Abstract:
A battery monitoring device for a battery having cells grouped in modules. The device includes a monitoring circuit for each module which monitors the voltage in each cell and the overall module voltage. The monitoring circuits can also detect module temperatures. The monitoring circuits are networked to a control computer. The device can be used with a power supply and relays for each module to interrupt charging when a fault condition is detected by the monitoring circuits. Other features of the device allow equalization of cells having excessive voltages.

Description:
STATEMENT OF GOVERNMENT INTEREST 
   The invention described herein may be manufactured and used by or for the Government of the United States of America for Governmental purposes without the payment of any royalties thereon or therefore. 

   BACKGROUND OF THE INVENTION 
   (1) Field of the Invention 
   The present invention relates generally to power management circuitry for a battery, and in particular to modular, digital power management circuitry. 
   (2) Description of the Prior Art 
   High power level rechargeable batteries are often necessary for specific applications. These batteries are made up of a plurality of series connected cells grouped in modules. In electric vehicle applications, batteries such as this are often capable of delivering in excess of 100 KW at a voltage of 400 VDC or above. A battery of this nature can have in excess of one hundred cells divided into individual modules or banks of cells. 
   A battery of this power level must be handled very carefully and monitored very closely during charge and discharge. This is particularly true of lithium ion batteries. The voltage of each cell must be monitored during discharge to ensure that no cell voltage is allowed to drop below approximately 2.1 VDC. Allowing a cell voltage below this level may cause irreversible damage to the cell. More importantly the voltage of each cell must be carefully monitored and controlled while charging. Overcharging a cell beyond approximately 4.3 VDC can result in catastrophic failure of the cell. 
   It is also desirable that all cells remain at the same voltage level, charge at the same rate, and reach the desired final voltage level at the same time. In practice, however, this is not the case; cell charge and discharge characteristics vary. All cells must be monitored very closely during charging. If any cell (or group of cells) reaches a predefined upper voltage limit prior to the rest of the cells, charging is suspended and the rogue cells must be discharged to the voltage level of the other cells. This process is referred to as “cell equalizing” or “cell equalization”. 
   Monitoring battery temperature during charge and discharge is also very important. Cell temperatures should not be allowed to exceed a predetermined temperature. If this temperature is reached, corrective action must be taken immediately. This corrective action can include shutting down the charge or discharge process or activating a cooling system. 
   These problems are specifically acute when using lithium-ion batteries; however, other battery chemistries have similar problems. In view of the prior art, there is a need for a battery monitoring and charging system that allows full monitoring and control of the battery. 
   SUMMARY OF THE INVENTION 
   One embodiment of the invention provides a battery monitoring device for a battery having cells grouped in modules. The device includes a monitoring circuit for each module which is connected in such a way as to monitor the voltage of each individual cell in the module in addition to the overall module voltage. The monitoring circuits can also detect module temperatures. The monitoring circuits are networked to a single control computer. During charging, each module&#39;s monitoring circuit is in direct control of a relay connecting the charge power supply to the battery and can interrupt charging when a fault condition is detected. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing invention will become readily apparent by referring to the following detailed description and the appended drawings in which: 
       FIG. 1  is a diagram providing an overview of an embodiment of the device and the associated battery; 
       FIG. 2  is a diagram showing the battery monitoring functions of the battery control board; 
       FIG. 3  is a diagram of the relay control apparatus for suspending battery charging in the event of a system fault or at charge completion; and 
       FIG. 4  is a diagram of the cell equalization circuitry. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENT 
   This embodiment provides a modular battery monitoring and charging system. The battery  10  is subdivided into a plurality of battery modules  12  each having a plurality of cells  14 . Each module  12  has a battery control board  16  associated with that module  12 . Each battery control board  16  has multiple temperature monitoring connections  18 , multiple cell connections  20  used for cell voltage monitoring and cell equalization, and a power relay connection  22 . Internal connections within module  12  and battery control board  16  will be shown hereinafter. Module  12  is only defined by its connection to a single battery control board  16 , and module  12  can have as many cells  14  as battery control board  16  can monitor. The battery control boards  16  are joined to a control computer  24  by a network  26 . For identifying the separate control boards  16  to the control computer  24 , each board  16  has a dip switch, memory, or other means for giving it a unique network address. This network  26  can be Ethernet™, wi-fi or any other networking technology. The charge control computer  24  provides coordination and control of the modules during discharge and charging. In order to charge battery  10 , there is a charging system  28  joined to network  26  and control computer  24 . Charging system  28  is further joined to power relay connection as discussed with relation to  FIG. 3 . Charging cables  30  and  32  are connected to the low and high side of battery  10 , respectively. 
