Patent Publication Number: US-11381103-B2

Title: Variable voltage charging system and method for a vehicle

Description:
FIELD 
     The present disclosure generally relates to charging systems and methods for vehicles, and more particularly to engine alternator systems and charging control methods for charging vehicle batteries. 
     BACKGROUND 
     The following U.S. patents are incorporated herein by reference, each in its entirety: 
     U.S. Pat. No. 5,448,152 discloses a battery management system maintains a charge on at least one auxiliary battery by switching the auxiliary battery automatically into parallel with the main battery charging circuit or with the auxiliary load. The system uses the AC component of the charging signal of a vehicle or boat charging system to trigger switching circuits coupled to operate relays or similar switching means which couple the auxiliary battery to the main charging circuit. When no charging signal is present, i.e., when the vehicle or boat engine is turned off, the auxiliary battery is switched automatically out of the charging system and is charged and in condition for use. A delay circuit can be provided for providing non-shorting operation, especially for use with more than one auxiliary battery in which batteries are charged in parallel and loaded in series, whereby the combined series voltage of the auxiliary batteries would exceed the vehicle or boat supply voltage. The timing circuits open the circuit from the auxiliary batteries to the vehicle or boat charging system prior to switching the batteries into a series configuration for use with a load requiring a voltage higher than the rated voltage of the charging system. The timing circuits thereby prevent momentary large currents upon changes of state. 
     U.S. Pat. No. 5,896,022 discloses a modification kit for the addition of an auxiliary battery charge management system for a marine or land vehicle having a starting battery and an auxiliary battery system. The kit includes a single pole breaker, a normally-on relay, and a two-way toggle switch. The single pole breaker is inserted in a circuit of the auxiliary battery system, and the normally-on relay and the toggle switch are inserted in the starting battery circuit of the vehicle to provide a dual mode charging system adapted to manual and automatic power regeneration of the starting battery and the auxiliary battery system. 
     U.S. Pat. No. 7,218,118 discloses a method for monitoring the condition of a battery of a marine propulsion system that provides the measuring of a voltage characteristic of the battery, comparing the voltage characteristic to a preselected threshold value, and evaluating the condition of the battery as a function of the relative magnitudes of the voltage characteristic and the threshold value. The voltage characteristic of the battery is measured subsequent to a connection event when a connection relationship between the battery and an electrical load is changed. The electrical load is typically a starter motor which is connected in torque transmitting relation with an internal combustion engine. The voltage characteristic is preferably measured at its minimum value during the inrush current episode immediately prior to cranking the internal combustion engine shaft to start the engine. 
     U.S. Pat. No. 9,054,555 discloses systems and methods for charging a rechargeable battery device on a marine vessel utilizing a rechargeable battery device, a charger charging the battery device, and a control circuit. The control circuit calculates an amount of current that is available to charge the battery device based upon an amount of current that is available from the shore power source and an amount of current that is being drawn from the shore power source by devices other than a voltage charger and limits the amount of current being drawn by the voltage charger to charge the battery device to an amount that is equal to or less than the calculated amount of current that is available to charge the battery device. The control circuit can repeatedly calculate the amount of current that is available to charge the battery device and limit the amount of current being drawn by a voltage charger to charge the battery device to thereby actively adjust an amount of charge applied to the battery device. 
     U.S. Pat. No. 10,097,125 discloses an alternator configured for use in a vehicle that includes a housing, a stator located within the housing, a field coil, a regulator, and a transceiver. The field coil is positioned in proximity to the stator and is configured for rotation relative to the stator. The regulator is electrically connected to the field coil and is configured to supply the field coil with an electrical signal based on a control signal. The transceiver is electrically connected to the regulator and is configured to wirelessly receive the control signal from an engine control module of the vehicle and to transmit the control signal to the regulator. 
