Patent Publication Number: US-2010117594-A1

Title: Strategy for maintaining state of charge of a low-voltage battery bank in a hybrid electric vehicle having a high-voltage traction battery bank

Description:
TECHNICAL FIELD 
     The technical field of this patent application concerns hybrid electric vehicles of the type in which the propulsion system comprises a combustion engine associated with an electric motor/generator that at times operates as a traction motor for propelling the vehicle and at times as a generator for maintaining state-of-charge (SOC) of a high-voltage traction battery bank. More particularly, the disclosure of this patent application relates to a strategy for recharging a low-voltage battery bank, typically a nominal 12-volt or 24-volt DC battery bank that unlike the traction battery bank, isn&#39;t used to propel the vehicle, when for whatever reason, the traction battery bank, acting through a DC-to-DC converter, becomes unable to maintain SOC of the low-voltage battery bank. 
     BACKGROUND OF THE DISCLOSURE 
     A hybrid electric vehicle whose propulsion system comprises an electric motor/generator associated with a combustion engine can operate with significantly greater fuel economy in comparison to a corresponding vehicle that is propelled only by a combustion engine because at certain times during operation of the vehicle, such as during vehicle braking for example, the motor/generator recovers kinetic energy from the vehicle by operating as a generator that generates electric current for recharging a high-voltage traction battery bank, and at other times during operation of the vehicle, the motor/generator operates as a traction motor that draws electric current from the traction battery bank to propel the vehicle either by itself, or to add additional torque to that being produced by the combustion engine. Because of the fuel economy improvements that can result, the added cost of such a propulsion system can have favorable cost implications for users such as commercial truckers. 
     The development of some hybrid electric vehicles begins by integrating a motor/generator with the powertrain of a more conventional motor vehicle that is propelled by an internal combustion engine, either gasoline or diesel. The existing electrical system of such a vehicle is a low-voltage one, such as a 12-volt DC system, that serves the electrical demands of many electrical devices in the vehicle. The battery bank of a low-voltage electrical system comprises one or more DC storage batteries whose SOC is maintained by an alternator that is driven by the engine when the engine runs. 
     Because a low-voltage motor/generator is generally considered unsuitable for use as a traction motor of a hybrid electric vehicle, the design of such a vehicle is predicated on the addition of a high-voltage electrical system, thereby endowing the vehicle with separate electrical systems, the usual low-voltage one and the additional high-voltage one. 
     The high-voltage electrical system comprises a high-voltage traction battery bank whose voltage can range as high as about 600 volts DC, with a 340-volt DC system being one example. 
     Certain non-hybrid vehicles depend on the low-voltage battery bank to operate certain accessory equipment when the vehicle is parked with the engine not running. In order to maintain SOC of the low-voltage battery bank because of the accessory load, the alternator may be operated by running the engine. 
     While the same may be true for a hybrid electric vehicle, the presence of a high-voltage traction battery bank provides an additional, and larger, source of energy that is available to operate the accessory equipment. Consequently the low-voltage and the high-voltage electrical systems may associated through a DC-to-DC converter that utilizes the SOC of the traction battery bank to maintain SOC of the low-voltage battery bank when the engine is shut off and low-voltage electrical accessory equipment is in use. One example of such accessory equipment is an electric power take-off (ePTO). 
     SUMMARY OF THE DISCLOSURE 
     Because the applicant has recognized the possibility that the high-voltage electrical system may, for whatever reason, become unable to maintain SOC of the low-voltage battery bank and that depletion of low-voltage battery bank SOC below some threshold level can begin to impact battery bank life and/or operation of accessories that are drawing electricity from the battery bank, this disclosure presents a solution for anticipating such depletion and taking steps to avoid it. 
     A hybrid electric motor vehicle comprises a combustion engine, a high-voltage electrical system comprising a high-voltage battery bank, a low-voltage electrical system comprising a low-voltage battery bank, an electric generator driven by the engine for recharging the low-voltage battery bank, a DC-to-DC converter for recharging the low-voltage battery bank from the high-voltage battery bank, a monitor for indicating voltage of the low-voltage battery bank, a recharge initiate timer that, while the high-voltage battery bank and not the generator is recharging the low-voltage battery bank via the DC-to-DC converter, is started by the monitor indicating that voltage of the low-voltage battery bank is below a recharge initiate threshold and whose rate of timing is a function of voltage indicated by the monitor as the timer times. If the timer times to a recharge initiate limit, the generator begins recharging the low-voltage battery bank. 