     FIG. 2  shows an embodiment of the voltage and temperature monitoring aspects of the battery control board  16 . As discussed above, each battery control board  16  is joined to a group of cells  14  arranged in a module  12  of battery  10 . Board  16  is joined to monitor the temperature of cells  14  in the module by thermistors  34  joined within module  12 . Thermistors  34  are dispersed among cells  14  to monitor their temperatures and are joined to temperature monitoring connection  18 . Each thermistor  34  is connected to a low pass filter  36  for reducing noise in the temperature signal. The outputs of low pass filters  36  are joined to a multi-channel temperature analog to digital converter  38 . At least one board temperature sensor  40  is also joined to the temperature analog to digital converter  38  through low pass filter  36 . Board temperature sensor  40  provides the local temperature of the battery control board  16  and of the equalization circuitry, shown in  FIG. 4 , that is installed on control board  16 . This embodiment is capable of utilizing as many thermistors  34  and temperature sensors  40  as necessary to monitor module  12 . As such, this disclosure should not be limited by the number of thermistors or sensors shown. Furthermore, other temperature sensing devices can be used in place of thermistors. These include thermocouples and other temperature sensing components. 
   Voltage monitoring connection  20  is required to monitor the voltage in each cell  14  as well as the overall voltage of the module  12 . Ground  42  for these purposes is the lowest voltage in module  12 . This is the voltage identified at  44 . In view of the series arrangement of battery  10  and modules  12 , ground  42  for the specific module  12  could be well above the base voltage of battery  10 . Components of the embodiment must be capable of operating with these voltage differentials. The overall voltage of the module  12  is measured by a voltage divider utilizing resistors  46  and  48  set up between the highest voltage in the module  12  and the ground  42 . The highest voltage for module  12  is that at point  50 . The voltage divider is connected to the input of an op amp configured as a low pass filter  52  for reducing noise in the overall voltage measurement. The filter output is connected to a voltage analog to digital converter  54 . 
   Cell voltages are measured by joining voltage monitoring connection  20  on both sides of each cell  14 . The voltage between the high voltage and low voltage of each side of cell  14  is measured by a differential amplifier  56 . Differential amplifier  56  can be any differential amplifier having a common mode voltage rating capable of handling the highest voltage  50  in the module  12 . The output of the differential amplifier  56  is connected to voltage analog to digital converter  54  through a low pass filter  58 . Differential amplifiers  56  convert the various differential measurement common mode cell voltage levels present in the module and reference them all to a common ground  42  preventing these voltages from adversely affecting analog to digital converter  54 . Low pass filters  58  eliminate high frequency noise that may be present on the cell voltage signals. Low pass filters  58  can be implemented as two pole active low pass filters or as a passive low pass filters. Cutoff frequency of these filters  58  should be chosen as is necessary to eliminate high frequency noise. 
   As described above, filtered voltage measurements from the cells and the overall module voltage divider output are provided to one or more voltage analog to digital converters  54 . The number of converters  54  is dictated by the number of cells  14  in module  12  and the number of channels in each analog to digital converter  54 . The digital output of voltage analog to digital converters  54  is serialized by a parallel to serial interface  60 . Temperature analog to digital converter  38  are serialized by a separate parallel to serial interface  62  because analog to digital converter  38  operates at the same ground level as the processor  64  and does not require isolation. Interfaces  60  and  62 , which may be integral to the analog to digital converter, convert signals from the analog to digital converters into a serial form that can be interfaced to the serial interface of processor  64 . The function of processor  64  can be implemented using a wide range of digital signal processors, microcontrollers, or microprocessors. The serial output of interface  60  is isolated from processor  64  by an isolator  66 . Isolator  66  allows the processor  64  to operate at a common, system wide, ground rather than the module specific ground  42  discussed above. This allows the processors  64  from all control boards  16  to be powered from a single power supply. 