     SUMMARY 
     This Summary is provided to introduce a selection of concepts that are further described below in the Detailed Description. This Summary is not intended to identify key or essential features of the claimed subject matter, nor is it intended to be used as an aid in limiting the scope of the claimed subject matter. 
     The disclosure relates to vehicle charging systems and methods configured to provide at least two different charge voltage outputs to charge different power storage devices on the vehicle. In one embodiment, a variable voltage charging system for a vehicle includes an alternator configured to generate electric power from a rotation output of an engine, wherein the alternator is configured to alternately output at least a low charge voltage to charge a low voltage storage device and a high charge voltage to charge a high voltage storage device. A switch is configured to switch between at least a first switch position connecting the alternator to the low voltage storage device and a second switch position connecting the alternator to the high voltage storage device. A controller is configured to control operation of the alternator and the switch between at least a low voltage mode and a high voltage mode. In the low voltage mode, the alternator outputs the low charge voltage and the switch is in the first switch position connecting the alternator to the low voltage storage device. In the high voltage mode, the alternator outputs the high charge voltage and the switch is in the second switch position connecting the alternator to the high voltage storage device. 
     A variable voltage charging system on a vehicle may be controlled to alternately charge a low voltage storage device and a high voltage storage device. The variable voltage charging system includes an alternator configured to alternately output at least a low charge voltage to charge the low voltage storage device and a high charge voltage to charge the high voltage storage device, and a switch configured to alternately connect the alternator to the low voltage storage device or the high voltage storage device. One embodiment of the control method includes operating the alternator and the switch in a low voltage storage mode to charge the low voltage storage device. Upon identifying a high voltage mode condition, the switch position is changed so as to connect to the high voltage storage device and a regulation setpoint of the alternator is changed from a low voltage setpoint to a high voltage setpoint. The alternator is then operated in the high voltage mode to output the high charge voltage to charge the high voltage storage device. When operating in the high voltage mode, a low voltage mode condition may be identified, and the system may then be reverted back to the low voltage mode. 
     Various other features, objects, and advantages of the invention will be made apparent from the following description taken together with the drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present disclosure is described with reference to the following Figures. 
         FIG. 1  schematically depicts a variable voltage charging system according to one embodiment of the present disclosure; 
         FIG. 2  depicts another embodiment of a variable voltage charging system according to the present disclosure; 
         FIG. 3  depicts another embodiment of a variable voltage charging system for a marine vessel according to the present disclosure; 
         FIG. 4A  is a graph depicting charging output power at various engine speeds; 
         FIGS. 4B-4C  are graphs depicting charging output power of exemplary embodiments of variable voltage charging systems at various engine speeds; 
         FIGS. 5-7  depict embodiments of methods of controlling a variable voltage charging system on a vehicle according to the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     The present inventor has recognized that 12 volt power systems are insufficient for some vehicle applications, particularly for marine vessels and recreational vehicles having amenities that have significant power needs and require relatively large energy stores. For example, the inventor has recognized that a 48 volt battery system is advantageous for powering house loads on such vehicles, for example, because it allows the use of smaller wire to transmit power to the various elements on the vehicle. For instance, the house load may include cabin lights, an air conditioner, appliances, and the like in the cabin of the vehicle. This results in a more efficient and compact unit, and also allows for more discrete wiring throughout the vessel. Additionally, 48 volt systems produce less heat and thus are more efficient because less energy is converted to heat rather than electrical power. Moreover, higher voltage alternators, such as 48 volt alternators, have a higher max charging output, primarily because the current level can be reduced and thus a smaller wire can be used without losing too much power to heat. 
     However, the inventor has also recognized that use of 48 volt, or similar higher voltage power systems can be problematic when the battery charging is provided by an alternator connected to the vehicle engine. This is because 48 volt systems do not provide sufficient battery charging at low engine speeds. In particular, alternators providing 48 volt charging output have cut in speeds that are above the idle speed of most engines. Cut in speed is the speed at which the alternator begins to provide charging output. 48 volt alternators have a higher cut in speed because the alternator needs higher RPM to produce 48 volts than it does to produce a lower voltage, such as 12 volts. Thus, high voltage alternators, such as 48 volt alternators, do not provide a charging output at low speeds, such as at idle. Since significant operation time on many vehicles is spent at idle engine speeds, this inability to charge at low engine speeds has a significant impact on the usefulness and viability of such high voltage charging systems. 
