the second contactor.

Preferably the failure state is a symmetric leakage of current from the battery pack to the reference point. Preferably the failure state is one in which one of the first and second contactors has failed in a closed position, and wherein the voltages are measured and the failure state detected whilst the 15 remaining contactors remain open.

Preferably the location is in the central third of the length of the plurality of cells. More preferably the location is the mid-point of the plurality of cells.

Preferably the battery system comprises a third contactor in parallel with the first contactor between the positive end of the battery pack and the positive terminal and the method comprises detecting a failure state involving at least one of the first, second and third contactors based on the measured voltages.

Preferably the method includes a step of measuring the first and third voltages with the first contactor closed and the remaining contactors open and measuring the second and third voltages with the second contactor closed and the remaining contactors open and comparing the sum of the measured voltages to the sum of the voltages of the plurality of cells. Such a step may provide a self-test of the battery system. If the sums are unequal the method may include entering a failure mode and preferably emitting an alert such as an audible, visual or electronically communicated alert. The method may include a step of obtaining the pack voltage, for example by measuring and summing the individual voltages of each cell in the battery pack, and comparing the pack voltage to one or more of the first, second or third voltages with at least one contactor closed. If everything is functioning normally, the first, second and third voltages are readily calculable functions of the pack voltage and the comparison can therefore detect faults within the measurement circuitry.

Preferably the method is performed when the battery system is turned on. That is, the method is performed when the battery system is requested to change from a state in which it is disconnected and supplying no power to a state in which it is connected and supplying power. Preferably the method is therefore a method of switching on a battery system. Preferably the method includes the steps of: a. instructing all contactors to open b. checking whether any contactors are stuck closed and whether there is any internal asymmetric current leakage c. instructing the first contactor (in other words, the positive contactor) to close d. checking whether the first contactor is stuck open, whether there is any internal symmetric leakage and whether there is any external leakage from the positive terminal e. instructing the first contactor to open f. checking whether any contactors are stuck closed g. instructing the second contactor (in other words, the negative contactor) to close h. checking whether the second contactor is stuck open and whether there is any external current leakage from the negative terminal i. instructing the third contactor (in other words, the pre-charge contactor, which is on the positive side) to close j. checking whether the third contactor is stuck open k. instructing the first contactor to close once pre-charge is complete I. optionally instructing the third contactor to open If any of the checks are failed, the system may stop executing the method and trigger an alarm. If all the checks are passed, the system closes the first contactor after the pre-charge is complete (step k). Optionally the system may open the third contactor after the first contactor is closed (step I).

It will be appreciated that features described in relation to one aspect of the invention may be equally applicable in another aspect of the invention. For example, features described in relation to the battery system of the invention, may be equally applicable to the method of the invention, and vice versa. Some features may not be applicable to, and may be excluded from, particular aspects of the invention.

Description of the Drawings

Embodiments of the present invention will now be described, by way of example, and not in any!imitative sense, with reference to the accompanying drawings, of which: Figure 1 is a circuit diagram of principle features of a prior art battery system; Figure 2 is a circuit diagram of the battery system of Figure 1, with a prior art battery management system monitoring for leakage currents and contactor failures; Figure 3 is a high voltage attenuator used in the voltage measurement of the battery management system of Figure 2 and in the voltage measurement of the invention; Figure 4 is a circuit diagram of a battery system according to the invention; Figure 5 is a circuit diagram representing a prior art measurement system with no leakage currents; Figure 6 is a circuit diagram representing a prior art measurement system with an asymmetric leakage current; Figure 7 is a circuit diagram representing a prior art measurement system with a symmetric leakage current; Figure 8 is a circuit diagram representing the measurement system of the invention with a symmetric leakage current; and Figure 9 is a circuit diagram of a battery system according to the invention.

Detailed Description

In figure 1 a battery system comprises a plurality of cells 4 arranged in series to form a cell pack 3. The cell pack 3 includes 7 cells 4 and can be said to have mid-points 5 either side of the central cell 4. The positive end 10 of the cell pack 3 is electrically connected via contactors 7 and 8 and precharge resistor 9 to positive terminal 1. Contactors 1 and 8 are arranged in parallel with each other between the positive end 10 of the cell pack 3 and the positive terminal 1 and precharge resistor 9 is arranged in series with contactor 8. The negative end 11 of the cell pack 3 is electrically connected to negative terminal 2 via contactor 6.