     FIG. 3  shows power relay control connection  22  that switches voltage to battery  10  during charging. Control computer  24  is joined by network  26  to each board  16  for controlling power distribution during charging and equalization. Control computer  24  is also joined by network  26  to an adjustable output charging power supply  68 . Charging power supply  68  is joined to charge battery  10  through a main power relay  70  and blocking diode  72  which prevents current from flowing from the battery back to the power supply  68  when the power supply  68  is turned off. The control coil of main relay  70  is energized by a fixed output power supply  74 . The control coil of main relay  70  and the fixed output power supply  74  are in series with a board relay  76  on each of the battery control boards  16 . If any one of the relays  76  is open then relay  70  is open and power supply  68  is disconnected from battery  10 . In the embodiment shown, power relay control connection  22  on boards  16  are connected in a series as a bus with the last connection having a terminator  78 . In other embodiments, the boards  16  could be connected directly in series with the last board being connected directly to relay  70  instead of through each board  16 , as shown. 
   On each board  16 , board relay  76  is connected to processor  64  through a driver  80 . Driver  80  is provided merely for giving the required control power for relay  76  and may not be necessary in some embodiments. Processor  64  is further connected to a network driver  82  in communication with network  26 . Processor  64  can control board relay  76  when it receives a command from control computer  24  or when it detects a fault through the sensors provided in relation to  FIG. 2 . 
   Adjustable output power supply  68  has a network driver  84  installed to communicate with network  26 . This allows control computer  24  to disable the adjustable output power supply  68  in case of fault or equalization. Network  26  is joined to a network hub  86 . Control computer  24  is joined to network hub  86  through network  26 . 
     FIG. 3  and  FIG. 4  show the circuitry necessary to equalize a cell  14  in a module  12 . Equalization is necessary when a cell  14  charges too fast and reaches a predefined upper limit before the rest of the cells  14 . This is detected first by the cells monitoring circuit board  16  and is communicated by board  16  to control computer  24  through network  26 . This equalization process is achieved by first opening main relay  70  thereby disconnecting power supply  68  from battery  10 . Processor  64  controls the main relay  70  by controlling its associated board relay  76 . Processor  64  then sends a signal to equalization driver  88  which activates equalization relay  90 . Equalization driver  88  and equalization relay  90  should cooperate to isolate processor  64  from ground  42 . Activation of equalization relay  90  places an equalization resistor  92  in parallel with cell  14 . Equalization relay  90  can be a photovoltaic relay, reed relay or other type of switch having the appropriate activation energy, current, and voltage ratings. Processor  64  monitors equalization voltage through voltage monitoring circuitry shown in  FIG. 2  until cell  14  voltage has been reduced to the desired level communicated by control computer  24 . Each cell  14  has its own equalization relay  90  and power resistor  92 . Once the cell or cells that were selected for equalizing have all reached the desired final voltage, all equalization relays  90  are opened, board relays  76  and main relay  70  are closed and charging resumes. 
   This embodiment features multiple layers of redundant fault sensing and control that are designed into the device. When charging, control computer  24  can be an external computer in constant communications with battery control boards  16  and with power supply  68  which has its own network interface  84 . Control computer  24  commands the power supply  68  voltage to 0 VDC and disables power supply&#39;s output  68  if it receives cell voltage or temperature data from one of the battery control boards  16  that is out of acceptable range specified by the user. Control computer  24  also disables power supply  68  if it loses communication with any of the battery control boards or receives a fault message from one of the boards. 
   The processor  64  on each battery control board is programmed to sample cell voltages and temperatures several times a second. Processor  64  compares these cell voltages to high and low voltage limits defined by the user and communicated to the processor  64  through a graphical user interface on control computer  24 . The graphical user interface can display cell voltage, module voltage, module temperatures and board temperatures. The graphical user interface can also provide user control for charging and equalization. Each processor  64  controls board relay  76  that controls the coil of main relay  70 . The voltage controlling main relay  70  and passing serially through board relays  76  on boards  16  is supplied from an external fixed output power source  74 . All processors  64  must close their respective board relays  76  in order for power to be applied to the main relay  70  that electrically connects the charge power supply  68  to the battery  10 . If any of the processors  64  senses a fault condition or loses communication with the control computer  24 , that processor  64  will command its board relay  76  to open which will also open the main relay  70  disconnecting the charge power supply  68  from the battery  10 . 
   It is to be understood that the foregoing description and specific embodiments are merely illustrative of the best mode of the invention and the principles thereof, and that various modifications and additions may be made to the invention by those skilled in the art, without departing from the spirit and scope of this invention, which is therefore understood to be limited only by the scope of the appended claims.