WO2013015817A1

WO2013015817A1 - Motor vehicle with multiple bus power system

Info

Publication number: WO2013015817A1
Application number: PCT/US2011/045763
Authority: WO
Inventors: Jay Bissontz
Original assignee: International Truck Intellectual Property Company, Llc
Priority date: 2011-07-28
Filing date: 2011-07-28
Publication date: 2013-01-31

Abstract

A motor vehicle including a hybrid electric drive train, a multiple bus power distribution system for the drive train and loads, and a control system including a plurality of switching devices allowing selective connection and disconnection of the buses from loads, the drive train and electrical power storage elements, provides for power flow control. Power flow control is responsive to the electrical machine entering its traction mode monitoring power flow from the electrical power storage device, to the electrical machine entering its generator mode monitoring the output voltage level of the electrical machine, to the electrical power storage device exceeding a maximum permitted rate reconfiguring the motor vehicle to deliver less power to the electrical machine, or to the voltage level from the electrical machine exceeding a maximum permitted level directing the electrical machine to reduce power output.

Description

MOTOR VEHICLE WITH

MULTIPLE BUS POWER SYSTEM

BACKGROUND

[001] Technical Field:

[002] The technical field relates generally to high voltage direct current (DC) power distribution systems for motor vehicles and, more particularly, to identifying and responding to voltage transients which can occur on power buses in such systems.

[003] Description of the Technical Field:

[004] The interest in meeting demand for improved motor vehicle fuel economy has seen ever greater penetration of hybrid vehicles into the motor vehicle market including the market for trucks. Various hybrid architectures exist including hybrid electric architectures employing an internal combustion engine, a three phase alternating current (AC) traction motor/generator, high voltage traction batteries, and a high voltage DC/AC converter/inverter electrically connected between the traction motor/generator and the traction batteries. The traction batteries are used to store electrical power from and supply electrical power to the DC/AC converter/inverter at potentials up to or exceeding 700 volts DC. Concurrent with increasing use of electric traction motors has been an increase in interest in using smaller high voltage electric motors to support accessories such as air conditioning, power steering and air compressors for pneumatic brake systems. The use of high voltage motors in accessory systems can conserve energy, both on conventional vehicles as well as electric hybrid vehicles. The voltage levels used for these accessory motors, while much higher than conventional contemporary 12 volt (DC) systems is typically lower than the voltage level supplied to the DC/AC converter/ inverter.

[005] Various power distribution architectures exist which allow the high voltage traction batteries to supply power both to the converter/inverter and to the high voltage accessory motors. An exemplary system has one 700 volt bus and two 350 volt accessory buses, with the first accessory bus operating between 350 volts and reference ground and the second accessory bus operating between 700 volts and 350 volts. Isolation contactors control power flow to the buses.

[006] Various issues arise on vehicles configured to supply high voltage direct current for traction or for the operation of electrified vehicle accessories. At a basic level these issues relate either to a hybrid traction motor/generator regenerating more power during dynamic braking than can be absorbed by the traction batteries or super capacitors (power storage devices) or to the high voltage traction motor/generator and/or electrified accessories drawing current at a greater rate than the power storage devices can supply. The limits of the storage devices relate to dynamics present within the storage devices themselves, for example: temperatures resulting from resistance losses exceeding cell or capacitor temperature limits, input/output voltages exceeding maximum/minimum voltage variances, a compromised internal component chemistry and the like. Long periods exceeding limits, or large, if transient, departures from limits in voltage levels or current draw levels, can reduce the performance and shorten the service life of the high voltage hybrid direct current storage devices. Generally high or low voltage conditions have been managed by communicating with controllers for the various electrical devices which are generating or consuming electrical energy to reduce their generation or consumption level when constraints are exceeded. When these methods are not successful in restoring voltage levels, the isolation contactors or insulated bipolar gated transistors (IGBTs), have been configured to isolate the high voltage direct current storage devices from the generation or power sink sources. A possible consequence of some isolation containment strategies is vehicle sub-systems may be de-energized. The de-energization of electrified accessory circuits such as the motors for power steering or for the brake system pneumatic compressor while the vehicle is in motion could compromise vehicle control.

SUMMARY

BRIEF DESCRIPTION OF THE DRAWINGS

[007] Fig. 1 is a side elevation of a truck and trailer system which may be equipped with a hybrid electric drive train.

