Abstract:
A method for operating a battery system having multiple battery packs. The method includes decoupling the output of a discharged battery pack from the vehicle load, reducing the voltage between an output of a charged battery pack and the vehicle load prior to coupling the output of the charged battery pack to the vehicle load.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority to, and hereby incorporates by reference, U.S. Provisional Application No. 61/260,393, filed Nov. 11, 2009 and entitled “Interlock Mechanism for a Multiple Battery Pack.” 
     
    
     TECHNICAL FIELD 
       [0002]    The present invention relates to battery systems. 
       BACKGROUND 
       [0003]    For electrical equipment that uses batteries and requires high power, such as electric or hybrid vehicles, the battery system will often contain multiple battery packs, only one of which will be powering the system at any time. The battery system therefore needs be able to switch from one pack to another in a manner that protects the battery packs and contactors, especially while the vehicle is in operation. Conventional systems with multiple packs generally switch to a different pack by shutting the load off completely regardless of the state of the operation and waiting for a long period to switch over. This can result in potentially dangerous problems including long switchover times that disable the vehicle operation for a long duration or switch over while the vehicle is midst of acceleration. 
         [0004]    An example of a battery system for an electric vehicle with N battery packs is shown in  FIG. 1 , where N is 2 or greater. In this system, only one of the battery packs is connected to the vehicle load at a time. When the charge in battery pack  1  is exhausted, the system may switch to the next battery pack with available charge. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0005]    The present invention is illustrated by way of example, and not limitation, in the figures of the accompanying drawings and in which like reference numerals refer to similar elements and in which: 
           [0006]      FIG. 1  illustrates an exemplary diagram of an existing multiple battery pack system; 
           [0007]      FIG. 2A  illustrates an exemplary diagram of a multiple battery pack system with reduced battery pack switchover delays; 
           [0008]      FIG. 2B  shows an exemplary system diagram for a battery management system; 
           [0009]      FIG. 3  illustrates an exemplary sequence of operations to switch between battery packs in a multiple battery pack system; 
           [0010]      FIG. 4  illustrates an exemplary graph of the relationship between voltage and time during a battery pack switch over; 
           [0011]      FIGS. 5A ,  5 B illustrate exemplary sequences of operations to determine if contactors are in a closed or open state; 
           [0012]      FIG. 6  illustrates an exemplary circuit to determine if contactors are in a closed or open state; and 
           [0013]      FIG. 7  illustrates a further exemplary sequence of operations to determine if contactors are in a closed or open state. 
       
    
    
