AU2008241371A1

AU2008241371A1 - Battery management system

Info

Publication number: AU2008241371A1
Application number: AU2008241371A
Authority: AU
Inventors: Rodney Colin Lawton Dilkes
Original assignee: Individual
Current assignee: Individual
Priority date: 2007-04-24
Filing date: 2008-04-24
Publication date: 2008-10-30
Anticipated expiration: 2028-04-24
Also published as: AU2008241371B2; WO2008128296A1; AU2008201759A1

Description

WO 2008/128296 PCT/AU2008/000565 Fall Safe Battery Management System Field of the Invention The present invention relates to a battery management system, particularly, although not exclusively, for batteries that comprise a number of Lithium Ion cells. Background Art The following discussion of the background art is intended to facilitate an understanding of the present invention only. The discussion is not an acknowledgement or admission that any of the material referred to is or was part of the common general knowledge as at the priority date of the application. It is known for batteries to consist or a number of cells connected in series and/or in parallel. Some batteries require controls to ensure that each cell remains between set upper and lower charge limits to prevent damage. In addition, overcharging or over discharging of the cells can lead to permanent damage of the cells, or even catastrophic failure. Lithium Ion batteries are a particular example of a requirement for a battery management system capable of preventing overcharge and over discharge. Lithium Ion batteries by themselves are generally not self balancing. That is, the cells will gradually take different charge levels over a number of charge cycles. During charging of a series connected battery some cells that may have a higher state of charge will become overcharged and possibly damaged. The same applies for deep discharges where damage can also result. Most inventions relating to Lithum Ion battery management revolve around centralised digital monitoring systems. These have wires that go to each cell and directly feed into a central processing unit. This can make for a sophisticated balancing and monitoring system however its long term reliability and fail safety can be questioned. It is essential that any battery management system exhibit a high degree of reliability in often hostile environmental conditions. Fail safe components are desirable, especially in situations such as electric vehicles where fires can result from battery failure.

WO 2008/128296 PCT/AU2008/000565 2 The scope of the current invention is to provide the simplest and most reliable battery balancing and safety monitoring system that will last for the entire life of the battery without failure. Disclosure of the Invention According to one aspect of the present invention, there is provided a battery management system for a battery comprising at least one cell, the battery management system comprising a master control unit coupled to charging and discharging means for charging and discharging the at least one cell, and at least one cell module coupled to the master control unit and to a respective cell, each cell having a respective cell module coupled thereto, whereby the cell module comprises a circuit operable to detect a change in state of the cell coupled thereto and to interrupt a control signal from the master control unit in response to the change of state such that the master control unit is operable to control charging or discharging of the battery in response to the control signal and the detected change in state. Preferably, the cell module includes circuit means for bypassing current when the voltage exceeds a predetermined amount. Preferably, the change of state is where the voltage across the cell terminals falls outside a predetermined range. The present invention provides a battery management system in which the cells within the battery are kept in a balanced state of charge thereby reducing the risk of permanent damage or failure. In addition, the battery management system operates in such a manner that if an individual cell fails or if the cell module circuit fails the master control unit will act to reduce or abort charging or discharging and in this manner will be fail safe. The battery management system is decentralised: operating at the cell level. That is, there is one cell module for each cell and it is attached between the positive and negative terminals. Each cell module operates independently of the others and is powered by the cell to which it is attached. This is an advantage over known battery management systems which manage the balancing of the cells at a centralised level.

WO 2008/128296 PCT/AU2008/000565 3 Brief Description of the Drawings The invention will now be described, by way of example only, with reference to the following drawings, of which: Figure 1 is a functional block diagram of a battery management system of a first embodiment of the present invention; Figure 2 is a circuit diagram for a cell module of Figure 1; Figure 3 is a schematic diagram of the master control unit of the battery management system of Figure 1; Detailed Description of a Best Mode for Carrying Out the Invention Throughout the specification and claims, unless the context requires otherwise, the word "comprise" or variations such as "comprises" or "comprising", will be understood to imply the inclusion of a stated integer or group of integers but not the exclusion of any other integer or group of integers. Throughout the specification and claims, unless the context requires otherwise, the word "cell" can apply to one or more smaller elements connected in parallel to effectively create a single cell of larger amp hour capacity. Throughout the specification and claims, unless the context requires otherwise, the word "optoisolator" can refer also to any similar device with an isolated output, such as an optocoupler or relay. In the present embodiment, a battery 1 comprises a plurality of cells 2 connected in series. In this embodiment, the cells are lithium ion cells, but other types and chemistries can also be used. The battery management system 100 of the present invention comprises a master control unit 3 and a number of cell modules 4: one cell module 4 for each individual cell 2. This is illustrated schematically in Figure 1.