When not in operation, contactors 6, 7 and 8 are open to isolate cell pack 3 from positive terminal 1 and negative terminal 2. In that way the risk of someone receiving an electric shock from the terminals 1 and 2, or of the cell pack 3 being drained by residual currents in devices connected between the terminals 1 and 2, is reduced. When power is required, the contactors 6 and 8 are closed to create a circuit including the cell pack 3 and the terminals 1 and 2. The circuit also includes the pre-charge resistor 9, which limits the flow of current whilst any capacitance in the circuit is charged. That prevents very large currents as the contactors 6 and 8 are closed and thus reduces the risk of damage to the contactors 6 and 8 or to the cell pack 3. After a short time the contactor 7 is closed and the normal operational circuit is complete. The battery system is not electrically connected to ground or to any local reference point such as a vehicle chassis and instead is allowed to float. That has the advantage that someone in contact with ground or the chassis will not receive an electric shock if they inadvertently touch one of the terminals 1 and 2. In order for that to be the case, it is important that there are no undesired current paths from the battery system to ground or the chassis as the case may be. A further potential failure mode is that a contactor 6, 7 or 8 could fail. A contactor 6, 7 or 8 could fail open, in which case circuits will not be completed as required, or a contactor 6, 7 or 8 could fail closed. In the latter case a potentially hazardous situation may result whereby a terminal 1 or 2 that is thought to be isolated from the cell pack 3 may in fact still be connected to the cell pack 3. Such a scenario increases the risk of someone receiving an electric shock. A contactor 6, 7 or 8 may fail in a closed state due to arcing as the contactor 6, 7 or 8 closes, which can effectively weld the contactor 6, 7 or 8 in the closed position.

In figure 2, the battery system includes a prior art battery management system 14, which monitors a first voltage between the positive terminal 1 and a reference point, such as ground or a vehicle chassis, and a second voltage between the negative terminal 2 and the reference point. The first voltage is monitored by a measurement circuit connected at a location 17 close to the positive terminal 1. The measurement circuit includes a high voltage attenuator circuit 12 which for example when used with a pack rated at 900V outputs a voltage in the range 0 to 5V in response to input voltages in the range -1000V to +1000V. The output voltage is read by an analogue to digital converter 15 and analysed by the battery management system 14. The second voltage is monitored via a measurement circuit connected at a location 18 close to the negative terminal 2. The measurement circuit also includes a high voltage attenuator circuit 12, the output of which is read by a second analogue to digital converter 16 and analysed by the battery management system 14.

In figure 3, an example high voltage attenuator circuit 12 has an input connection 19, and output connection 21 and a reference point connection 20. The reference point connection 20 may for example be connected to ground or to a vehicle chassis. The input connection 19 is connected to the location at which the voltage relative to the reference point is to be measured and the output connection 21 is connected to the battery management system 14. Four 270 kla resistors R1, R2, R3 and R4 are connected in series to the input connection 19. The output from the four resistors R1, R2, R3 and R4 is connected via a 5 kfl resistor R5 to a +5V reference voltage and via another kO resistor R7 to the reference point connection 20. The output is also connected via 270 M.) resistor R6 to the input of an amplifier Ul, with a 10 nF capacitor Cl connected between the input of amplifier U1 and the reference point connection 20. The attenuator operates to divide the incoming voltage and the reference voltage between the resistors R1, R2, R3, R4; R5 and R7 resulting in an output via resistor R6 in the range 0 to 5V for input voltages of around -1000V to +1000V. The output is smoothed by capacitor C1 and output to the battery management system 14 via the output connection 21.

The battery management system 14 monitors the first and second voltages and controls the contactors 6, 7 and 8. With the contactors 6, 7 and 8 open, the first and second voltages should be zero. Also, if a single contactor 6, 7 or 8 is closed, both voltages should still be zero, because there is still no complete circuit with a single contactor 6, 7, or 8 closed. In order to test whether a contactor 6, 7 or 8 is failed in an open position, the battery management system can close contactor 6 and either contactor 7 or 8 and monitor the first and second voltage. If both contactors 6 and 7 or 8 are operating correctly non-zero voltages should be recorded for both the first and second voltage. If no voltage is recorded, then one of the contactors 6 and 7 or 8 is failed open.

