Patent Publication Number: US-2021175485-A1

Title: System and method for operating a dual battery system

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The present application claims priority to U.S. Provisional Application No. 62/520,468, entitled “SYSTEM AND METHOD FOR OPERATING A DUAL BATTERY SYSTEM”, and filed on Jun. 15, 2017. The entire contents of the above-listed application are hereby incorporated by reference for all purposes. 
    
    
     TECHNICAL FIELD 
     The present description relates to methods and systems related to a dual battery system. 
     BACKGROUND AND SUMMARY 
     Auxiliary (Aux) dual battery systems can provide cost effective designs for battery applications where both long term and short term energy storage and dissipation are desirable. For example, in a hybrid vehicle a low-cost, traditional lead acid battery may be coupled with a small, high power lithium ion battery. Whereas the lead acid battery is utilized primarily for engine cranking, the smaller lithium ion battery allows for higher power for charge recuperation during regenerative braking and discharge power for cold cranking. 
     However, the inventors herein have recognized potential disadvantages with the above approach. The charge voltage of lead acid batteries increases as temperature decreases, and is higher than the charge voltage of certain configurations of lithium ion batteries at low temperatures. Applying these high charge voltages to the lithium ion batteries can degrade the lithium ion battery, for example, because of lithium metal plating at the battery electrodes. Some conventional dual battery systems utilize a lithium titanate (LTO) battery coupled with a lead acid battery because LTO batteries can be more tolerant to plating at cold temperatures as compared with other lithium ion battery types. However, LTO batteries are more costly to produce, and are less compact than other types of lithium batteries, which can raise manufacturing costs. 
     One approach that at least partly addresses the above issues includes a battery system comprising: a first battery and a second battery electrically connected in parallel, the second battery comprising a plurality of battery cells and a heater thermally coupled to the plurality of battery cells; and a controller on board the second battery, including executable instructions to, in response to a charge voltage being greater than a threshold voltage, diverting a portion of the charge voltage in excess of a threshold voltage from the second battery to the heater. 
     By diverting voltage from the second battery to a heater thermally coupled to one or more battery cells of the second battery, degradation of the second battery due to high charge voltages can be reduced. Furthermore, diverting voltage to the heater can aid in increasing the temperature of the second battery, further reducing degradation of the second battery. Further still, reducing degradation of the second battery, including at colder temperatures, facilitates utilizing lower-cost higher-density lithium battery chemistries, such as lithium iron phosphate (LFP), the dual battery system. 
     The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings. 
     It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a schematic view of an exemplary assembly of a battery cell stack; 
         FIG. 2  shows a schematic view of an exemplary battery cell; 
         FIG. 3  shows a simplified schematic diagram of an exemplary dual battery system; 
         FIG. 4  shows a plot of battery charging profiles; 
         FIG. 5  shows a partial schematic view of the battery system of  FIG. 3  including an external heater 
         FIG. 6  shows an example schematic of a voltage detection and control system; 
         FIG. 7  shows an example flow chart for a method of operating the battery system of  FIG. 3  including the battery system of  FIG. 5 . 
         FIG. 8  shows an example timeline for operating the battery system of  FIG. 3  including the battery system of  FIG. 5 . 
     
    
    
     DETAILED DESCRIPTION 
     The present description is related to methods and systems for a dual battery system, including a first battery electrically coupled to a second battery, as shown in  FIG. 3 . In one embodiment, the battery pack of the second battery may be comprised of one or more battery cell stacks, one of which is illustrated in  FIG. 1 , and the battery cell stacks may be comprised of a plurality of battery cells, one of which is illustrated in  FIG. 2 . The second battery may further include a voltage detection and control system, as shown in  FIG. 6 . As shown in  FIG. 4 , the first battery and the second battery may exhibit distinct charging profiles with respect to temperature. By adding a heater external and adjacent to the battery cells of the second battery as shown in  FIG. 5 , and by diverting higher charge voltages from the second battery to the heater, degradation of the second battery can be reduced. A method and timeline for operating the dual battery system of  FIG. 3  is illustrated in  FIGS. 7 and 8 , respectively. 
     Referring now to  FIG. 1 , an exemplary assembly of a battery cell stack  200  is shown. Battery cell stack  200  is comprised of a plurality of battery cells  202 . In some embodiments, the battery cells may be lithium-ion battery cells such as (lithium iron phosphate) LFP or (lithium titanate) LTO battery cells, for example. In the example of  FIG. 1 , battery cell stack  200  is comprised of ten battery cells  202 . Although battery cell stack  200  is depicted as having ten battery cells  202 , it should be understood that a battery cell stack  200  may include more or less than ten battery cells. For example, the number of cells in a battery cell stack  200  may be based on an amount of power desired from the battery cell stack  200 . Within a battery cell stack  200 , battery cells  202  may be coupled in series to increase the battery cell stack voltage, or battery cells  202  may be coupled in parallel to increase current capacity at a particular battery voltage. Further, a battery pack may be comprised of one or more battery cell stacks  200 . As shown in  FIG. 1 , battery cell stack  200  further includes cover  204  which provides protection for battery interconnects (not shown) that route charge from the plurality of battery cells  202  to output terminals of a battery pack. 