     While 12 volt systems provide good cut in speeds, and thus provide good charging output at lower engine RPMs, 12 volt alternators are large and heavy and wiring for 12 volt systems is bulky.  FIG. 4A  depicts the relationship between cut in speed and max output comparing 12 volt and 48 volt systems. The graph represents an alternator output at three different output voltage levels. For example, the graph may represent power output of a variable voltage alternator running at three different output voltages, including at a 12 volt charging output, a 24 volt charging output, and a 48 volt charging output. As illustrated in the graph, 48 volt systems have a higher maximum power output than lower voltage systems, including 12 volts systems, at higher engine speeds. However, 48 volt alternators have a much higher cut in speed where output is initiated. In this example, the cut in speed  40  for the 12 volt alternator is around 500 RPM, compared to the cut in speed  44  for the 48 volt alternator which is above 1500 RPM and well above idle speed of the exemplary marine engine in this particular example. However, the maximum power output  45  of the 48 volt alternator in this example is over three times greater than the maximum power output  41  of the 12 volt alternator. 
     In view of the above described recognition of the problems and benefits of low voltage versus high voltage power systems, the inventor endeavored to provide a variable voltage system combining the low cut in speed benefits of 12 volt systems and the high max power output benefit of higher voltage systems, such as a 48 volt system. While systems do exist that utilize two alternators on the same engine, one running at 12 volts and the other running at 48 volts, the inventor has recognized that such systems are not ideal for many vehicle applications where there is insufficient room for connection of two separate alternators to an engine. Thus, the inventor developed the disclosed system and method for allowing the same alternator to function in both a 12 volt system and a 48 volt system, and thus to utilize the strengths of both. Namely, at low engine speeds and other low voltage mode conditions described herein, the alternator operates in a low charge voltage output mode and is connected to a low voltage storage device, such as a 12 volt lead acid battery. At higher engine speeds above an engine speed threshold, the alternator operates in a high voltage mode in order to provide a high charge voltage to charge a high voltage storage device, such as a 48 volt battery, series of four 12 volt batteries, or two 24 volt batteries. This allows the system to take advantage of the high maximum charge output  45  of the higher voltage system, such as the 48 volt system, as well as to provide smaller and simpler wiring throughout the vehicle, such as throughout the marine vessel or recreational vehicle. 
     Typically, alternators designed for 48 volt are not compatible with 12 volt systems because they contain smaller wires which cannot handle the current generated for charging a 12 volt battery, particularly the current levels generated by the alternator at high engine speeds. In particular, alternators designed for high voltage outputs, such as 48 volts, have a stator comprised of smaller wire that can overheat at the high current levels generated in a 12 volt mode. Thus, operating a 48 volt alternator to charge a 12 volt battery will likely overheat the alternator, particularly if charging occurs at high engine speeds, or even lower engine speeds for an extended period of time. Accordingly, the disclosed system and method only operate the high-voltage-compatible alternator in the low voltage mode, such as providing charging output appropriate for a 12 volt battery, at low engine RPMs and otherwise provide current limiting functionality and/or temperature-based control that prevents overheating of their alternator in the low voltage mode. 