     A method for recharging a low-voltage battery bank of a low-voltage electrical system in a hybrid electric vehicle whose propulsion system comprises an electric motor/generator associated with a combustion engine such that at certain times during operation of the vehicle, the motor/generator recovers kinetic energy from the vehicle by operating as a generator that generates electric current for recharging a high-voltage traction battery bank in a high-voltage electrical system of the vehicle, and at other times during operation of the vehicle, the motor/generator operates as a traction motor that draws electric current from the traction battery bank to propel the vehicle either by itself, or to add additional torque to that being produced by the combustion engine The vehicle further comprises an electric generator driven by the engine and a DC-to-DC converter through which the low-voltage battery bank is recharged by the high-voltage battery bank. 
     The method comprises monitoring voltage of the low-voltage battery bank and when the monitored voltage is below a recharge initiate threshold, while the high-voltage battery bank and not the generator is recharging the low-voltage battery bank via the DC-to-DC converter, starting a recharge initiate timer and as the timer times, controlling the timer&#39;s rate of timing as a function of monitored voltage. Once the timer has timed to a recharge initiate limit, the generator is caused to begin recharging the low-voltage battery bank. 
     The foregoing summary, accompanied by further detail of the disclosure, will be presented in the Detailed Description below with reference to the following drawings that are part of this disclosure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of a representative propulsion system of a hybrid electric vehicle. 
         FIG. 2  is a schematic wiring diagram of portions of high- and low-voltage electrical systems in the hybrid electric vehicle. 
         FIGS. 3A and 3B  collectively comprise a strategy diagram that embodies the aforementioned solution. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows a portion of an exemplary propulsion system  10  of a hybrid electric vehicle  12  as background for ensuing explanation of the other Figures. Not all mechanical detail of propulsion system  10  is shown. Vehicle  12  is shown, by way of example, as a rear wheel drive type vehicle, in which propulsion system  10  is configured such that an output shaft of an internal combustion engine  14  and a rotor of a rotary DC electrical machine (i.e. a motor/generator)  16  are suitably coupled to an input shaft of a transmission  18  such that either or both engine  14  and motor/generator  16  can propel vehicle  12  via a drivetrain in which an output of transmission  18  is coupled via a driveshaft  20  to a differential  22  of a rear axle  24  having wheels  26  attached to outer ends of respective shafts, and such that when kinetic energy of the vehicle is to be recovered, the drivetrain can operate motor/generator  16  as a generator to re-charge a high-voltage traction battery bank  28  ( FIG. 2 ) that stores the recovered energy for later use in operating motor/generator  16  as a motor. 
     Battery bank  28  is a constituent of a high-voltage electrical system (negative ground, 340 VDC for example) in vehicle  12 . A low-voltage electrical system (negative ground, 12 VDC for example) in vehicle  12  comprises a low-voltage battery bank  30  ( FIG. 2 ) of one or more batteries whose SOC is maintained by an alternator  32  ( FIG. 1 ), or any equivalent electric generator, that is driven by engine  14  through any suitable coupling such as a belt and sheaves to generate electricity for keeping battery bank  30  properly charged. 
       FIG. 2  shows an electronic system controller (ESC)  34 , a remote power module (RPM)  36 , an in-cab dash panel  38 , a hybrid control module (HCM)  40 , a transmission control module (TCM)  42 , a push button shift console  44 , and a DC-to-DC converter  46 . 
     A CAN (computer area network) data link  48  provides a data communication path between ESC  34  and RPM  36 . A CAN data link  50  provides a data communication path between ESC  34  and various controls and displays of dash panel  38 . A CAN data link  52  provides a data communication path between ESC  34  and both HCM  40  and TCM  42 . A CAN data link  54  provides a data communication path between HCM  40  and DC-to-DC converter  46 . A CAN data link  56  provides a data communication path controls of shift console  44  and TCM  42 . 