     Turning now to  FIG. 2 , an exemplary embodiment of an individual battery cell  300  is shown. Battery cells  202  may be represented by the battery cell  300  in  FIG. 2 . Battery cell  300  includes cathode  302  and anode  304  for connecting to a bus (not shown). The bus routes charge from a plurality of battery plates to output terminals of a battery pack and may be coupled to bus bar support  310 . Battery cell  300  further includes prismatic cell  308  that contains electrolytic compounds. Prismatic cell  308  is in communication with heat sink  306 . Heat sink  306  may be formed of a metal plate with the edges bent up 90 degrees on one or more sides to form a flanged edge. In the example of  FIG. 2 , the bottom edge, and sides, each include a flanged edge. 
     When a plurality of cells is put into a stack, the Prismatic cells may be separated by a compliant pad (not shown). Thus, a battery cell stack is built in the order of heat sink, Prismatic cell, compliant pad, Prismatic cell, heat sink, and so on. One side of the heat sinks (e.g., flanged edges) may then contact the cold plate to increase heat transfer. In some embodiments, the compliant pads separating the Prismatic cells may include heating coils or heating pads for transferring heat to the battery cells  300  (see  FIG. 5 ). 
     Referring now to  FIG. 3 , it illustrates a simplified schematic of a dual battery system  400 , comprising a first battery  410  and a second (auxiliary) battery  420 . In one example embodiment, the dual battery system  400  may comprise a lead acid battery as the first battery  410  and a lithium ion battery (such as an LTO or LFP battery) as the second battery  420 . The second battery  420  may comprise one or more battery packs  200  including one or more battery cell stacks  200  as described with reference to  FIGS. 1 and 2  above. In the dual battery system of  FIG. 3 , the first battery and the second battery are electrically coupled in parallel to each other and to one or more power sources  404 , one or more loads  460 , and a motor  402 . 
     Power source  404  may comprise one or more power sources such as an alternator coupled to an internal combustion engine and a motor coupled to a regenerative braking system. The power source  404  may be used to charge one or both of the first battery and the second battery. The charging of one or both of the first battery and the second battery by the power source  404  may be dependent on the type of power generated by the power source  404 . In some examples, the one or more power sources  404  may be used to charge one or both of the first battery  410  and the second battery  420 . For example, an alternator may be used to charge both the first battery  410  and the second battery  420 , whereas a motor driven by a regenerative braking system may be used to charge the second battery  420 . For example, if the power source  404  comprises a flywheel generating power from regenerative braking in a vehicle, power from power source  404  may primarily charge the second battery (e.g., a lithium ion battery) since the charging rates are higher. In another example, the motor  402  may drive a power source  404  such as an alternator, which can be used to more slowly charge the first battery  410  (e.g., a PbA type battery). 
     One or both of the first battery  410  and the second battery  420  may provide power to the one or more loads  460 , depending on the power discharge rate. Loads  460  requiring higher discharge rates, for example a motor powering propulsion of vehicle, may be provided primarily by the second battery  420 , whereas loads  460  requiring lower discharge rates may be powered primarily by the first battery  410 . The dual battery system  400  may reside on board a vehicle for powering loads  460  such as auxiliary loads such as vehicle lights, HVAC, audio/visual accessories, vehicle seat positioners, seat warmers, and the like. 
     Dual battery system may comprise one or more battery management systems  414  and  424 . As shown in  FIG. 3 , a battery control module or battery management system (BMS)  414  may be electrically connected proximally to the first battery  410  and may aid in regulating or measuring voltage and/or current supplied to and dissipated from the first battery  410 . In some examples, the first battery  410  may not include a BMS. In other examples, first battery  410  may include an intelligent battery sensor (IBS). BMS  424  may reside on board the second battery  420 , as illustrated in the example of  FIG. 5 , and may control modules for regulating voltage and/or current supplied to and dissipated from individual battery cells  202  in the battery cell stack  200  of the second battery  420 . In other embodiments, BMS  414  and BMS  424  may be integrated into a single BMS for regulating voltage and/or current supplied to and dissipated from both first battery  410  and second battery  420 . Further, the BMS may be comprised of a microprocessor having random access memory, read only memory, input ports, real time clock, and output ports. Various sensors such as temperature sensors may communicate internal environmental conditions of battery pack  200  to BMS  424 . The BMS may further aid in regulating voltage and/or current supplied to and dissipated from the battery cell stack  200 . For example, during charging of the battery pack  200 , the BMS may regulate voltage levels to each individual battery cell in the battery cell stack  200  to balance the charging of each battery cell and to reduce overcharging of the battery cells, which can cause degradation of the battery cell stack. 