WO 2008/128296 PCT/AU2008/000565 4 Each cell module 4 comprises electronic circuitry which will be described in further detail below. Each cell module 4 is connected to positive and negative terminals of its respective cell 2 via a suitable conductor 6a, 6b such as wire or metal plate and is - in this embodiment - located near to its respective cell 2, although this is not necessary. Each cell module 4 is also coupled to adjacent cell modules 4 via signal conducting means such as wires 11 or other suitable signal conductors such as optical fibres. The master control unit 3 controls the overall charge and discharge currents to the battery. In this regard, the master control unit 3 is coupled to a charge controller 8 and a discharge controller 9. The charge controller 9 and the discharge controller 8 can be any suitable charge controller and discharge controller respectively, as would be known to any person skilled in the art. In its simplest form the discharge controller 8 will be a relay or electrical contactor controlled by the master control unit 3 . If one or more cell modules 4 in the battery registers a fault to the master control unit 3, the master control unit 3 will cause the relay to open and disconnect the load 7 from the battery. A more sophisticated example of a discharge controller 8 may be a device that progressively reduces the current to the load 7 while the master control unit is signaling a fault. Another example of a discharge controller 8 may be to illuminate an alarm light or sound a warning buzzer. In the present embodiment of this invention the master control unit 3 can disconnect the battery charger 10 from the battery in the event of a battery fault. In this case the charge control unit 9 becomes part of the integrated master control unit 3a. The cell modules 4 are also coupled to the master control unit 3 as illustrated in Figure 1. In the present embodiment, each cell module 4 is coupled to the master control unit 3 and to each other by a one-wire interface - comprising the signal conductors 11. The one-wire interface comprises a one-wire circuit 11 providing a series connection of the cell modules 4 signal outputs 19. The one-wire circuit 11 is coupled to a voltage monitor signal input 17 of the master control unit 3. This will be described in further detail below.

WO 2008/128296 PCT/AU2008/000565 5 This single signal wire 11 connects to all of the cell modules 4 in series. Under normal conditions the voltage monitor circuit of each cell module 4 will be such that the ouput optoisolator 19 or relay will be in an active state or "closed circuit". With the cell modules 4 connected together in series thus, a small signal current supplied by the master control unit 3 can be made to flow through the signal wire 11. If the voltage of any cell 2 goes outside outside a safe predetermined range the cell module 4 voltage monitor circuit will cause its optoisolator 19 or relay to become inactive or "open circuit". In this manner the master control unit 3 will no longer be able to cause a current to flow in the signal wire 11 and will thus detect an error with the battery 1. The same also applies if the voltage monitor circuit of any cell module 4 fails. In this way the battery management system 100 can operate in a fail safe manner. The master control unit 3 controls the overall charge and discharge currents to the battery 1 in that it controls the charging and discharging directly or supplies signals to the charge controller 9 and discharge controller 8 to control the charging and discharging of the cells 2. The battery charger 10 is operable to provide a constant current/voltage of the appropriate level for the battery 1 such that it provides a constant current up to a predetermined voltage limit and then a lower constant current until the peak charging voltage for the battery 1 is reached, and then a constant voltage for a predetermined time or until the charging current reduces to a predetermined limit. The charging cycle is then complete, as is known to persons skilled in the art. In one functional aspect of the battery management system 100, the master control unit 3 is operable to reduce or cut off the charging current or the discharging current to the cells 2 if the voltage of one of the cells 2 goes outside predetermined limits, such as those considered being safe for the battery to be subject to. The battery management system 100 thus provides a fail safe operation when all the individual cells 2 are operating within a predetermined safe voltage in order to activate charging. If one of the cells 2 fails or begins to operate outside the safe limits, then master control unit 3 will detect an open circuit in the signal wire 11 and the master control unit 3 interrupts charging.