       FIGS. 1 and 2  demonstrate exemplary embodiments of a dual voltage charging system  100  having an alternator  116  that alternately connects to a low voltage storage device  102  and a high voltage storage device  108  through a switch  114 , which may be a mechanical or solid state switch. In the depicted example, the low voltage storage device  102  is a 12 volt battery and the high voltage storage device  108  is a 48 volt battery. In other embodiments, the storage devices may be at other voltage levels where one is at a high voltage level than the other. Furthermore, the system may be used to switch between more than two voltage levels, such as between three voltage levels (e.g., 12 volts, 24 volts and 48 volts) or even more voltage levels. In the depicted example, the 12 volt battery powers the propulsion-related functions, such as ignition, steering, etc., and the 48 volt battery powers the house loads  120 ; however, other arrangements may be provided and within the scope of the present disclosure. For example, the 12 volt and 48 volt batteries may both be configured to power the house loads  120 , with a converter in between. 
     The alternator  116  is configured to generate electric power from a rotation output of the engine  104 , and provides a variable charging output capability such that it can be controlled to output at least two charging voltages. One exemplary alternator providing such capability is a 48 volt (or higher voltage output, such as 56 volts) LIN alternator. The alternator  116  is configured and controlled to alternately output a low charge voltage to charge the low voltage storage device  102  and a high charge voltage to charge the high voltage storage device  108 . 
     The switch  114  is configured to switch between a first switch position  114   a  wherein it connects the alternator  116  to the low voltage storage device  102 , and a second switch position  114   b  wherein it connects the alternator to the high voltage storage device  108 . The switch may be a mechanical switch or a solid state switch controllable by a controller. The switch may reside outside or inside the alternator housing  124  ( FIG. 2 ). A controller  112  is configured to control operation of the alternator  116  and switch  114  so as to change between a high voltage mode operation and a low voltage mode operation. In the high voltage mode, the alternator  116  outputs the high charge voltage and the switch connects the alternator  116  to the high voltage storage device  108 . In the low voltage mode, the alternator  116  outputs the low charge voltage and the switch  114  connects the alternator  116  to the low voltage storage device. For example, where the high voltage storage device is a 48 volt battery the high charge voltage may be in the range of 53 to 56 volts, for example. Where the low voltage storage device is a 12 volt battery, the low charge voltage may be in the range of 14-14.5 volts, for example. 
       FIG. 2  depicts another embodiment of a variable voltage charging system  100 , which includes an engine  104  and an electrical system comprising a low voltage storage device  102  and a high voltage storage device  108  connected to an electrical load  120  on the vehicle. The term “vehicle,” as used herein, refers to any device configured to carry or to transport something or someone, including, without limitation, cars, trucks, buses, boats or other marine vehicles, recreational vehicles, trains, and planes. The term “engine,” as used herein, includes any type of internal combustion engine suitable for powering the vehicle. The term “storage device,” as used herein includes any type of battery suitable for supplying electrical energy to the vehicle and the engine. 
     The alternator assembly  116  includes a housing  124 , a rotor  128  including a field coil  132  mounted thereon, a stator  136 , a rectifier assembly  140 , and a voltage regulator assembly  144 . In the embodiment of  FIG. 2 , the rotor  128  is located at least partially within the housing  124  and is configured for rotation relative to the housing  124  and the stator  136 . A coupling device  148 , such as an endless belt, couples the rotor  128  to the rotational output of the engine  104 . 
     The stator  136  is also located at least partially within the housing  124 . The stator  136  is fixed in position with respect to the housing  124 . The stator  136  typically includes a plurality of windings  152 . As shown  FIG. 2 , the stator  136  includes at least three windings  152  and is referred to as a three-phase stator. The stator  136  is configured to output a three-phase AC signal in response to rotation of the rotor  128  by the engine  104 . 
     The rectifier assembly  140  is a three-phase full-wave bridge rectifier, but in other embodiments is provided as any desired type of rectifier. The rectifier assembly  140  includes a plurality of diodes  156  electrically connected to the stator  136 , the electrical load  120 , the voltage regulator assembly  144 , and the connected one of the storage devices  102 ,  108 . The diodes  156  are configured to rectify the three-phase AC signal generated by the stator  136 . The rectified signal (i.e. the output of the rectifier assembly  140 ) is typically a single-phase DC signal that is suitable for charging the storage devices  102 ,  108  and powering the load  120 . 