     Dual battery system may further include various sensors, such as temperature sensors  624 , as described above with reference to  FIG. 5 , which can transmit signals to the one or more BMSs  414  and  424 . Various switches and/or relays may include a cranking disconnect  470 . In one example, cranking disconnect may be used for decoupling a motor  402  such as a starting motor from an engine after an engine has been started. A switch or relay  474  may be used to decouple the second battery  420  from a power source  404 , for example, when a charge voltage is greater than a threshold voltage, to reduce a risk of degrading the second battery  420 . 
     Referring now to  FIG. 4 , it illustrates example plot  500  showing charging profiles  510  and  520  versus temperature for a lead acid (PbA) battery and a lithium iron phosphate (LFP) battery, respectively. As shown by the lead acid battery charging profile  510 , at lower temperatures the charge voltage for the lead acid battery is high and greater than a cold temperature lithium plating voltage  530 . In the example plot  500 , the cold temperature lithium plating voltage  530  is approximately 14.4 V below 0° C. Furthermore, the charge voltage of the lead acid battery does not decrease below the cold temperature lithium plating voltage until the temperature increases above a threshold temperature  540  (e.g., approximately 20° C.). As such, at temperatures less than 20° C., charging a dual battery system comprising a lead acid battery and a LFP battery coupled in parallel can lead to lithium plating and degradation of the LFP battery since the charge voltage applied to the dual batteries is given by the charge profile of the PbA battery. 
     As the temperature is increased, the charge voltage of the PbA battery tends to decrease, whereas the charge voltage of the LFP battery tends to increase. Accordingly, heating the dual battery system, in particular heating the LFP battery, can reduce a risk of degradation of the second battery, and also increase charging performance since the charging of the LFP battery can be performed at higher charge voltages (but still less than the cold temperature lithium plating voltage  530 ). At temperatures above 20° C., the charge voltage for the PbA battery is less than the lithium plating voltage, and the heater may not be utilized. 
     Referring now to  FIG. 5 , it illustrates an example battery pack  600  including one or more heaters  620  positioned intercellularly between each battery cell in the battery cell stack  200 , and at the ends of the battery cell stack  200 . The heaters may be positioned adjacent and external to the battery cells, and apart from the electrolyte within the battery cells. In this way, existing battery pack designs can be retrofitted easily with the heaters  620 . For example existing compression pads or compliant pads between the battery cells can be replaced or outfitted/augmented with heaters  620 . In one embodiment, battery pack  600  can be an LFP battery pack, wherein the heaters  620  are used to heat LFP battery cells in the LFP battery cell stack. Heaters  620  may comprise flat sheet compression pad type heaters, resistance heaters, or another type of compact heater that can efficiently and uniformly transfer heat to the battery cells. Heaters  620  may be electrically coupled to the BMS  608 . Furthermore, although not shown, battery pack  600  may further include one or more temperature sensors  624  and one or more voltage sensors (see  FIG. 6 ) to measure and/or imply the temperature and voltage of each battery cell of battery cell stack  200 , respectively. In this way, the temperature of and voltage applied to each of the battery cells can be determined and communicated to BMS  608 . 
     Furthermore, the BMS  608  can direct voltage and/or current to one or more of the battery cells in battery cell stack  200  responsive to the one or more temperature and voltages at the battery cells. For example, in response to a charge voltage being greater than a threshold voltage, the BMS may divert a portion of the charge voltage in excess of the threshold voltage from the battery cells of battery cell stack  200  to the one or more heaters  620  adjacent and external thereto. The threshold voltage may correspond to an electrode plating voltage, such as cold temperature lithium plating voltage  530 . As such, diverting the portion of the charge voltage in excess of the threshold voltage may reduce a risk of degradation of the dual battery system. In another example, the threshold voltage may vary with temperature and state of charge, and can be determined based on a charge voltage profile  520  for the battery and a temperature of the battery. Diverting excess voltage from the battery to one or more heaters  620  generates heat at the heater  620 , thereby increasing the battery cell temperature. In the case of charge voltage profile  520 , increasing the battery temperature can increase the threshold voltage. A higher threshold voltage raises the effective charge voltage of the battery (since only voltage excess to the threshold voltage is diverted), thereby reducing a risk of degradation and increasing a charging power. 