WO 2008/128296 PCT/AU2008/000565 6 The structure and operation of the battery management system 100 and the modules within the battery management system 100 will now be described in more detail. The cell module 4 circuit will now be described. Each cell module 4 comprises a shunt regulator circuit 12 and a voltage monitor circuit 13. This is illustrated in Figure 2. The shunt regulator circuit 12 is operable to bypass current from the cells 2 when the voltage across any one cell 2 reaches a predetermined limit, thus enabling the battery 1 state of charge to be balanced. The voltage monitor circuit 13 is operable to provide feedback to the master control unit 3 via signal wire 11 when cells are operating outside safe voltage limits. The operation of the shunt regulator circuit 12 will now be described. A circuit diagram for the shunt regulator circuit 12 is shown in Figure 2. As described above, each cell module 4 is coupled to its respective cell 2 via conductors 6a, 6b. The shunt regulator circuit 12 is coupled to the conductors 6a, 6b and is operable to bypass some or all of the charging current to its respective cell 2 when the voltage across the cell 2 reaches a predetermined level. The bypass level is set using a resistive voltage divider 14 provided in the shunt regulator circuit 12. This voltage divider acts to reduce the cell voltage applied to the reference input of shunt regulator IC 15 to a suitable level to activate the shunt regulator IC 15 at a predetermined voltage. The shunt regulator IC 15 when activated will cause transistor 23 to conduct current between the cell terminals 6a and 6b. Thus the circuit acts in total as a shunt regulator. The voltage at which bypassing commences is set to the same level for all cells 2 and is in the in the range 3.5 volt to 4.2 volt for Lithium Ion batteries. This level is determined by the battery cell chemistry. Other values may suit other types of cell. The bypass level will normally be set at or close to what is considered to be the fully charged voltage of the cell. This voltage can be arbitrarily assigned according to safety and lifecycle considerations. When the voltage across the cell 2 reaches the predetermined limit, some proportion of the charging current is bypassed through the shunt regulator circuit 12. In this way the first cell 2 to reach this voltage will have less available charging current as some of the WO 2008/128296 PCT/AU2008/000565 7 current is being lost to heat through the shunt regulator. This will allow other cells 2 at a lower state of charge to catch up or equalize their state of charge. Charging of the cells will then continue until all the cells 2 reach the same state of charge and become balanced. The fully charged and balanced voltage is determined by the battery charger 10 peak voltage. This voltage will normally equal the number of cells 2 in the battery 1 multiplied by the cell fully charged voltage value. The shunt regulator circuit 12 includes an LED 18 which lights when charging current is bypassed by the shunt regulator circuit 12. This indicates that the relevant cell 2 has reached its predetermined voltage. The voltage monitor circuit 13 will now be described. This enables the battery management system 100 to detect when at least one of the cells 2 is operating outside safe limits. As can be seen from Figure 2, the voltage monitor circuit 13 is also coupled to the conductors 6a, 6b and in parallel with the shunt regulator circuit 12. The voltage monitor circuit 13 is coupled to the one-wire circuit 11 via an optoisolator 19. The voltage monitor circuit 13 detects whether the voltage across the respective cell 2 is within predetermined upper and lower limits. These upper and lower limits are set using second and third resistive voltage dividers 20, 21 respectively. The upper maximum voltage is determined by voltage divider 20 and the lower minimum voltage is determined by voltage divider 21. Voltage dividers 20 and 21 act in unison with their respective regulator ICs 20a, 21 a such that the optoisolator 19 will be active when the voltage between conductors 6a and 6b is between the predetermined upper and lower limits. In this embodiment, the upper limit is 4.2 volt and the lower limit 2.5 volt. These limits have been arbitrarily assigned according to the cell type and chemistry and as such the values are examples only. These limits are defined by altering the values of the resistors that make up the potential dividers in the voltage monitoring circuit component.

WO 2008/128296 PCT/AU2008/000565 8 Each of the optoisolators 19 are coupled in series via the one-wire circuit 11 to adjacent cell modules 4 in a logical "AND" arrangement. In other words all of the optoisolators must be in a conducting state for current to flow in the on-wire circuit 11. When the voltage across a cell 2 is within the predetermined upper and lower limits, then the optoisolator 19 will conduct current. This represents the "OK" state. If all the optoisolators 19 are conducting, then current will flow along the one-wire circuit 11 and a closed circuit will be detected at the voltage monitor signal terminals 17 of the master control unit 3. If any of the cells 2 begin to operate outside the predetermined upper and lower limits i.e. outside a safe range, then the respective optoisolator 19 will cease to conduct and the signal current supplied by the master control unit 3 will cease to flow along the one wire circuit 11, the master control unit 3 via signal terminals 17 will thus detect an open circuit and so the master control unit 3 will determine that one of the cells is operating outside safe limits. In response to this, the master control unit 3 will be operable to interrupt charging or discharging. This is therefore a fail safe arrangement. As with the shunt regulator circuit 12, an LED 22 is provided coupled to the optoisolator 19 to provide a visual indication of safe operation on the cell module. In this embodiment of the cell module 4, the cell module 4 has an added advantage that when the shunt regulator circuit 12 is bypassing current, heat generated is directed to the terminals of the cell: thus the cell 2 acts as a heat sink. This means that no external heat sink is required for the cell module 4. Figure 3 is a circuit diagram for the master control unit 3. The master control unit 3 wil now be described. The master control unit has three primary circuits. The power circuit 24 contains the power supply and cell module monitoring function. The load control circuit 31 contains an output relay 29 to control the load. The charge control circuit 9 contains functionality to disconnect the charge current in event of an abnormal state.