Keeping contactor 6 closed and switching between the contactors 7 and 8 and repeating the test should allow determination of which contactor 6, 7 or 8 is stuck in an open position. In order to test whether a contactor 6, 7 or 8 is failed closed a single contactor 6, 7 or 8 can be closed by the battery management system 14 and the first and second voltages monitored. If non-zero voltages are detected then one of the two supposedly open contactors 6, 7 or 8 is failed in a closed position. By switching between single closures of the different contactors 6, 7 or 8 each of the remaining contactors 6, 7 or 8 can be tested. This technique however has the disadvantage that the detection method necessarily involves a completed circuit involving the high voltage terminals 1 and 2 and, in the event of a failed contactor being present, the terminals 1 and 2 will be connected to the cell pack 3 as part of the detection method. That can result in unexpected high currents in apparatus connected to the terminals 1 and 2 and also in the contactors 6, 7 and 8, which could increase the risk of a further contactor 6, 7 or 8 failing.

In order to detect leakage currents, the battery management system compares the first and second voltages when the contactors 6 and 7 are closed. Since the circuit is symmetric, the first and second voltages will be equal in the absence of any leakage currents to the reference point. Asymmetric leakages will alter that balance and be detectable as a difference between the first and second voltages. However symmetric leakages will not remove the balance and may thus go undetected. That is illustrated in figures 5, 6 and 7. In figure 5 a cell pack is represented as two half-packs V1 and V2 of equal voltage. The measurement circuits from the positive and negative terminals have total resistances R10 and R11 respectively and the voltages in each case are measured relative to a reference point 23. Since the overall resistance of the measurement circuits R10 and R11 are equal, the voltages dropped across the two measurement circuits R10 and R11 are equal. Turning to figure 6, an asymmetric leakage to the reference point will result in a second path to the reference point 23 on one side of the circuit, with the resistance R12 of that path being in parallel to the resistance R10 of the measurement circuit. The two halves of the circuit will thus have different resistances and the voltages dropped across the two measurement circuits R10 and R11 will no longer be equal. Turning to figure 7, a symmetric leakage, which may result from leakages near the centre of the cell pack as shown or from symmetric leakages on either side of 30 the cell pack, results in a total leakage resistance R13 that does not affect the balance of the two halves of the circuit and that may therefore go undetected.

In figure 4, a battery system according to the invention comprises a plurality of cells 104 arranged in series to form a cell pack 103. The cell pack 103 includes 7 cells 104 and can be said to have mid-points 105 either side of the central cell 104. The positive end 110 of the cell pack 103 is electrically connected via contactors 107 and 108 and precharge resistor 109 to positive terminal 101. Contactors 101 and 108 are arranged in parallel with each other between the positive end 110 of the cell pack 103 and the positive terminal 101 and precharge resistor 109 is arranged in series with contactor 108. The negative end 111 of the cell pack 103 is electrically connected to negative terminal 102 via contactor 106. The battery system includes a battery management system 114, which monitors a first voltage between the positive terminal 101 and a reference point, such as ground or a vehicle chassis, a second voltage between the negative terminal 102 and the reference point and a third voltage between a location part way along the cell pack 103 and the reference point. The first voltage is monitored by a measurement circuit connected at a location 117 close to the positive terminal 101. The measurement circuit includes a high voltage attenuator circuit 112 which outputs a voltage in the range 0 to 5V in response to input voltages in the range -1000V to +1000V. The output voltage is read by an analogue to digital converter 115 and analysed by the battery management system 114. The second voltage is monitored via a measurement circuit connected at a location 118 close to the negative terminal 102. The measurement circuit also includes a high voltage attenuator circuit 112, the output of which is read by a second analogue to digital converter 116 and analysed by the battery management system 114. The third voltage is monitored via a measurement circuit connected at a location part way along the cell pack 103. In this case, the location is a mid-point 105 of the cell pack 103. The measurement circuit also includes a high voltage attenuator circuit 112, the output of which is read by a third analogue to digital converter 122 and analysed by the battery management system 114. The high voltage attenuators 112 could, for example, be high voltage attenuators 12 as described with reference to figure 3. It will be appreciated however that other high voltage attenuators or voltage measurement circuits could be used to measure the first, second and third voltages.