     Referring now to  FIG. 6 , a schematic diagram of a voltage detection and management system  700  is shown. The voltage detection and management system  700  may reside within a battery, such as the battery  420  as shown in  FIG. 3 , or the battery pack  600  as shown in  FIG. 5 , and reside on board the BMS. As depicted, the system includes a plurality of battery cells  712 , voltage detectors  702 , charge reducing circuitry for each battery cell, a power supply  704 , non-volatile storage  710 , and a microcontroller  706  that is in communication with a BMS by way of communication channel  708 . Power supply  704  may be activated by voltage detectors or by the BMS. In some examples, one or more of the voltage detectors  702 , power supply  704 , micro-controller  706 , non-volatile storage  710 , and communication channel  708  may be integrated into the BMS. 
     In the example of  FIG. 6 , each of the plurality of battery cells  712  is shown in communication with a voltage detector  702  which includes voltage detection circuitry. Voltage detector circuits  702 , power supply  704 , microcontroller  706 , non-volatile storage  710 , load resistor  714 , transistor switch  716 , and communication channel  708  are incorporated into the BMS. Once the BMS is coupled to the battery cell stack  200 , the battery cells are continuously monitored by the voltage detector circuits. The voltage detector circuitry may be powered by the battery cells in the battery cell stack. Thus, the battery cell stack may become self-regulating during some conditions. In one embodiment, voltage detector circuitry  702  may be comprised of a comparator referenced to a threshold balancing voltage. If the input to the comparator exceeds the threshold balancing voltage the comparator changes state from a low voltage output to a higher voltage output. The higher voltage output provides an indication that the particular battery cell is charged to a level greater than a desired level. Further, the outputs of the voltage detection circuits may be tied together in an OR arrangement so that a high level signal is present at a power supply located on the BMS whenever one of the plurality of battery cells is greater than a threshold balancing level. 
     When a particular battery cell voltage or voltage range is detected, voltage detector circuitry  702  outputs a high level signal to power supply  704 . For example, if the voltage of an individual battery cell is greater than a threshold balancing value, voltage detector circuitry  702  may send a signal to power supply  704 , thereby activating the power supply. Power supply  704  is in communication with microcontroller  706 . As such, microcontroller  706  may be activated once power supply  704  is turned on. Microcontroller  706  may include digital inputs and outputs as well as one or more A/D inputs, read only memory, random access memory, and non-volatile storage. 
     As shown in  FIG. 6 , the microcontroller  706  provides a communication channel  708  for the battery pack. In one embodiment, communication channel  708  may be a CAN link. The battery pack controller may be a battery control module (BMS), as described above with reference to  FIG. 3 , for example. Via the communication channel  708 , microcontroller  706  may communicate a variety of information. As one example, the microcontroller  706  may update the BMS regarding battery cells that have been discharged while the BMS is unavailable. 
     Microcontroller  706  may include non-volatile storage  710 . As such, microcontroller  706  may save data regarding the plurality of battery cells to the non-volatile storage  710 . For example, non-volatile storage  710  may save data regarding the voltage states of the battery cells including data regarding charge draining from the one or more battery cells that exceed the threshold voltage (e.g., amount of charge drained, number of times charge is drained from a particular battery cell, time and date of battery cell discharge etc.). In this manner, the microcontroller  706  may communicate battery cell information to the BMS when conditions are more favorable. 
     Once activated, microcontroller  706  may output a signal to turn on battery cell charge reducing circuitry which includes a load resistor  714  and a switch  716 . For example, a digital output from the microcontroller  706  may close switch  716 . As an example, switch  716  may be a transistor such as a field-effect transistor. Thus, when the switch  716  is closed, current may be allowed to flow through the charge reducing circuit. Battery cell charge may be dissipated by load resistor  714 . In the example of  FIG. 6 , each battery cell of the plurality of battery cells is coupled in parallel with a switch (e.g., each battery cell is in communication with a switch). Once the charge of a particular battery cell is less than a threshold level, the output of voltage detector  702  coupled to the battery cell changes state to indicate that the charge of the particular battery cell is less than the desired level. 
     The appropriate switch (e.g., switch  716 ) may be set to an open condition by microcontroller  706  when battery cell voltage as measured by an A/D convertor and input to microcontroller  706  is less than the desired threshold voltage. Further, power supply  704  may be latched in an on condition by an output from the microcontroller (e.g., microcontroller  706 ). The microcontroller may hold a digital output high to keep the power supply activated until charge of each battery cell in the battery cell stack  200  is less than a threshold. Further, the microcontroller may keep the power supply activated until it has completed a scheduled task that was initiated by activating power supply  704  (e.g., after writing battery cell event data to non-volatile storage). 