[008] Figs. 2 and 2B are high level block diagram of a control and power distribution system for the truck of Fig. 1.

[009] Fig. 3 is a high level flow chart illustrating operation of the system.

DETAILED DESCRIPTION

[0010] In the following detailed description, like reference numerals and characters may be used to designate identical, corresponding, or similar components in differing drawing figures. Furthermore, example sizes/models/values/ranges may be given with respect to specific embodiments but are not to be considered generally limiting.

[0011] Referring now to the figures and in particular to FIG. 1, a truck/trailer combination 10 comprising a truck 12 with a trailer 14 attached thereto along the axis of a fifth wheel 20 is shown. Trailer 14 rides on a plurality of wheels 16. Truck 12 rides a combination of wheels 16 and drive wheels 18. Drive wheels 18 are connected to a hybrid electric drive train 19 for locomotion. The rotation of wheels 16 and drive wheels 18 can be retarded to stop the vehicle through service brake system 99 which is actuated using a pneumatic system. The rotation of drive wheels 18 can also be retarded by using them to back drive the hybrid electric drive train 19 to generate electricity.

[0012] Figs. 2 and 2B are high level schematic of an electric power distribution system and an associated control system. The power distribution system is representative of systems which can be used with hybrid electric drive train 19 and to supply power to high voltage DC motors and DC/DC converters for accessory/auxiliary vehicle systems. Power flow is routed using a high voltage distribution box 37 to which are attached two high voltage battery sub-packs 38 and 39, a high voltage AC/DC inverter/converter 46, a plurality of motor controllers 31, 56 and 58 for DC electric accessory motors 32, 57 and 59 and a pair of bi-directional DC/DC converters 62. DC/DC converters support an low (12 volt) DC vehicle electrical system which includes 12 volt chassis batteries 60, 61.

[0013] Hybrid electric drive train 19 is represented as a parallel system, though the present disclosure is not limited to such systems. The hybrid electric drive train 19 includes a thermal/internal combustion engine 48 and a dual mode electrical machine (tractor motor/generator) 47. Electrical machine 47 may be run in an electric traction motor mode or it may be back driven from drive wheels 18 (or thermal engine 48) for operation in an electrical generator mode. Electrical machine 47 is a three phase alternating current (AC) synchronous machine. Connection between a traction bus 11 and the electrical machine 47 is through a high voltage inverter/converter 46 which operates on 700 volts DC on its direct current power distribution system side and high voltage, variable frequency, three phase alternating current on the electrical machine 47 side. High voltage distribution box 37 includes a ground fault detector 35 which provides for detecting voltage leakage from the traction bus 11 or one of the accessory buses 13 or 15 to the vehicle ground reference.

[0014] Traction batteries are installed in high voltage battery sub-packs 38, 39. These receive power generated by the dual mode electrical machine 47 in its generator mode, supply power to the electrical machine 47 in its traction motor mode and stabilize power distribution system voltage. Each battery sub-pack supports a 350 volt DC potential difference and are connectable in series using high voltage switching devices 67 (here illustrated as isolation contactors) to traction bus 11 to the high voltage inverter/converter 46 to supply 700 volts DC to the inverter/converter 46. High voltage battery sub-pack 38 is connected to accessory bus 13 to supply power on the bus at 350 volts (the difference between the positive terminal of the sub- pack and reference ground. Accessory bus 15 is connected across the positive terminal of high voltage battery sub-pack 38 (which is connected to the negative terminal of high voltage battery sub-pack 39) and the positive terminal of high voltage battery sub-pack 39. Use of a split battery plant, that is two high voltage battery sub-packs 38, 39, allows distribution of direct current (DC) power through the high voltage distribution box 37 to accessory motors at 350 volts DC on accessory buses 13, 15.

[0015] High voltage battery sub-packs 38 and 39 include battery management systems (BMS) 70 which report traction battery voltage and current flow into and out of the sub-packs over hybrid CAN data link 25. Additional data may be reported such as battery temperature. The BMS 70 units can also calculate an estimated battery state of charge (SOC) which relates to the ability of the high voltage battery sub-packs 38, 39 to accept current inflows (charging) and support current outflows (discharging). These rates may also vary with battery temperature and other variables. If loads or sources on the system exceed a maximum acceptable rate, for example by driving excess current into the high voltage battery sub-packs 38, 39 stemming from an over voltage condition, damage to the high voltage battery sub-packs 38, 39 may result.