When not in operation, contactors 106, 107 and 108 are open to isolate cell pack 103 from positive terminal 101 and negative terminal 102. In that way the risk of someone receiving an electric shock from the terminals 101 and 102, or of the cell pack 103 being drained by residual currents in devices connected between the terminals 101 and 102, is reduced. When power is required, the contactors 106 and 108 are closed to create a circuit including the cell pack 103 and the terminals 101 and 102. The circuit also includes the pre-charge resistor 109, which limits the flow of current whilst any capacitance in the circuit is charged. That prevents very large currents as the contactors 106 and 108 are closed and thus reduces the risk of damage to the contactors 106 and 108 or to the cell pack 103. After a short time the contactor 107 is closed and the normal operational circuit is complete. The battery system is not electrically connected to ground or to any local reference point such as a vehicle chassis and instead is allowed to float. That has the advantage that someone in contact with ground or the chassis will not receive an electric shock if they inadvertently touch one of the terminals 101 and 102. In order for that to be the case, it is important that there are no undesired current paths from the battery system to ground or the chassis as the case may be. A further potential failure mode is that a contactor 106, 107 or 108 could fail. A contactor 106, 107 or 108 could fail open, in which case circuits will not be completed as required, or a contactor 106, 107 or 108 could fail closed. In the latter case a potentially hazardous situation may result whereby a terminal 101 or 102 that is thought to be isolated from the cell pack 103 may in fact still be connected to the cell pack 103. Such a scenario increases the risk of someone receiving an electric shock. A contactor 106, 107 or 108 may fail in a closed state due to arcing as the contactor 106, 107 or 108 closes, which can effectively weld the contactor 106, 107 or 108 in the closed position.

If all contactors 106, 107 and 108 are open then the first, second and third voltages should be zero.

However, if any one of the contactors 106, 107 or 108 is closed then the third voltage and either the first or second voltage will be non-zero. Thus the battery management system 114 can detect failure states in which any of the contactors 106, 107 or 108 are stuck open by closing the contactor 106, 107 or 108 individually and checking for a non-zero third voltage. Because the test can be performed with a single contactor 106, 107 or 108 closed, the test can be performed without powering any apparatus located between the positive 101 and negative 102 terminals. Similarly, if any one of the contactors 106, 107 or 108 fails closed, that will result in a non-zero third voltage, without any other of the contactors 106, 107 or 108 needing to be closed. The identity of the failed contactor 106, 107 or 108 can be determined first by observing which of the first and second voltages are non-zero, with a non-zero second voltage indicating failure of contactor 106 and a non-zero first voltage indicating a failure of contactor 107 or 108, and then, in the latter case, by attempting to close contactor 107. If contactor 107 is failed, then closing contactor 107 will have no effect. If contactor 108 is failed then closing contactor 107 will remove pre-charge resistor 109 from the measurement circuit, resulting in a change in the measured voltages. Since contactor 106 can remain open, and is known to be open due to the zero second voltage, closing contactor 107 does not create a risk of an unintentional completion of the circuit including the positive 101 and negative 102 terminals.

The battery management system 114 can detect asymmetric leakages in the same way as the prior art system using the first and second voltages, but can advantageously also detect leakage currents without closing contactors 106, 107 and 108 on both sides of the cell pack 103. If only contactor 106 is closed for example, a leakage that is asymmetric to that part of the battery system between the locations at which the second and third voltages are measured can be detected by comparing the second and third voltages, which will be equal in the absence of any leakage. Similarly, closing contactor 107 would allow detection of leakages asymmetric to the part of the battery system between the locations at which the first and third voltages are located. A further advantage is that leakages that are symmetric to the first and second voltages measurement circuit will be asymmetric to the first and third or second and third voltages measurement circuits. That is illustrated in figure 8, where the symmetric leakage that could not be detected with the system illustrated in figure 7, can now be detected because the symmetric leakage resistance R13 sits in parallel to, and therefore modifies the voltage dropped across, the measurement circuit resistance R14 for measuring the third voltage. The presence or absence of a path R13 can therefore be detected by comparing the ratio of the voltages dropped across R14 and R10 (that is, the third and first voltages) or across R14 and R11 (the third and second voltages).