     The voltage detection and management system  700  may be utilized to balance or redistribute charges and mitigate overcharging amongst individual battery cells within a battery stack during battery charging. Typically, the individual cells in a battery have somewhat different capacities and may be at different levels of state of charge (SOC). Without redistribution, discharging stops when the cell with the lowest capacity is empty (even though other cells are still not empty); this limits the energy that can be taken from and returned to the battery. Without balancing, the battery cell having the lowest capacity becomes limiting to other battery cells; it can be easily overcharged or over-discharged while cells with higher capacity undergo only partial cycle. Balancing charges bypasses the lower capacity battery cells; so that in a balanced battery, the cell with the larger capacities can be more fully charged while reducing overcharging any smaller capacity battery cells; conversely, in a balanced battery, battery cells with larger capacities can be more fully discharged while reducing over-discharging any smaller capacity battery cells. Battery balancing (e.g., a balancing mode) comprises transferring voltage (exceeding the threshold balancing voltage) from or to individual cells, until the SOC of the cell with the lowest capacity is equal to the battery&#39;s SOC. 
     Turning now to  FIG. 7 , it illustrates a method  800  of operating a dual battery system  400  including a first battery  410  and a second battery  420  (such as battery pack  600 ). In one embodiment, the first battery  410  may comprise a lead acid battery and the second battery  420  may comprise a lithium ion battery such as an LTO or LFP battery. Method  800  may comprise executable instructions on board a controller such as BMS  608 . In other examples, method  800  may comprise executable instructions on board a controller external to the second battery  420 , but electrically coupled to the dual battery system  400 . Method  800  may be executed independently from a balancing mode, the balancing mode comprising when a voltage detection and management system  700  is balancing charges amongst individual battery cells as described above with reference to  FIG. 6 . Thus method  800  may be executed while a balancing mode is active or while a balancing mode is inactive. 
     Method  800  begins at  802  where battery system conditions such as temperatures of the first and second batteries (T 1 , T 2 ), state of charge of the first and second batteries (SOC 1 , SOC 2 ), and the like are estimated and/or measured. As described above, T 1  and T 2  may be measured using one or more temperature sensors positioned external to the battery cells but mechanically coupled to the battery cells. In other embodiments, the T 1  and/or T 2  may be inferred using one or more temperature sensors. Method  800  continues at  810 , where the controller connects the first and second batteries in parallel. As described above with reference to  FIG. 3  and  FIG. 6 , the battery system may comprise various circuitry components such as switches, transistors, and the like, which can be actuated by the controller to electrically couple the first and second batteries in parallel. At  814 , the controller may similarly actuate various connect circuitry components such as switches, transistors, and the like, to connect one or more motors, generators, and loads in parallel with the first and second batteries. 
     Next, method  800  continues at  818  where one or more heaters external to the cells of the second battery are coupled to the cells of the second battery  818 . Coupling the one or more heaters external to the cells of the second battery may comprise positioning the one or more heaters adjacent and external to the battery cells of the second battery, but within the 2 nd  battery pack. In this way, heat that is generated at the external heaters can be more efficiently and more rapidly transferred to the battery cells of the second battery. Furthermore, by positioning the one or more heaters adjacent and external to the battery cells, existing battery packs can be retrofitted with the external heaters inexpensively, as compared to installing heaters internal (intracellularly) to the battery cells. 
     Method  800  continues at  820  where the controller determines a charge voltage, V c , based on the temperature of the first battery, T 1 . In one example, T 1  may be determined from a charge voltage profile  510 , a lookup table, and the like. In this way, V c  may be temperature dependent. At  830 , the controller may determine a threshold voltage, V TH , based on a temperature of the second battery, T 2 . T 2  may be determined from a battery charge voltage profile  520  of the second battery, a lookup table and the like. In this way, the threshold voltage, V TH , for the second battery may be temperature dependent and may correspond to the charge profile for the second battery. In another example, V TH  may correspond to a voltage above which the rate of battery degradation is increased. For example, V TH  may correspond approximately to the cold temperature plating voltage of −14.4 V for a LFP battery. 
     At  850 , the controller determines if a first condition is met. The first condition may comprise when V c  applied to one or more of the battery cells in the second battery  420  is greater than V TH . For example, if the second battery  420  comprises an LFP battery, V TH  may be determined from charging profile  520  and may be a function of the temperature of the second battery. Furthermore, if the first battery comprises a PbA battery, V c  may be determined from the charging profile  510  and may be a function of the temperature of the first battery. Referring to  FIG. 4 , plot  500  clearly illustrates that V c  given by charging profile  510 , is greater than V TH  given by charging profile  520 , when the temperatures of the first and second battery are less than the threshold temperature  540 , T TH . Accordingly, the first condition may further comprise when one or both of the temperatures T 1  and T 2  are less than a threshold temperature T TH . 