     ESC  34  is in a low-voltage electrical system of vehicle  12  and controls and monitors various aspects of vehicle operation including engine  14 . The controls and displays of dash panel  38  are also in the low-voltage system. Communication among ESC  34 , dash panel  38 , HCM  40 , TCM  42 , and controls of shift console  44  provides for coordinated control of propulsion system  10  to propel vehicle  12  by engine  14  operating alone, by motor/generator  16  operating alone, or by motor/generator  16  operating to supplement operation of engine  14 , while enabling kinetic energy of vehicle  12  to be recovered and re-used via motor/generator  16  in conjunction with high-voltage battery bank  28 . 
     DC-to-DC converter  46  has an input coupled to high-voltage battery bank  28  and an output coupled to low-voltage battery bank  30 .  FIG. 2  shows a cable  58  running from a positive terminal of battery bank  28  to a positive input terminal of DC-to-DC converter  46  and a cable  60  running from a positive output terminal of converter  46  to a positive terminal of battery bank  30 . Both battery banks are commonly grounded to a ground  62 . Voltage of battery bank  30  can be monitored by ESC  34  in any suitably appropriate way, such as by a direct connection  64 . 
     A strategy  66  for anticipating and avoiding charge depletion of battery bank  30  when alternator  32  is not delivering recharging current to it is presented in  FIGS. 3A and 3B . While the strategy is intended to become active in a situation where vehicle  12  is parked with engine  14  not running and electrical accessory equipment is being supplied with current from battery bank  30 , it may be come active in other situations. An example of one situation is that of the chassis being placed in the ePTO mode of operation (step  68  in  FIG. 3A ) that allows an electric power take-off to operate by drawing electric current from battery bank  30 . 
     A step  70  shows that ESC  34  monitors the voltage of battery bank  30  (sometimes referred to as chassis battery voltage). As long as the monitored voltage remains above a preset minimum threshold value (sometimes referred to as a recharge initiate threshold) as determined by a step  72 , then ESC  34  keeps a minimum voltage proportional timer (sometimes referred to as a recharge initiate timer) inactive and engine  14  off, as indicated by steps  74  and  76 . 
     Should the monitored voltage become less than the preset minimum threshold value as determined by a step  78 , then ESC  34  starts the minimum voltage proportional timer as indicated by a step  80 . Assuming that the monitored voltage continues to remain less than the preset minimum threshold value until the timer times out after having timed a presettable recharge initiate limit (step  82 ), ESC  34  causes engine  14  to be cranked and started (step  84 ,  FIG. 3B ) upon timer time-out, starting alternator  32  in the process. With alternator  32  being directly connected to battery bank  30 , battery bank  30  begins to be recharged by the alternator. 
     As long as the voltage of battery bank  30  remains less than the preset minimum threshold value (step  86 ), engine  14  continues to run (step  88 ), continuing the re-charging of battery bank  30  by alternator  32 . 
     Should the voltage of battery bank  30 , during re-charging by alternator  32 , become greater than a preset maximum threshold value (step  90 ), then ESC  34  starts a maximum voltage timer (sometimes referred to as a recharge stop timer) (step  92 ). As long as the monitored voltage continues to remain greater than the preset maximum threshold value until the maximum voltage timer expires, or times out, after having timed for a preset length of time (step  94 ), then engine  14  is shut down upon the timer timing out (step  96 ), causing alternator  32  to cease recharging battery bank  30  when that happens. 
     Had the monitored voltage dropped to a voltage less than the maximum threshold value while the maximum voltage timer was timing (step  98 ), then the maximum voltage timer would have been stopped (step  100 ), allowing engine  14  to continue running (step  102 ). When the monitored voltage becomes greater than the maximum threshold value (step  90 ), then the maximum voltage timer is reset, and then re-started (step  92 ) to begin timing the preset length of time and as a consequence cause the generator to continue recharging the low-voltage battery bank until the preset length of time elapses without the monitored voltage becoming less than the maximum threshold value during timing. 
     Had voltage of battery bank  30  risen above the minimum threshold value (step  104 ) after step  80  had started the minimum voltage proportional timer, then that timer would have been stopped (step  106 ), and engine  14  would have remained off (step  108 ). Should the monitored voltage again become less than the minimum threshold value (step  78 ), then the minimum voltage proportional timer is reset to zero and re-started (step  80 ). 
     The ability to preset any one or more of the minimum threshold value, the recharge initiate limit, the maximum threshold value, and the length of time that the maximum voltage timer times before battery bank  30  is considered sufficiently recharged to allow continued recharging by alternator  32  allows a user to select appropriate values for the user&#39;s particular situation.