The battery management system 114 can carry out a self-test by measuring the voltages of the individual cells 104 and summing them to determine the cell pack voltage. The cell pack voltage can be compared to the sum of the first and third voltages with the contactor 107 closed and the second and third voltages with the contactor 106 closed to verify that the measurement circuitry is operating as expected. As with the other measurements, the self-test can be performed without ever creating a complete circuit via the positive 101 and negative 102 terminals.

The operation of the battery management system may be further understood by reference to tables 1, 2 and 3, below. The voltages obtained in normal operation and in various failure states when all 10 contactors should be open are shown in table 1.

Table 1

Fault First Voltage Second Voltage Third Voltage None 0 0 0 First contactor stuck closed +Pack Voltage/4 0 -Pack Voltage/4 Third contactor stuck closed +Pack Voltage/4 0 -Pack Voltage/4 Second contactor stuck closed 0 -Pack Voltage/4 +Pack Voltage/4 First and Second contactors stuck closed +Pack Voltage/2 -Pack Voltage/2 0 Second and third contactors stuck closed +Pack Voltage/2 -Pack Voltage/2 0 Leakage path from centre of pack to reference 0 0 0 Leakage path from positive end of pack to reference 0 0 -V depending on leakage resistance Leakage path from negative end of pack to reference 0 0 +V depending on leakage resistance External leakage path from positive terminal to reference 0 0 0 External leakage path 0 0 0 from negative terminal to reference The voltages obtained in normal operation and in various failure states when the third contactor should be closed are shown in table 2. The third contactor in this case is across the first contactor, i.e. at the positive end of the pack.

Table 2

Fault First Voltage Second Voltage Third Voltage None (Just third contactor closed) +Pack Voltage/4 0 -Pack Voltage/4 Third contactor stuck open 0 0 0 Leakage path from centre of pack to reference +V depending on leakage resistance (> +Pack Voltage/4) 0 -V depending on leakage resistance (> Pack Voltage/4) Leakage path from positive end of pack to reference +V depending on leakage resistance (< +Pack Voltage/4) 0 -V depending on leakage resistance (< Pack Voltage/4) Leakage path from negative end of pack to reference +V depending on leakage resistance (> +Pack Voltage/4) 0 +V depending on leakage resistance External leakage path from positive terminal to reference +V depending on leakage resistance (< +Pack Voltage/4) 0 -V depending on leakage resistance (< Pack Voltage/4) External leakage path from negative terminal to reference +Pack Voltage/4 0 -Pack Voltage/4 The voltages obtained in normal operation and in various failure states when the second contactor should be closed, with the third contactor now open, are shown in table 3.

Table 3

Fault First Voltage Second Voltage Third Voltage None (Just second contactor closed) 0 -Pack Voltage/4 +Pack Voltage/4 Second contactor stuck open 0 0 0 Leakage path from centre of pack to reference 0 -V depending on leakage resistance (< -Pack Voltage/4) +V depending on leakage resistance (< +Pack Voltage/4) Leakage path from positive end of pack to reference 0 -V depending on leakage resistance (< -Pack Voltage/4) -V depending on leakage resistance Leakage path from negative end of pack to reference 0 -V depending on leakage resistance (> -Pack Voltage/4) +V depending on leakage resistance (> +Pack Voltage/4) External leakage path from positive terminal to reference 0 -Pack Voltage/4 +Pack Voltage/4 External leakage path from negative terminal to reference 0 -V depending on leakage resistance (> -Pack Voltage/4) +V depending on leakage resistance (> +Pack Voltage/4) From table 1, it can be confirmed that no contactors are stuck closed and that there are no leaks from the ends of the packs to the reference. Note that the example uses the ends of the packs, but any asymmetric leakage would be detected. Moving to table 2, it can be confirmed that the third contactor is not stuck open and leakages from the centre of the pack (symmetric leakages) and external leakages from the positive terminal can be detected. Note that any internal asymmetric leakage would have been detected in table 1, so the comparison of the table 1 and table 2 results enables the external leakage to be identified. Moving to table 3, it can be confirmed that the second contactor is not stuck open and external leakages from the negative terminal can be identified.