     In response to V c  being greater than V TH  (or when the first condition is met at  850 ), then the controller continues at  852 , where a portion of the V c  in excess of V TH  is diverted from the second battery to the one or more external heaters  620 . At  852 , the controller may actuate one or more switching circuit components (e.g., switch or relay  474 ) to aid in diverting the excess voltage from all battery cells in the second battery subject to V c &gt;V TH . Furthermore, the controller may divert a portion of the V c  in excess of V TH  from all the battery cells of the second battery to the one or more external heaters  620  in response to V c  being greater than V TH  (or when the first condition is met at  850 ), without diverting any voltage from the battery cells of the first battery. 
     Next, at  854 , heat may be generated at the external heaters from the portion of V c  in excess of V TH  diverted thereto from the second battery. Since the external heaters  620  are positioned adjacent and external to the battery cells of the second battery, the generated heat may be transferred to the battery cells of the second battery at  856 , thereby increasing T 2 ; and at  858 , the controller may adjust V TH  based on the new value of T 2 . Accordingly, for the case where the second battery comprises an LFP battery, and where V TH  is determined based on the charging profile  520 , V TH  will increase in response to diverting excess voltage to the external heaters, since the charging voltage increases with increasing temperature. Consequently, diverting the charge voltage V c  applied to the second battery in excess of V TH  may reduce a risk of degradation of the second battery since overcharging the battery is reduced. Furthermore, diverting the charge voltage V c  applied to the second battery in excess of V TH  may increase a charging performance of the second battery since T 2  is increased, thereby increasing V TH , and the voltage at which all battery cells of the second battery can be charged. 
     After  850  for the case where V c &lt;V TH , method  800  continues at  860  where the controller applies V c  to the second battery without diverting any portion thereof therefrom. Since V c &lt;V TH , V c  can be applied to all the battery cells of the second battery without increasing a risk of battery degradation. After  860 , and following 858 method  800  continues at  870  where the controller applies V c  to the first battery without diverting voltage to the external heaters. As described above, the controller may actuate one or more switching circuitry components to direct V c  to the first battery and the second battery in steps  860  and  870  respectively, without diverting any voltage to the external heater. After  870 , method  800  ends. 
     As described above, method  800  may be executed by the controller independently of balancing mode operations, as described with reference to  FIG. 6 . Furthermore, in method  800 , the portion of V c  in excess of V TH  is diverted for all the battery cells of the second battery where V c &gt;V TH . In this way the method  800  is distinct from the balancing operations of  FIG. 6  because the balancing operations divert voltage from individual battery cells based on the state of charge or remaining battery capacity. Furthermore, the steps of method  800  are executed by the controller independently of battery capacity. As such, the steps of method  800  may be executed when the battery capacity of the second battery is higher than a threshold battery capacity, and when the battery capacity of the second battery is lower than a threshold batter capacity. 
     In this manner, a method for a battery system may include applying a charge voltage to first battery and a second battery electrically connected in parallel, diverting a portion of the charge voltage in excess of a threshold voltage from all battery cells of the second battery to a heater coupled externally to the second battery, and transferring heat from the heater to the second battery, the heat generated from the portion of the charge voltage. In a first example of the method, in the absence of diverting the portion of the charge voltage in excess of the threshold voltage from all battery cells of the second battery to the heater, degradation of an electrode in the second battery would occur upon applying the charge voltage to the second battery. A second example of the method includes the first example and further includes, wherein the portion of the charge voltage in excess of the threshold voltage may be diverted from all battery cells of the second battery to the heater independently of a charge capacity of the second battery. A third example of the method includes one or more of the first and second examples and further includes, wherein the portion of the charge voltage in excess of the threshold voltage may be diverted from the second battery to the heater independently from balancing voltages of the plurality of battery cells of the second battery. A fourth example of the method includes one or more of the first through third examples and further includes generating heat at the heater resulting from diverting the portion of the charge voltage in excess of the threshold voltage from the second battery to the heater, and transferring the heat from the heater to the second battery, thereby raising a temperature of the second battery. A fifth example of the method includes one or more of the first through fourth examples and further includes raising the threshold voltage in response to an increase in the temperature of the second battery. A sixth example of the method includes one or more of the first through fifth examples and further includes lowering the charge voltage in response to an increase in a temperature of the first battery. 