     The controller  112  for controlling the variable voltage charging system  100 , including the mode switching operation described herein, may be an engine control module (ECM) for the engine  104 . Alternatively, the controller  112  may be a dedicated controller for the alternator  116 , or may be some other control device, such as a propulsion control module, helm control module, etc. The controller  112  receives information from various sensors on the vehicle, including the temperature sensor  160  that senses a temperature of the alternator  116 . The controller  112  may further receive information from a current sensor and/or a voltage sensor that sense the output of the alternator. For example, a wired connection  161  ( FIG. 1 ) makes a connection from the output stud to an input on the voltage regulator to facilitate voltage sensing. The sensing is done inside the alternator and used as a reference for the voltage regulator to determine if more power or less power is needed to maintain the voltage setpoint. This wire could also go to the ECU, thereby providing information to the controller  112  indicating the voltage output of the alternator  116 , for example such that the controller  112  can confirm that the transition has occurred to the commanded battery before ramping up the power. The controller  112  is configured to identify and command either the high voltage mode operation or the low voltage mode operation of the system  100 . 
     The controller  112  controls the alternator  116  and the switch  114 , such as via one or more communication links. In the depicted embodiment, the controller  112  communicates control commands to the alternator  116  via communication link  172 , and communicates commands to the switch  114  via communication link  173 . In various embodiments, the communication links  172  and  173  for communicating with the alternator  116  and the switch  114 , respectively, may be the same or different communication means. The communication links  172 ,  173  may be physical links, such as wired data buses, or may be wireless links operating via any appropriate wireless protocol. For example, one or the other of the communication links  172 ,  173  may be via a local interconnect network (LIN) bus or via a controller area network (CAN) bus, such as a CAN Kingdom network. Alternatively, the switch  114  may be connected by a wire to ground inside the controller  112  to control the switch  114 . Alternatively, the switch could reside inside the alternator. 
     In the depicted embodiment, the communication link  172  between the controller  112  and the alternator  116  is via a LIN bus. Additionally, one or more of the vehicle sensors may self-communicate with the controller  112  via LIN. The LIN communication protocol is a serial network protocol that is configured to operate with one master license and several slave devices. Thus, the controller  112  may be established as the master device, with the alternator  116  as a slave device. In this embodiment, the voltage regulator  176  communicates with the controller  112  via LIN bus  172  so as to control the alternator  116  in the high voltage mode and the low voltage mode. LIN voltage regulators are known and have a LIN terminal  170  configured to transmit and receive data according to the LIN communication protocol. 
     The voltage regulator  176  is configured to optimize the output voltage of this stator  136  (i.e., the output voltage of the alternator assembly  116 ), by adjusting the voltage supplied to the field coil  132 . Alternatively or additionally, the voltage regulator  176  may be configured to control the excitation current of the stator  136  in order to control output and prevent the alternator from overheating. Additionally, the voltage regulator  176  may be configured to control a rate of change, or ramp up and ramp down, of the alternator charge output. This control functionality may be provided in response to instructions communicated to the voltage regulator  176 , and particularly to the LIN terminal  170 , by the controller  112 . Control instructions for controlling the output voltage, excitation current, and ramp rate for the alternator  116  are established according to the LIN protocol. 
     The switch  114  switches between the low voltage storage device  102  and the high voltage storage device  108  so that both storage devices are charged by the system  100  at different times. The low voltage storage device  102  may be connected to the propulsion system  105 , which includes the engine  104 , in order to power the propulsion-related loads. For example, the low voltage storage device  102  may be a 12 voltage lead acid battery configured to power 12 volt loads relating to the propulsion system  105 , including the engine starting, steering system, propulsion-related sensing system, etc., as is typical in many vehicle applications. The 12 volt battery  102  may also be configured to power other 12 volt loads on the vehicle. The high voltage storage device  108 , such as a 48 volt battery or battery bank, may be configured to power corresponding 48 volt loads, such as a vehicle air conditioning system, appliances and other house loads. 