[0016] Electrical power to drive the dual mode electrical machine 47 as a traction motor is delivered to the dual mode electrical machine through the inverter/converter 46 from a high voltage distribution box 37 across traction bus 11. Power is supplied to the high voltage distribution box 37 from high voltage battery sub-packs 38, 39. Power generated by the dual mode electrical machine 47 when in its generator mode passes through the inverter/converter 46 back to high voltage battery sub-packs 38, 39 for storage as chemical energy. This can occur due to regenerative/dynamic braking or due to the electrical machine being back driven by the thermal engine 48 up to the rate of charge limits and total charge capacity of the high voltage battery sub-packs 38, 39. The rates at which battery sub-packs 38, 39 can accept or deliver current are functions of battery temperature and battery SOC. [0017] Isolation contactors 67 and accessory contactors 34 provide for power routing to the hybrid electric drive train 19 and the accessories. Associated with isolation contactors 67 are a plurality of pre-charge resistors 64 for limiting current inflow during system initialization. The operation of the pre-charge resistors 64 is conventional with the pre-charge resistors being switched out of the circuit by reconfiguring the isolation contactors 67 after the initialization period. Located within the high voltage distribution box 37 is a ground fault detector 35. Ground fault detector 35 is connected to insert signals on the traction bus 11 or onto the accessory buses 13, 15 and from there into the high voltage inverter/converter 46, the accessory motors 32, 57, 59 and to the DC/DC converters 62. Reference ground can be monitored for appearance of these signals. A remote power module (RPM) 22, which functions as an extension of a electronic system controller (ESC) 40, a type of body computer, controls the states of sets of isolation contactors 67 and accessory contactors 34 as directed by the ESC 40. Accessory contactors 34 provide power couplings to motor controllers 31, 56 and 58 and thereby to accessory motors 32, 57 and 59 and to bi-directional DC/DC converters 62 through which power is transmitted to, and drawn from, first and second twelve-volt chassis batteries 60, 61.

[0018] Vehicle control is implemented through a plurality of data links and controllers. There are two data buses which provide the back bones for a drive train controller area network (CAN) data link 23 and a hybrid controller area network (CAN) data link 25, respectively. Data links 23, 25 and the controllers connected thereto conform to the physical requirements of the Society of Automotive Engineers J 1939 standard and implement a communications protocol conforming to the same standard. There is a lower capacity J1708 data bus 63 conforming to the SAE J 1708 protocol used to convey switch state information from a dash panel 49 to ESC 40. A driver display 41 relating to hybrid system condition is connected to hybrid data link 25 over which it receives data relating to power distribution system operation for display to an operator.

[0019] A plurality of programmable controllers is interconnected by data links 23, 25 or both. The controllers generally relate to major vehicle systems as identified by their names, for example, the anti-lock brake system (ABS) controller 43. ABS controller 43 measures the rotational speed of wheels 16, 18 and provides data allowing involved in control over the truck/trailer combination 10 service brake system 99 and control over individual brakes. The service brake system 99 is a conventional pneumatic system for a truck allowing extension of the system to trailer 14. The pneumatic system operates as a vehicle accessory system driven by compressor motor 32 and pneumatic compressor 33. Compressor motor controller 31 and the compressor motor 32 draw electrical power from the traction batteries or the dual mode electrical machine 47. The pneumatic system includes a pneumatic compressor 33 which supplies compressed air to compressed air supply tanks 27, 28 and 29 and an air dryer 26. A manifold solenoid valve controller (MSVA) 30 allows use of compressed air from the supply tanks 28, 29 to operate purge valves for the dryer tank 26, to supply air to the service brake system 99 and other tasks. Pressure sensors 66 for the supply tanks 28, 29 communicate air pressure readings to a supervisory controller (for example ESC 40 or hybrid control unit (HCU) 51) for the compressor motor controller 31 and MSVA 30. The demand level for compressed air will be seen to be proportional to current drawn by compressor motor 32 to maintain pneumatic system pressure.

[0020] Other controllers include a transmission control unit (TCU) 42, an engine valve controller 44, an engine control unit (ECU) 45, BMS controllers 70 associated with high voltage battery sub-packs 38 and 39 and HCU 51. In addition, ESC 40 provides integration functions and handles control over the states of the accessory contactors 34 and isolation contactors 67 of the high voltage distribution box 37 through programmable remote power modules (RPM) 24 and 22. Another RPM 36 may be programmed by the ESC 40 to allow extension of ESC control over secondary cooling loop 54. In addition ESC 40 provides supervisory control over MSVA 30. Various vehicle sensors 98 may be directly connected to ESC 40.