Thus by stepping through the three tables, the battery system can identify any fault before closing the first contactor. The ability to test the full system, including differentiating between intemal and external leakages and detecting asymmetric and symmetric internal leakages and contactor faults without ever having to complete the full circuit through the device being powered is a significant advantage in the present invention. The system will normally also measure the voltages of every cell in the pack. These can be summed to give the pack voltage. The pack voltage can be compared to the first, second and third voltages with at least one contactor closed. In the absence of a fault, the first, second and third voltages should be simple functions of the pack voltage (e.g. +Pack Voltage/4, see the tables above). Those comparisons may give a robust self-check of the circuitry with any single fault within the circuitry being detected by those comparisons.

In figure 9, a battery system comprises 2 battery packs with their respective contactors and voltage measurement circuits (indicated by 100a and 100b respectively). The first battery pack is represented as two half-pack voltages V1 and V2 and the second battery pack as two half-pack voltages V3 and V4. Voltage measurements are made in relation to V1 and V2 by measurement circuits indicated by resistors R10, R11 and R13. Contactors 106a and 107a, one of which may have a pre-contactor across it, are associated with battery pack V1 and V2. The second battery pack has associated with it voltage measurement circuits R15, R16 and R17 and contactors 107b and 106b. The first and second battery packs are connected in parallel to provide power via HV+ terminal 101 and HV-terminal 102. All voltage measurements are made relative to the same reference 23. Because the first and second battery packs are connected via the terminals 101 and 102, the measurements R10 and R15 and the measurements R11 and R16 are connected. However, the measurements R13 and R17 are specific to the first and second battery packs respectively. Thus, the system in figure 9 can advantageously test both parts of the system 100a and 100b simultaneously by making use of the measurements R13 and R17 to distinguish between the parts of the system 100a and 100b for any fault detected. For example, both packs can do the initial test for all associated contactors open set out in table 1 above at the same time. As the third voltage measurements R13 and R17 are unique to each pack then internal asymmetric leakage and any associated open contactor stuck closed can be determined (see table 1). If all the parallel packs then close an associated single contactor simultaneously, by reference to table 2 or 3 it will be seen that the third voltage measurements R13 and R17 (which are unique to each pack) allow the detection of associated contactors stuck open and internal symmetric leakage.

In prior art systems, detecting a single open contactor typically requires each pack in turn closing just one contactor, which results in excessive turn on time for the pack. Detecting in which pack a single closed contactor fault is located using a prior art system is even more complex. With the present invention the multiple packs in parallel can have the same start-up time as a single pack and the same excellent single fault coverage. The provision of battery systems with multiple packs in parallel has advantages in the mass-production of the packs, which can then be combined in any number for any given application. The battery system of the present invention advantageously permits those packs to be mass-produced with associated voltage measurement circuitry that allows a system including multiple packs to simultaneously test each of the packs and their associated contactors. With a number of packs combined in parallel, the time saving on start-up resulting from simultaneous testing can be significant.

It will be appreciated by persons skilled in the art that the above embodiments have been described by way of example only, and not in any limitative sense, and that various alterations and modifications are possible without departure from the scope of the invention as defined by the appended claims. For example, different attenuator circuits or measurement techniques could be used to provide the first, second and third voltages to the battery management system 114. Also, the checks performed in tables 1, 2 or 3 could be performed in another order, or a different set of checks, with a different sequence of contactor opening and closing, could be used.