     DETAILED DESCRIPTION 
       [0014]    In various embodiments disclosed herein, a pre-charge device and contactor test device are used to reduce the delay to switch between battery packs in a multiple battery pack system compared to delays required in prior-art techniques. The reduction in switchover delay provides many significant benefits, including, for example, improved electric/hybrid vehicle drivability and safety as an interruption in power during a critical time period is less likely. Moreover, battery life is improved because a) multiple battery packs are prevented from being connected at the same time, and b) the intrinsic capacitance of the vehicle load is prevented from becoming discharged, both of which could cause excessive current flow and damage to the battery system. While the foregoing benefits apply particularly to electric/hybrid vehicles, any system or apparatus which requires switchover between battery packs may benefit from the techniques described herein. 
         [0015]      FIG. 2A  illustrates one embodiment of a battery pack system with contactor test  240  and pre-charge  220  devices to reduce the time taken to switch between battery packs. The system is controlled by a battery management system (BMS)  270 . The vehicle management system (VMS)  280  communicates with the BMS via a controller-area network (CAN) bus and monitors and/or controls the vehicle load  260 . The BMS, shown in  FIG. 2B , includes a processor, memory, and I/O block coupled together with an interconnect bus. The BMS is capable of executing the sequences of operations disclosed herein. One embodiment of the pre-charge device is a resistor with a relay that is controlled by the BMS. The resistance is calculated to a) allow current flow that can charge the intrinsic capacitance of the vehicle load  260  within a desired time limit, and; b) limit the current flow from the battery pack  210  below a maximum limit. An alternative embodiment of the pre-charge device could be a current source. The contactor test device  240  is used to determine if one or both of the contactors connected to a battery pack are either open or closed. In one embodiment, this information is used to prevent multiple battery packs being connected to the vehicle load at the same time. The diode  230  and diode contactor  225  are used to allow the depleted battery pack to provide a low level of power during the pre-charge period of switching between battery packs. A contactor may be any type of switching element including, for example and without limitation, a relay (or any other mechanical switch), a semiconductor device, or an electrically-isolating switching element. 
         [0016]      FIG. 3  illustrates an exemplary sequence of operations used to switch between battery packs in a multiple battery pack system. At step  315  the process waits until the charge in the current battery pack (in this example pack  1 ) becomes sufficiently low that a pack switchover is required. At step  325  contactor NB (from  FIG. 2A ) is closed to connect the negative terminals of the two battery packs. The diode protected positive contactor DC 1  is closed on the battery pack currently in use in step  330 . This allows current to continue flowing from the depleted battery pack but prevents any reverse current. At  335  the BMS monitors the vehicle power demand, and when it drops below a threshold (programmable based on the characteristics of the contactors and other factors), it opens Contactor  1 A in step  340 . This will cause the power draw to immediately switch to Contactor DC 1   225  (through the diode). Next in  345 , the contactor test device  240  is used to determine if Contactor  1 A has opened successfully. One embodiment of the contactor test device is shown in  FIG. 6  described below. 
         [0017]    At step  350  the vehicle is completely operating from power supplied through Contactor DC 1  which prevents any current from flowing into the Battery  1  through diode D 1 . The pre-charge for the next battery pack to be used (in this case pack N) is enabled to allow current to flow. This will allow the pack voltage to increase until it reaches the voltage of battery pack N. 
         [0018]    As long as the vehicle continues to draw a lower level of power the pre-charge will be successful; however, if the vehicle starts drawing more power than the pre-charge circuit can accommodate, the pre-charge will be reset and the vehicle will return to operation using battery pack  1 . Different embodiments may have different pre-charge circuits that are capable of operating at different (higher or lower) levels of power drawn by the vehicle. This sequence will continue until the vehicle remains in a low power state long enough to fully pre-charge the pack to the pre-charge level of pack N (see  FIG. 4 , described below). When the pre-charge level of pack N reaches a predetermined threshold (which may be stored in a plurality of programmable registers in the BMS) in step  355 , in this example defined as when the voltage at V load + reaches 90% of the battery pack N voltage (although other methods could be used to determine successful pre-charge), contactor NA is closed and the pre-charge for battery pack N is disabled in step  360 , allowing the pack to draw full power from pack N. The contactor test device is again used to determine if contactor NA has actually closed in step  365 . If contactor NA is determined to not be closed, then the sequence continues back to step  350  to repeat the pre-charge and contactor closing operations. The diode D 1  prevents energy from flowing from pack N into pack  1 . Next in step  370  contactors  1 B and DC 1  are opened. The state of contactors  1 B and DC 1  state is verified, again using a contactor test device, in step  375 . If both contactors are verified (or confirmed) to be open, the vehicle is then completely switched from battery pack  1  to battery pack N. Otherwise, the sequence loops back to attempt to open the contactors again. 
         [0019]      FIG. 4  is a graph of the changing voltage across the vehicle load  260  as the switchover process progresses, as well as the effect of increasing the vehicle load during the pre-charge process. In the example shown, the connection to battery pack N completed when the V load  voltage reaches the predetermined threshold, in this example 90% of the battery pack N voltage. 
         [0020]      FIGS. 5A and 5B  provide some exemplary sequences of steps to determine (or confirm or verify) if a contactor is open or closed. Referring to  FIG. 5A , beginning at step  505 . First the contactor (in this case  1 A) is opened. Next the pre-charge for that contactor is enabled in step  515 . If the voltage across the vehicle load (V load ) rises as checked in step  520 , then it may be inferred that the contactor is open as the depleted battery pack would otherwise hold the voltage low against the pre-charge. This inference may be taken as a confirmation or verification for the purposes of battery pack switchover. Similarly a confirmation or verification that a contactor has been closed can be performed as shown in  FIG. 5B . First the pre-charge is disabled in step  545 . Then the contactor (in this example contactor NA) is closed in step  550 . Next V load  is measured over time (as the load changes) and evaluated for stability in step  555 . If the V load  is stable, then the contactor is closed and the charged battery is providing power to the system. 
         [0021]      FIG. 6  shows one embodiment of a circuit that detects whether contactors are in an open or closed state. The circuit consists of load A and load B, test switch A and test switch B and voltage sensors  510  to temporarily connect a load between the two terminals of the battery pack and detect current flowing through the load. One embodiment may use resistors for loads and MOSFETs for switches although other components with similar capabilities could be used. 
         [0022]      FIG. 7  illustrates an exemplary sequence of operations that may be used to determine whether contactors are closed and thus implement contactor test  240 . Referring to both  FIG. 6  and  FIG. 7 , the sequence begins by closing test switch B in operation  720 . If voltage sensor  510  indicates that current is flowing through load B (i.e., a voltage drop greater than a predetermined or programmable threshold is detected), then it may be inferred that both contactors A and B are closed as no current would flow from the battery pack if both contactors A and B were open. Conversely, if voltage sensor  510  indicates that no or negligible current is flowing through load B, then processing continues with step  740  where test switch A is closed. If current then flows through load A as checked in step  750 , then contactor A is closed. Otherwise contactor A is open. 
         [0023]    The embodiments described herein may be applied to any type of device that can store energy, rechargeable or non-rechargeable, including, but not limited to, alkaline, lithium-ion, nickel-cadmium, lead-acid, flow and atomic batteries, fuel cells, and capacitors. 
         [0024]    In the foregoing description and in the accompanying drawings, specific terminology and drawing symbols have been set forth to provide a thorough understanding of the present invention. In some instances, the terminology and symbols may imply specific details that are not required to practice the invention. For example, the term “coupled” is used herein to express a direct connection as well as a connection through one or more intervening circuits or structures. Device or system “programming” may include, for example and without limitation, loading a control value into a register, one-time programmable-circuit (e.g., blowing fuses within a configuration circuit during device production) or other storage circuit within an integrated circuit device of the host system (or host device) and thereby control an operational aspect of the host system or establish a host system configuration. The terms “exemplary” and “embodiment” are used to express an example, not a preference or requirement. Signal paths that appear as single conductors may include multiple conductors and vice-versa, and components shown as being included within or forming part of other components may instead be disposed separately from such other components. With regard to flow diagrams and the like, the order of operations may be different from those shown and, where practical, depicted operations may be omitted and/or further operations added. 
         [0025]    While the invention has been described with reference to specific embodiments thereof, it will be evident that various modifications and changes may be made thereto without departing from the broader spirit and scope. For example, features or aspects of any of the embodiments may be applied, at least where practicable, in combination with any other of the embodiments or in place of counterpart features or aspects thereof. Accordingly, the specification and drawings are to be regarded in an illustrative rather than a restrictive sense.