[0021] The controllers connected to ESC 40 over one or both of the data links 23, 25, and sensors directly connected to ESC 40 or which can communicate to ESC 40 through another controller, provide data relating to truck 12 operating variables. These in turn relate to expected power consumption by dual mode electrical machine 47, one the accessory motors 32, 57, 59 or the DC/DC converters 62. To take an example, data from either the ABS controller 43 or TCU 42 may be used to generate an estimate of vehicle speed. Vehicle speed is in turn inversely related to power consumption by the power steering accessory motor 59 provided the rate of change in the angle of the wheels used for turning is constant. Another example is demand on HVAC compressor accessory motor 57 to support cabin cooling. Power consumption by compressor accessory motor 57 is related to outside ambient temperature and the cabin temperature request made by the operator.

[0022] Gauge cluster and controller 53 and the engine valve controller 44 are connected only to the drive train data link 23. The HCU 51 and ECU 45 communicate directly with one another and over the hybrid data link 25 and drive train data link 23, respectively, with other controllers. The BMS controllers 70 for the high voltage battery sub-packs 38, 39 are connected to the hybrid data link 25 only, as is a heating, ventilation and air conditioning (HVAC) pusher fan controller 52. RPMs 22, 24 and 36 are controlled over the hybrid data link 25 from ESC 40. Networked interaction made possible by CAN technology gives ESC 40 access to data relating to a number of vehicle operating conditions such as vehicle speed (which relates to power steering power demands), ambient temperature (which relates to air conditioner compressor power demands, and so on.

[0023] RPM's 24 and 22 provide essentially direct ESC 40 control over accessory contactors 34 and isolation contactors 67, respectively. ESC 40 controls motor controllers 58, 56 and 31 over hybrid data link 25 and thus controls the compressor motor 32 which is the prime mover for pneumatic compressor 33.

[0024] Reconfigurable software installed for execution by the ESC 40 and the CAN based control architecture allows voltage levels generated by electrical machine 47 and the voltage level on and current sourced by the high voltage battery sub-packs 38, 39 to be monitored in near real time. The amount of electrical energy being discharged from one or more high voltage battery sub-packs 38, 39 to support the operation of the electrical machine is known. If either the voltage levels generated, or the current levels drawn, by electrical machine 47 exceed predetermined levels as defined in the software controls of ESC 40 the ESC 40 can request that the HCU 51 command the electrical machine 47 to modify its output voltage or current draw characteristics to acceptable levels as defined by the high voltage direct current storage devices energy performance specifications. This can involve reducing vehicle acceleration or engaging the thermal engine 48 to carry some or all of the power demanded. This may be of interest where the driving characteristics of the vehicle are to remain constant regardless of the battery SOC. Where excess power is being generated this may be accomplished by reducing self excitation of the electrical machine 47. Under these circumstances ABS controller 43 may be instructed to compensate for loss of dynamic braking capacity by increased use of the service brake system 99 or thermal engine 48 braking may be engaged. It may be noted that this action results in increased use of the compressor motor 32 which in turn absorbs some of the excess generated power from electrical machine 47. Because the permitted maximum power outflow and charging rates to the high voltage battery sub-packs 38, 39 change with battery SOC and temperature, and because driving conditions are not constant all adjustments are dynamic.

[0025] If the electrical machine (traction motor/generator) 47 does not respond to the HCU 51 commands, the HCU will direct isolation contactors 67 to a state interrupting the flow of electrical power between the high voltage direct current storage devices (e.g. high voltage battery sub-packs 38, 39) and the electrical machine 47. At the same time, because of the multiple bus configuration of the hybrid electric vehicle's high voltage distribution system, the HCU 51 and ESC 40 have the option not to interrupt high voltage DC power to other electrified components of sub-systems. These components include particularly the accessory motors 32 and 59 for the pneumatic system and power steering, DC-to-DC converters 62 for the twelve volt DC power distribution system, secondary cooling loops 54 and to a lesser extent HVAC compressor accessory motor 57. Operator control over vehicle operation is thereby eased while the operator responds to driver display 41 directions including maneuvering the vehicle off the road.