     In other embodiments, a DC-DC converter may be provided between the low voltage storage device  102  and the high voltage storage device  108 .  FIG. 3  schematically depicts such an embodiment configured for a marine vessel. Here, the converter  117  connects the 48 volt battery  108  to the 12 volt battery  102  to enable power transfer there between. In such an embodiment, the converter  117  may be configured as a two way converter between the low voltage and high voltage levels so that power can be shared between the two systems. In the depicted example, the converter  117  allows power sharing between the 48 volt and 12 volt systems, where power generated at low engine speeds may be utilized by the 48 volt system and power generated by the 48 volt system may be utilized by the 12 volt system. Thereby, the converter  117  only needs to supply the average current rather than the peak, with the battery  108  as a buffer. 
       FIGS. 4B and 4B  depict exemplary charging outputs of embodiments of the disclosed charging system  100 .  FIGS. 4B and 4B  graphically depict exemplary charging output profiles  101  imposed on the graph of  FIG. 4A  depicting alternator outputs at three different voltage levels. In these examples, the system provides a low voltage charging output for charging a 12 volt battery and a high voltage charging output for charging a 48 volt battery. In other embodiments, the 24 volt alternator and storage device configuration may be utilized as either the high voltage system portion (paired with a 12 volt or other lower voltage system) or as the low voltage system portion (paired with the 48 volt or some other higher voltage system). 
     The depicted embodiment has benefits in that it utilizes the strength of both systems, including the low cut in speed of the 12 volt system and the high maximum charging output of the 48 volt system. Switching between the storage devices  102  and  108  for charging purposes is controlled by the controller  112  and performed based on engine speed and other factors, as described herein.  FIG. 4B  illustrates a charging profile  101   a  of one exemplary alternator configuration. In the example, the system switches between the low voltage charging function  180  and the high voltage charging function  190  at transition point  185 , which is at the intersection point  92  of the 12 volt charge profile and the 48 volt charge profile. The transition point  185  between the low voltage mode  180  and the high voltage mode  190  may be based on engine RPM, alone or in conjunction with assessment of other factors such as alternator temperature, battery charge levels, etc. For example, a threshold RPM, which in the depicted embodiment is around 2000 RPM, may be established where the controller controls the alternator  116  and the switch  114  to switch between the low voltage mode  180  functionality to charge the low voltage storage device  102  and the high voltage mode  190  functionality to charge the high voltage storage device  108 . For example, the controller  112  may communicate with the alternator  116  via LIN bus  172  to control the voltage output of the alternator  116  as appropriate for charging either the low voltage storage device  102  or the high voltage storage device  108 . 
     In certain embodiments, the controller may also control the excitation current of the alternator, which may be particularly useful in the low voltage mode  180  in order to prevent overheating of the alternator  116  by reducing the field current in the stator  136 .  FIG. 4C  depicts a charging output profile  101   b  wherein excitation current is limited. Excitation current of the alternator, and thus the alternator output, may be limited for a portion  182  of the low voltage mode  180 , such as based on engine RPM. For example, the alternator  116  may be controlled in a limited output mode  182  to limit the max output of the alternator above, for example 1,000 RPM. Here, the transition point  184  may be set to a lower RPM than the above-described intersect  90 ,  92  in order to further limit the duration of operation in the low voltage mode  180  in order to prevent overheating. Alternatively or additionally, a time limit for operation in the low voltage mode  180  may be set to further prevent over heating of the alternator  116 . In some embodiments, a temperature sensor  160  may be positioned to sense temperature of the alternator  116  and the alternator output may be controlled based thereon. For example, the temperature sensor  160  may be positioned within the alternator housing  124 , such as to provide information regarding the temperature of the stator  136  so that overheating can be prevented. For example, if the temperature sensed by the temperature sensor  160  exceeds a high temperature threshold, then the controller  112  may change the operation of the alternator  116  to either reduce the excitation current even further or to switch to the high voltage mode  190  which will automatically reduce the current flowing in the stator  136 . 