     In this manner, a method for a battery system may include connecting a first battery and a second battery in parallel, coupling a heater externally to a plurality of battery cells of the second battery, and applying a charge voltage to the first battery and the second battery. During a first condition, comprising when the charge voltage is greater than a threshold voltage, the method may include diverting a portion of the charge voltage in excess of the threshold voltage from the second battery to the heater, and applying the charge voltage to the first battery without diversion of any portion of the charge voltage away from the first battery. In a first example of the method, coupling the heater to the second battery may include positioning the heater directly adjacent but external to the plurality of battery cells of the second battery. A second example of the method optionally includes the first example and further includes wherein diverting the portion of the charge voltage in excess of the threshold voltage is further in response to when a temperature of the second battery is less than a threshold temperature. A third example of the method optionally includes the first and second examples and further includes, wherein diverting the portion of the charge voltage in excess of the threshold voltage may be performed independently from balancing voltages of the plurality of battery cells of the second battery. A fourth example of the method optionally includes the first through third examples and further includes, connecting a generator in parallel to the first battery and the second battery, and generating the charge voltage from the generator. 
     Turning now to  FIG. 8 , it illustrates an example timeline  900  illustrating operation of a dual battery system  400  according to method  800 . Timeline  900  includes trend lines for V c    910 , V TH    912 , the effective charging voltage for the first battery, V c1    918 , the effective charging voltage for the second battery, V c2    916 , T 1    920 , T 2    926 , and a balancing mode status  950 . Also shown is the threshold temperature, T TH    922 . As described above the charge voltage V c  applied to the first battery and the second battery may be determined from the charging voltage profile of the first battery. For example, for the case when the first battery comprises a PbA battery, V c  can be determined from a charging profile such as the charging profile  510 . The times t 1 , t 2 , and t 3 , may correspond to discrete instances in time when the controller receives transmitted data from various battery system temperature and voltage sensors and when calculated values such as T TH  and V c  may be determined. 
     Prior to time t 1 , both T 1  and T 2  are less than T TH . As described above, T TH  may correspond to a threshold temperature  540 , below which a charging voltage V c  applied to the first and second batteries is greater than V TH . V TH  may be determined from a charging profile of the second battery. For the case where the second battery comprises a LFP battery, V TH  may be determined based on the charging profile  520  and T 2 . Responsive to V c &gt;V TH , the controller diverts the portion of V c  in excess of V TH  from the second battery to the external heaters, thereby generating heat at the external heaters. Because the voltage in excess of V TH  is diverted from the second battery to the heater, the effective charge voltage applied to the second battery, V c2    916 , matches V TH    912  (in  FIG. 8 , V c2    916  and V TH    912  are slightly staggered on the voltage access for illustrative purposes). Furthermore, because the voltage in excess of V TH  is diverted from the second battery to the heater without diverting any voltage from the first battery, the effective charge voltage applied to the first battery, V c1    918 , matches V c    910  (in  FIG. 8 , V c1  and V c  are slightly staggered on the voltage access for illustrative purposes). Because the external heaters are positioned adjacent and external to the battery cells of the second battery, the generated heat is transferred to the battery cells of the second battery and T 2    926  is increased. Prior to time t 1 , T 1  also increases gradually because the charging process for the PbA battery is exothermic. 
     At time t 1 , owing to the increase in T 2 , V TH    912  increases, and owing to the increase in T 1 , V c    910  decreases. However, because V c  remains greater than V TH  between time t 1  and time t 2 , a first condition is met and the controller, in response, continues to divert a portion of the voltage V c  in excess of V TH  from the second battery, to reduce a risk of degradation of the second battery. As such, heat is generated in the external heaters adjacent to and external to the battery cells of the second battery, thereby increasing T 2  between time t 1  and time t 2 . T 1  also increases gradually between time t 1  and time t 2  because the charging process for the PbA battery is exothermic. Because the voltage in excess of V TH  is diverted from the second battery to the heater, the effective charge voltage applied to the second battery, V c2    916 , matches V TH    912 ; furthermore, because the voltage in excess of V TH  is diverted from the second battery to the heater without diverting any voltage from the first battery, the effective charge voltage applied to the first battery, V c1    918 , matches V c    910 . 
     Owing to the increase in T 1    920 , V c    910  decreases at time t 2 . Similarly, owing to the increase in T 2 , V TH    912  increases at time t 2 . At time t 2 , T 2  increases above T TH , however T 1  still remains below T TH . Timeline  900  uses the example case where T TH  corresponds to the threshold temperature  540 , and the charging voltage profile of the first battery and the charging voltage profile of the second battery are as given by  510  and  520 , respectively, in  FIG. 4 . At time t 2 , V c &gt;V TH  since the charging voltage of the second battery reaches a cold temperature lithium plating voltage  530  at temperatures greater than T TH , whereas the charging voltage of the first battery is greater than the cold temperature lithium plating voltage  530  at T 1 &lt;T TH . Responsive to V c &gt;V TH , upon applying V c  to the first and second batteries, the controller diverts a portion of V c  in excess of V TH  from the second battery to the external heater, to reduce a risk of degradation of the second battery, without diverting any voltage from the first battery. Accordingly between time t 2  and time t 3 , the effective charge voltage to the first battery V c1    918  is equal to the applied charge voltage V c , and the effective charge voltage to the second battery V c2    916  is equal to the threshold voltage V TH    912 . 