Claims

Claims 1. A battery system comprising: a battery pack comprising a battery pack comprising a plurality of cells arranged in series; a positive terminal electrically connected via a first contactor to a positive end of the battery pack; a negative terminal electrically connected via a second contactor to a negative end of the battery pack; and a controller configured to measure a first voltage between the positive terminal and a reference point, a second voltage between the negative terminal and the reference point, and a third voltage between a location part way along the plurality of cells in the battery pack and the reference point and, based on the measured voltages, to detect a failure state of the battery system.
2. A battery system according to claim 1 wherein the failure state is selected from leakage of current from the battery system to the reference point, failure of the first contactor or failure of the second contactor.
3. A battery system according to any preceding claim wherein the location is in the central third of the plurality of cells.
4. A battery system according to claim 3 wherein the location is the mid-point of the plurality of cells.
5. A battery system according to any preceding claim, wherein the failure state is a symmetric leakage of current from the battery pack to the reference point.
6. A battery system according to any preceding claim, wherein the controller is configured to detect a failure state in which one of the first and second contactors has failed in a closed position, wherein the failure state is detected whilst the remaining contactors remain open.
7. A battery system according to any preceding claim in which the battery system comprises a third contactor in parallel with the first contactor between the positive end of the battery pack and the positive terminal and the controller is configured, based on the measured voltages, to detect a failure state involving at least one of the first, second and third contactors.
8. A battery system according to any preceding claim wherein the reference point is ground.
9. A battery system according to any of claims 1 to 7 wherein the battery system is for use in a vehicle having a chassis and the reference point is the chassis.
10. A battery system according to any preceding claim, wherein the controller is configured to compare a sum of the first, second and third voltages with a sum of the voltages of the plurality of cells so as to detect a failure state of the battery system.
11. A battery system according to any preceding claim, wherein the controller is configured to obtain a pack voltage, being the sum of the voltages of the plurality of cells, and compare the pack voltage to one or more of the first, second or third voltages with at least one contactor closed.
12. A battery system according to any preceding claim, wherein the system comprises multiple battery packs, each battery pack comprising a plurality of cells arranged in series; the positive terminal being electrically connected via respective first contactors to positive ends of each of the battery packs; the negative terminal being electrically connected via respective second contactors to negative ends of the battery packs; and the controller being configured to measure at least one first voltage between the positive terminal and a reference point, at least one second voltage between the negative terminal and the reference point, and respective third voltages between a location pad way along each of the plurality of cells in the battery packs and the reference point and, based on the measured voltages, to detect a failure state of the battery system.
13. A vehicle comprising a battery system according to any preceding claim.
14. A method of checking for a failure state in a battery system comprising: a battery pack comprising a plurality of cells arranged in series; a positive terminal electrically connected via a first contactor to a positive end of the battery pack; and a negative terminal electrically connected via a second contactor to a negative end of the battery pack, the method comprising measuring a first voltage between the positive terminal and the reference point, measuring a second voltage between the negative terminal and the reference point, measuring a third voltage between a location part way along the plurality of cells in the battery pack and the reference point and, based on the measured voltages, checking for the failure state.
15. A method according to claim 14, wherein the failure state is selected from leakage of current from the battery system to the reference point, failure of the first contactor or failure of the second contactor,
16. A method according to claim 15, wherein the failure state is a symmetric leakage of current from the battery pack to the reference point.
17. A method according to claim 15, wherein the failure state is one in which one of the first and second contactors has failed in a closed position, and wherein the voltages are measured and the failure state checked for whilst the remaining contactors remain open.
18. A method according to any of claims 14 to 17 wherein the location is in the central third of the length of the plurality of cells.
19. A method according to claim 18 wherein the location is the mid-point of the plurality of cells.
20. A method according to any of claims 14 to 19 in which the battery system comprises a third contactor in parallel with the first contactor between the positive end of the battery pack and the positive terminal and the method comprises checking for a failure state involving at least one of the first, second and third contactors based on the measured voltages.
21. A method according to any of claims 14 to 20, wherein the method includes a step of measuring the first and third voltages with the first contactor closed and the remaining contactors open and measuring the second and third voltages with the second contactor closed and the remaining contactors open and comparing the sum of the measured voltages to the sum of the voltages of the plurality of cells.
22. A method according to any of claims 14 to 21, wherein the method includes a step of obtaining a pack voltage, being the sum of the voltages of the plurality of cells, and comparing the pack voltage to one or more of the first, second or third voltages with at least one contactor closed.
23. A method according to any of claims 14 to 22, wherein the system comprises multiple battery packs, each battery pack comprising a plurality of cells arranged in series; the positive terminal being electrically connected via respective first contactors to positive ends of each of the battery packs; and the negative terminal being electrically connected via respective second contactors to negative ends of the battery packs, the method comprising measuring at least one first voltage between the positive terminal and a reference point, at least one second voltage between the negative terminal and the reference point, and respective third voltages between a location part way along each of the plurality of cells in the battery packs and the reference point and, based on the measured voltages, checking for a failure state of the battery system.
24. A method according to claim 23, wherein the method comprises simultaneously checking for faults associated with any of the battery packs.
25. A battery system substantially as herein described with reference to the accompanying figures 3, 4, 8 and 9.
26. A method substantially as herein described with reference to the accompanying figures 3, 4, 8 and 9.