       FIGS. 5-7  depict embodiments of methods  200  of controlling a variable voltage charging system  100  on the vehicle. After engine start at step  202 , the system  100  is operated in the low voltage mode  180  to charge the low voltage storage device  102 . Upon detecting a high voltage mode condition at step  206 , operation of the system is changed at step  208  so as to operate in the high voltage mode  190 . The high voltage mode condition may be defined based on one or more parameters, including engine RPM, alternator temperature, and/or charge level of one or more of the storage devices  102 ,  108 . At the transition point  185 , the operation of the alternator  116  and the switch  114  are transitioned. Exemplary steps for carrying out the transition are described below. When operated in the high voltage mode  190 , the controller detects low voltage mode conditions at step  210 . The low voltage mode condition for switching operation of the system  100  to the low voltage mode  180  may be based on engine RPM and/or charge level of the low voltage storage device  102 , for example. Upon detection of the low voltage mode condition, the controller changes operation of the system  100 , including transitioning the switch  114  and the alternator  116  to operate in the low voltage mode  180 . 
       FIG. 6  depicts another embodiment of the method  200  of controlling the variable voltage charging system  100  on a vehicle. The engine is started at step  220  and operated at idle, which is typical engine operation at start. The charging system  100  is thus operated in the low voltage mode  180  to charge the low voltage storage device  102 , which in the depicted examples is a 12 volt battery. Engine RPM, charge level, and alternator temperature are monitored to determine whether the high voltage mode condition has occurred. In the depicted example, instructions are executed at step  224  to determine whether engine speed has exceed a threshold RPM for a predetermined amount of time. Since transitioning between charging modes  180  and  190  significantly changes the load on the engine  104 , it may be desirable to reduce the frequency and number of mode changes by requiring that the engine speed demonstrate consistency above or below the threshold before transitioning so that the user does not experience frequent changes in engine  104  performance. Thus, the transition from low voltage mode  180  to high voltage mode  190 , and the reverse transition, may only be provided if the engine  104  is operated at a relatively consistent engine speed above or below the transition threshold, respectively. 
     If the engine speed does not exceed the threshold RPM at step  224 , the controller  112  may further be configured to assess battery charge level and/or alternator temperature to consider whether to continue operation in the low voltage mode  180 . Instructions are executed at step  226  to determine whether the charge level of the 12 volt battery indicates that transitioning to the high voltage mode  190  is appropriate. If the 12 volt battery is fully charged, then the controller  112  may be configured to transition to the high voltage mode  190 , such as to avoid risk of overheating the alternator  116  by continuing operation in the low voltage mode  180 . If the low voltage storage device  102  could use further charge, and thus continuing operation in the low voltage mode  180  is beneficial, then instructions may be executed at step  228  to assess the temperature of the alternator  116 . If the alternator temperature remains below the high temperature threshold a step  228 , the system  100  may continue operation in the low voltage mode  180 . On the other hand, if the alternator temperature exceeds the high temperature threshold at step  228 , then the controller  112  may send a command to the alternator  116  to reduce the excitation current setpoint of the alternator  116  in order to reduce the current in the stator  136  and prevent over heating of the alternator  116 . 