[0026] The system provides passive monitoring of the state of high voltage isolation contactors 67 and accessory contactors 34 by ESC 40 and HCU 51, particularly the conductive states of the contactors relative to the operation of the electrical machine (traction motor/generator) 47 all in conjunction with the performance characteristics of the high voltage storage devices (battery sub-packs 38, 39). Driver display 41 is used to provide an operator with direct graphic, text and audible indications depending on the state and status of the contactors 67, 34, the state of operation of the electrical machine (traction motor/generator) 47 and the performance variables of the high voltage storage devices 38, 39. Audible indication may take the form of an in cab alarm or audio messaging communicated through the entertainment radio system.

[0027] A possible process to implement such monitoring is illustrated in Fig. 3 where step 102 indicates division of a monitoring process based on whether electrical machine 47 is in its generator mode or its traction mode. Following the generator mode the voltage level of the electrical machine 47 is monitored (step 106) for high going transients. Following the traction mode branch from step 102 the high voltage battery sub-packs 38, 39 are monitored for power discharge. Decision steps 110 and 108, respectively, reflect process branching depending upon whether unacceptable transients or levels are detected. If not the NO branches from steps 108, 110 reflect ongoing monitoring based on transitions between modes of the electrical machine 47. Step 112 provides for setting all contactors for normal operation. The YES branches from steps 108 and 110 reflect steps taken to adjust operation of components connected to the power distribution buses, that is traction bus 11 and accessory buses 13 and 15, to restore acceptable voltage levels or to limit current drawn. Step 114 following step 108 attempts to deal with excessive current draw from the high voltage battery sub-packs 38, 39 by reducing power consumption by the electrical motors or DC-DC converters 62. Usually the easiest way to achieve this result is to reduce power consumption by electrical machine 47 by directing ECU 45 to increase thermal engine 48 output.

[0028] It may be possible under some circumstances to achieve power consumption reduction by stepping down air conditioner operation or reducing power steering boost, among other steps. Step 116 following step 110 reflects operational changes intended to reduce power output of the electrical machine 47 in its generator mode. Normally this would occur as a result of regenerative or dynamic braking since the ECU 45 is unlikely to request the thermal engine 48 back drive the electrical machine 47 to the point where it generates more power than can be used to recharge the high voltage battery sub-packs 38, 39. [0029] Conventionally regeneration power can be diminished by transferring braking load to the service brake system 99, possibly by reducing self excitation of the electrical machine 47. Step 118 follows both steps 114 and 116 to determine if the steps were effective. If not the HCU 51 can implement a strategy to isolate the electrical machine 47 from the high voltage battery sub-packs 38, 39 while maintaining power to the high voltage accessory buses 13, 15. This step may include opening accessory contactors 34 which are deemed non-essential to immediate vehicle operation such as the accessory contactor 34 for HVAC compressor accessory motor 57 and possibly the accessory contactors for the bi-directional DC to DC converters 62. A strategy of isolating accessories from drawing power where power draw is excessive may be implemented under appropriate circumstances. Direction is given to the driver display 41 to emit the appropriate alarms for this case. Following the YES branch from step 118 or following step 120 the process returns to step 102.

Claims

What is claimed is:

1. A motor vehicle comprising: a drive train including an electrical machine having a traction mode and a generator mode; an electrical power storage device; a multiple bus power distribution system; a control system with a plurality of controllers including a supervisory controller and a data link connecting the plurality of controllers for exchange of data; a plurality of switching devices allowing selective connection and disconnection of the buses of the multiple bus power distribution system to and from the electrical machine and the electrical power storage device; a source of voltage level measurements from the electrical machine in its generator mode and a source of power discharge measurements from an electric power storage system, the sources being connected to the data link to supply the measurements to the supervisory controller; and the supervisory controller being responsive to voltage level measurements from the electrical machine operating in its generator mode exceeding a maximum permitted level or to a power discharge rate from the electrical power storage device exceeding a maximum permitted rate when the electrical machine is operating in its traction mode for directing altered operation of the electrical machine to reduce the voltage level or the power discharge rate.

2. A motor vehicle as set forth in claim 1, further comprising: the supervisory controller being further responsive to failure of direction to the electrical machine to reduce to the voltage level or power discharge rate below the respective maximum allowed level or rate for directing changes in the states of the plurality of switching devices to isolate the electrical machine from the electrical power storage device.