     At time t 3 , T 1    920  has increased above T TH . Referring to the example case of  FIG. 4 , when both the temperatures of the first battery and the temperature of the second battery are greater than T TH , the charge voltage for the second battery  520  becomes greater than the charge voltage for the first battery  510 . Consequently, at time t 3 , the applied charging voltage V c    910  to the first and second batteries matches the voltage charging profile for the second battery  520 . Accordingly, after time t 3 , V c    910  matches V TH    912 . Furthermore, since V c =V TH , the first condition is not satisfied. In response, the controller does not divert any voltage from the second battery, and does not divert any voltage from the first battery. Thus, the effective applied voltage to the second battery  916  also matches V c    910  and V TH    912  after time t 3 . Since T 1  and T 2  are both greater than T TH , the effective applied voltage to the first battery V c1    918  matches the charge voltage according to the charging profile for the first battery  510 , dropping to a value below V c , V TH , and V c2 . As shown from timeline  900 , the steps of method  800  may be conducted independently of the balancing mode status  950 . In other words, method  800  may be executed while a balancing mode is active or while a balancing mode is inactive. 
     In this manner, a battery system may include a first battery and a second battery electrically connected in parallel, the second battery comprising a plurality of battery cells and a heater thermally coupled to the plurality of battery cells, and a controller on board the second battery, including executable instructions to, in response to a charge voltage being greater than a threshold voltage, diverting a portion of the charge voltage in excess of a threshold voltage from the second battery to the heater. In a first example of the battery system, the executable instructions may include determining the threshold voltage based on a temperature of the second battery. A second example of the battery system optionally includes the first example and further includes, wherein the executable instructions may include determining the charge voltage based on a temperature of the first battery. A third example of the battery system optionally includes one or more of the first and second examples and further includes, wherein the executable instructions may include raising the threshold voltage in response to an increase in the temperature of the second battery. A fourth example of the battery system optionally includes one or more of the first through third examples and further includes, wherein the executable instructions may include lowering the charge voltage in response to an increase in the temperature of the first battery. A fifth example of the battery system optionally includes one or more of the first through fourth examples and further includes, wherein the heater may be positioned external to the plurality of battery cells and apart from an electrolyte of the second battery. A sixth example of the battery system optionally includes one or more of the first through fifth examples and further includes, wherein the first battery comprises a lead acid battery and the second battery comprises a battery other than a lead acid battery. A seventh example of the battery system optionally includes one or more of the first through sixth examples and further includes, wherein the second battery comprises a lithium iron phosphate battery. 
     In this way, the technical effect of reducing degradation of the second battery due to high charge voltages can be achieved by diverting voltage from the second battery to a heater thermally coupled to one or more battery cells of the second battery when the applied charge voltage is greater than a threshold voltage, especially at colder temperatures. Furthermore, diverting voltage to the heater can aid in increasing the temperature of the second battery, further increasing performance of the second battery. Further still, reducing degradation of the second battery, including at colder temperatures, facilitates utilizing lower-cost higher-density lithium battery chemistries, such as lithium iron phosphate (LFP), the dual battery system. Further still, the methods and systems described herein may be executed independently of battery capacity and independently from battery charge balancing. Further still, the methods and systems described herein may be applied to heterogeneous dual battery systems comprising batteries of different chemistries, especially batteries having mismatched charging voltage temperature profiles, such as when a charging profile of a first battery monotonically decreases with temperature and while a charging profile for a second battery monotonically increases with temperature. Further still, the systems and methods may be applied to existing dual battery systems relatively inexpensively by retrofitting the second battery with one or more external heaters positioned adjacent and external to the battery cells of the second battery. 
     The subject matter of the present disclosure includes all novel and nonobvious combinations and subcombinations of the various systems and configurations, and other features, functions, and/or properties disclosed herein. 
     The following claims particularly point out certain combinations and subcombinations regarded as novel and nonobvious. These claims may refer to “an” element or “a first” element or the equivalent thereof. Such claims should be understood to include incorporation of one or more such elements, neither requiring nor excluding two or more such elements. Other combinations and subcombinations of the disclosed features, functions, elements, and/or properties may be claimed through amendment of the present claims or through presentation of new claims in this or a related application. Such claims, whether broader, narrower, equal, or different in scope to the original claims, also are regarded as included within the subject matter of the present disclosure.