     If the engine speed exceeds the threshold RPM at step  224 , then the controller  112  may instruct the alternator  116  and the switch  114  to transition to the high voltage mode functions at step  232 . Exemplary steps for controlling the switch are those traded at  FIG. 7 . The system is then operated in the high voltage mode at step  234  to charge the high voltage storage device  108 , which in the depicted examples is a 48 volt battery. Instructions are executed at step  236  to determine whether the engine is again being operated in the low speed range. If the engine speed is below the threshold RPM for a pre-determined time at step  236 , then the controller  112  may assess whether the charge level of the low voltage storage device  102  justifies switching to the low voltage mode  180 . For example, if the charge level of the 12 volt battery is above a charge threshold at step  238  indicating that the 12 volt battery is sufficiently charged, then the system  100  may continue to operate in the high voltage mode  190 . However, if the engine speed remains below the threshold RPM and the low voltage storage device  102  is in need of charge, then the controller  112  may instruct a change in the operation of the alternator  116  and the switch  114  in order to change to the low voltage mode  180  at step  240 . 
       FIG. 7  depicts exemplary steps for switching between modes  180  and  190 . Since the transition will enact a noticeable change on the load exerted on the engine  104 , the controller  112  may be configured to execute transition procedures that slowly ramp down the draw by the alternator prior to the switch and then slowly ramp the alternator output back up so that the operator does not notice sudden changes in engine mode. Additionally, ramping down the charge output by the alternator to zero or a very low output will put less stress on the switch  114 , and thus prolong the life of the switch. The controller  112  may be configured to send a ramp down command to the alternator  116  via the communication link  172 . For example, the controller  112  may send a LIN command to the LIN terminal  170  of the voltage regulator  176  that commands the voltage regulator  176  to effectuate a decrease in charge output by the alternator  116  at a particular rate of change—i.e., a ramp down rate. For example, the voltage regulator  176  may be commanded to decrease the charge output to zero output at the commanded ramp down rate. The ramp down rate is sufficiently slow so that the operator does not notice a sudden or drastic change in engine performance. 
     Once the alternator output reaches zero or some preconfigured minimal output, at step  244 , then the controller  112  may command the switch  114  to change positions at step  246 . In certain embodiments, the communication link  173  between the controller  112  and the switch  114  may be by different means than the communication link  172  between the controller  112  and the alternator  116 . For example, the communication link  173  to the switch  114  may be via a CAN bus or some other serial communication bus, or alternatively may be a direct and dedicated connection for communicating between the controller and the switch  114 , such as a wire connected to ground inside the controller  112 . If the system  100  is in the low voltage mode  180  and a high voltage mode condition is detected, then the switch  114  will be commanded to change from the low voltage switch position  114   a  to the high voltage switch position  114   b . Conversely, if the system  100  is operating in the high voltage mode  190  and a low voltage mode condition is satisfied then the switch  114  is commanded to change to the low voltage position  114   a.    
     The controller  112  also sends a corresponding regulation setpoint command to the alternator at step  248 . The regulation setpoint command commands the voltage regulator  176  to change its maximum voltage output as appropriate for the charge mode  180 ,  190 . If transitioning to the high voltage mode  190 , then the commanded regulation setpoint will provide a voltage output appropriate for the high voltage load  120 . For example, with a high voltage load  120  is a 48 volt battery, the regulation setpoint for the voltage regulator  176  may be a typical charge voltage for charging a 48 volt battery, such as a value in the range of 53 to 56 volts. If transitioning to the low voltage mode  180 , then the regulation setpoint command will instruct the voltage regulator  176  to provide a charging output that is appropriate for the low voltage storage device  102 . Where the low voltage storage device  102  is a 12 volt battery, the regulation setpoint will provide an appropriate charge voltage that is standard for charging a 12 volt battery (e.g., a value in the range of 14 to 14.5 volts). Once the regulation setpoint of the alternator  116  has been changed, then the controller commands the alternator at step  250  to ramp its output up to the new setpoint. 
     This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to make and use the invention. Certain terms have been used for brevity, clarity and understanding. No unnecessary limitations are to be inferred therefrom beyond the requirement of the prior art because such terms are used for descriptive purposes only and are intended to be broadly construed. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have features or structural elements that do not differ from the literal language of the claims, or if they include equivalent features or structural elements with insubstantial differences from the literal languages of the claims.