3. A motor vehicle as set forth in claim 2, further comprising: accessory loads connected to one of the multiple buses by accessory contactors; and the supervisory controller being further responsive to failure of direction to the electrical machine selectively to reduce the power discharge rate to below the maximum allowed rate for directing reduced power consumption by one or more accessory loads or opening accessory contactors for the accessory loads.

4. A motor vehicle as set forth in claim 1 wherein the maximum allowed voltage level and maximum allowed discharge rate are dynamic responsive to condition of the electric power storage device.

5. A motor vehicle as set forth in claim 4 wherein the electric power storage device comprises a plurality of traction battery sub-packs.

6. A motor vehicle as set forth in claim 1, further comprising: a display device connected to the data link for receiving data relating to operational state of the electrical machine, the power storage device, the switching devices including accessory contactors and the multiple bus power distribution system.

7. A motor vehicle as set forth in claim 4 further comprising an inverter/converter connecting the multiple bus power distribution system to the electrical machine.

8. In a motor vehicle including a drive train with an electrical machine having a traction mode and a generator mode, an electrical power storage device, a multiple bus power distribution system, a control system with a plurality of controllers including a supervisory controller and a data link connecting the plurality of controllers for communication of data, and a plurality of switching devices allowing selective connection and disconnection of the buses of the multiple bus power distribution system to and from the electrical machine and the electrical power storage device, a method of controlling power flow into and out of the electrical power storage device comprising the steps of: responsive to the electrical machine entering its traction mode monitoring power flow from the electrical power storage device; responsive to the electrical machine entering its generator mode monitoring the output voltage level of the electrical machine; responsive to the power flow from the electrical power storage device exceeding a maximum permitted rate reconfiguring the motor vehicle to deliver less power to the electrical machine; and responsive to the voltage level from the electrical machine exceeding a maximum permitted level directing the electrical machine to reduce power output.

9. The method of claim 8, further comprising the steps of: monitoring a state of charge of the electrical power storage device; monitoring the temperature of the electrical power storage device; dynamically varying the permitted maximum voltage level and the permitted power outflow rate as functions of the state of charge and the temperature.

10. The method of claim 9, comprising the further step of: responsive to failure of the power flow to decrease below the permitted maximum rate or the voltage level to decline to a level below the permitted maximum level electrically isolating the electrical machine from the electrical power storage device.

11. The method of claim 10, comprising the further step of: subsequent to isolating the electrical machine from the electrical power storage device selecting accessories for isolation or continued energization from the electrical power storage device.

12. A motor vehicle with hybrid drive train and control system comprising: an internal combustion engine; an electrical machine having a traction mode and a generator mode couple-able with the internal combustion engine for providing traction power; a pair of traction battery sub-packs each supporting equal voltage levels; a power distribution system providing a traction bus having a voltage level equal to the sum of the voltage levels of the pair of traction battery sub-packs and a pair of accessory buses each of which is connectable across a different one of the pair of traction battery sub-packs, the power distribution system providing for energizing the pair of accessory buses while isolating the traction bus; a control system with a plurality of controllers including a supervisory controller and a data link connecting the plurality of controllers for exchange of data; a plurality of switching devices allowing selective connection and disconnection of the buses of the multiple bus power distribution system; a source of voltage level measurements from the electrical machine in its generator mode and a source of power discharge measurements from an electric power storage system, the sources being connected to the data link to supply the measurements to the supervisory controller; the supervisory controller being responsive to voltage level measurements from the electrical machine operating in its generator mode exceeding a maximum permitted level or to a power discharge rate from the electrical power storage device exceeding a maximum permitted rate when the electrical machine is operating in its traction mode for directing altered operation of the electrical machine to reduce the voltage level or the power discharge rate.

13. A motor vehicle as set forth in claim 12, further comprising: the supervisory controller being further responsive to failure of direction to the electrical machine to reduce to the voltage level or power discharge rate below the respective maximum allowed level or rate for directing changes in the states of the plurality of switching devices to isolate the electrical machine from the electrical power storage device.

14. A motor vehicle as set forth in claim 13, further comprising: accessory loads connected to one of the accessory buses by accessory contactors; and the supervisory controller being further responsive to failure of direction to the electrical machine selectively to reduce the power discharge rate to below the maximum allowed rate for directing reduced power consumption by one or more accessory loads or opening accessory contactors for the accessory loads.

15. A motor vehicle as set forth in claim 12 wherein the maximum allowed voltage level and maximum allowed discharge rate are dynamic responsive to condition of the electric power storage device.