WO 2008/128296 PCT/AU2008/000565 9 In the present embodiment of this invention the master control unit 3 is powered directly from the battery 1 via connectors 5a, 5b. This feeds a dc-dc converter 25 which has at its output 12 volt DC. A signal current output 17 is connected to the cell modules 4. This current is limited to a small amount by resistor 26 which is attached to the base of transistor 27. If all the cells 2 are within the predetermined voltage range the cell module signal circuit 11 will conduct some current which will in turn cause transistor 27 to switch on and conduct current through relay 28. If the voltage of one or more cells 2 falls outside the predetermined range the signal output 17 will appear as a high resistance or open circuit. In this case the transistor 27 will switch off and not conduct current through relay 28. If all of the cell 2 voltages are within the predetermined range the normally open relay 28 will be active and thus the output contacts will be closed. This provides +12 volt to the coil of the normally open load control relay 29. In turn this relay will be closed which will indicate to the load control device 8 that conditions are "OK" and the load can operate normally. LED 30 will also illuminate at this time. If all the cell 2 voltages are within the predetermined range the relay 28 will provide +12 volt to the charge control section 9. When the charge reset button 33 is pressed, the normally open charge reset relay 34 will be activated and remain activated until a cell module 4 indicates a change of state or the master control unit is switched off via switch 35. This will in turn activate the normally open charge control relay. 36 which will indicate to the battery charger 10 that conditions are "OK". In the event of a change of state to "abnormal" the charge control relay 36 will deactivate and stay deactivated until the charge reset button 33 is again pressed. When the charge control relay 36 is active an LED 37 will illuminate to indicate that the charging circuit is active. It can be seen in this instance that the charge control circuit 9 directly controls the charge current which will be disconnected in the event of an abnormal battery 1 state. In this manner the master control unit 3 and charge control unit 9 are integrated into one device 3a which directly controls the battery charger. All of the relays in the master control unit 3 require a +12V supply to keep them in an active or "OK" state. It can be seen that any circuit failure or power failure will result in both the charge control relay 36 and load control relay 29 becoming inactive and thus WO 2008/128296 PCT/AU2008/000565 10 inhibiting charging and discharging. In this manner the master control unit 3 exhibits a fail safe characteristic. Variations are possible within the scope of the invention. For example alternative circuit arrangements, upper and lower cell voltage limits and/or component and component values can be used. Different voltage ranges within the master control unit 3 could also be applied. In particular the master control unit could differ from the preferred embodiment by the use of digital technology to detect continuity in the circuit 11. The battery management system can work with any number of cells and cell-types. In particular a variation within the scope of this invention may include optical means of signal transmission between the cell modules 4 and the master control unit 3 rather than the electrical circuit 11.

Claims

1. A battery management system for a battery comprising at least one cell, the battery management system comprising a master control unit coupled to means for charging and discharging the at least one cell, and at least one cell module coupled to the master control unit and to a respective cell, each cell having a respective cell module coupled thereto, whereby the cell module comprises a circuit operable to detect a change in state of the cell coupled thereto and to supply a control signal to the master control unit in response to the change of state such that the master control unit is operable to control charging or discharging of the cell in response to the control signal and the detected change in state.

2. A battery management system according to claim 1, wherein the cell module includes circuit means for bypassing current when the voltage exceeds a predetermined amount.

3. A battery management system according to claim 1, wherein the change of state is where the voltage across the cell falls outside a predetermined range.

4. A battery management system according to claim 3, wherein the cell module comprises a circuit operable to change the state of an optoisolator or relay according to the said change of state in claim 3.

5. A battery management system according to claim 4, wherein the cell module optoisolator or relay is connected in series with the optoisolators or relays of the other cells comprising the battery.

6. A battery management system according to claim 5, wherein the master control unit can detect a change of state in one or more cell modules by attempting to pass a signal through the series connected optoisolators or relays of the cell modules.

7. A battery management system according to claim 6 wherein as a result of detecting a change of state the master control unit can control or interrupt the charging and discharging means.