Patent Publication Number: US-2022224125-A1

Title: Battery management system for parallel charging of battery modules

Description:
FIELD OF THE INVENTION 
     The invention relates to battery charging systems, particularly to battery charging systems for automatic charging of multiple battery modules arranged in a battery system. 
     BACKGROUND OF THE INVENTION 
     Battery powered machines like floor-cleaning machines may comprise a battery system which can include one or more battery modules. In some situations, where two or more battery modules are included in the battery system, the battery modules may have different performance characteristics. The different performance characteristics may be due to different cell capacity, different charge cut-off, different impedance, different cell technology, different age and other. 
     Efficient charging of such battery modules having different performance characteristics may be a challenge. Particularly, fast, reliable and safe charging of such battery modules may not be achievable with existing battery management systems. 
     EP 2 575 235B1 discloses a system for controlling the charging and discharging of one or more battery packs or battery modules connected to a power source or an apparatus driven by the battery packs. Each battery pack comprises a number of battery cells connected to two or more terminals for establishing an electrical connection with the power source or the apparatus. The electronic system for controlling the charging of the battery pack and the electronic system for controlling the operation of the apparatus are integrated into the battery pack (8). The battery pack comprises a communications interface for communicating with other battery packs and generates a charging and discharging pool, where the most effective battery pack to charge or discharge is charged or discharged first. 
     Thus, EP 2 575 235B1 discloses a system where the battery modules are charged or discharged one by one. 
     SUMMARY 
     It is an object of the invention to improve battery charging systems to alleviate one or more of the above-mentioned problems, and therefore to provide a battery management system capable of providing fast but still reliable and safe charging of a battery system which may consist of battery modules with different battery characteristics. 
     In a first aspect of the invention there is provided a battery module comprising
     at least one battery cell,   a battery module terminal arranged to detachably connect with a connection arrangement, where the connection arrangement is arranged to electrically connect a plurality of the battery modules in parallel to enable parallel charging/discharging via individual switches,   a master controller arranged to determine a current control signal for controlling and adjusting a charging current from a current source,   a slave control unit configured to monitor a battery condition of the battery module, where the slave control unit and/or the master controller is arranged to determine a battery event based on the battery condition, where the master controller is configured to   determine the current control signal dependent on the battery event so as to cause a reduction or increase of the charging current, and to determine the current control signal dependent on battery module capacities of the one or more battery modules being connected to the current source via the individual switches.   

     When two or more battery modules are connected in parallel via the battery system connector, each slave control unit of the battery modules are capable of generating battery conditions which could generate a battery event. Since the current control signal is determined based the battery event from any battery module, the reduction of the current is adapted dependent on any of the parallel charged or discharged battery module. Advantageously, the adjustment of the charging current supplied to the parallel connected battery modules dependent on battery events from any of the battery modules will optimize charge performance of each module. 
     Advantageously, since the battery modules are charged in parallel, with a suitable charging power, it may be possible to improve the charging speed compared with a system where battery modules are charged sequentially one by one, due to the limited maximal charging current of a single battery module. The improved charging speed may be achieved while reliability and safety are maintained since the charging current is adjustable and dependent on any battery event. 
     While the reduction of the charging current is dependent on the battery event, increases may be independent on the battery event but dependent on other conditions such as dependent on a timer signal or dependent on an allowed time condition. Alternatively, the master controller may be configured to determine the current control signal dependent on the battery event so as to cause an increase of the charging current. For example, changes in the battery module temperature could generate a temperature-based battery event which could allow an increase of the charging current. 
     Increase or reduction of the charging current may comprise corresponding changes in the charging current dependent on predetermined changes or changes which are determined according to predetermined rules. The adjustments may be performed according to predetermined times where current adjustments are allowed. 
     The battery condition may comprise a battery module temperature, a cell voltage of the at least one battery cell, a battery module voltage measured over the at least one battery cell, a battery module charging current flowing into one of the battery modules and/or a comparison result of the battery module charging current or the charging current, or derivatives thereof, with a current threshold. For example, a derivative of the module charging current or the charging current in the form of a time average may be compared with a current threshold for accessing a fully charged condition of the battery module. 
     The term “battery event” is meant to denote a measurement of a battery condition or battery parameter outside a specified range. Thus, a battery event is an indication or warning that at least one battery condition or battery parameter exceeds or lies below a predetermined range. Battery events may relate to any appropriate battery parameter, such as e.g. voltage, current, temperature, or state of charging of battery modules. 
     Thus, as an example, a battery event may be determined in response to one or more of:
     determining a maximum cell voltage event when the cell voltage has reached a maximum voltage,   determining a fully charged battery module event indicating that the battery module is fully charged, and   determining a maximum battery module charging current event when the battery module charging current exceeds a maximum current.   

     As another example, a battery event may be determined in response of determining that the battery module voltage is below a given voltage limit, is within a given voltage range or is the lowest battery module voltage among other battery modules voltage. This battery event may be used during an initial charging process where battery modules may be charged individually or in groups dependent on the battery module voltages in order to equalize battery module voltages among the connected battery modules. For example, the battery modules with the lowest module voltage is connected to the current source first. The other modules, i.e. the battery modules which are not connected to begin with, are connected in parallel with the first-connected modules automatically when the modules voltages of the initially connected modules reach the voltage level of modules with higher module voltages. 
     Any appropriate battery condition relevant to the battery module and/or the charging thereof may be specified to lie within a predetermined range, and a battery event may be provided in the case where the battery condition lies outside the predetermined range. 
     It should be noted, that the battery module comprises both a slave control unit as well as a master controller. However, the battery module may be part of a battery system with more battery modules and/or in a battery system with a separate master controller. In the case, where the battery system comprises more than one master controller, one of the master controllers is appointed as the active master controller, whilst the other(s) is/are passive. 
     According to an embodiment a magnitude of the reduction or the increase of the charging current is determined dependent on the battery module capacities of said one or more battery modules. Advantageously, the magnitude of the charging current is adapted dependent on the remaining capacity of the parallel connected battery modules so that the charging current matches the allowed total charging current of the still not fully charged battery modules. 
     According to an embodiment, the master controller is configured to determine the current control signal so as to cause and possibly continue the increase of the charging current only in the absence of the battery event. Advantageously, the battery events, which require a reduction of the charging current may be prioritized over current increases. This may prevent too high charging currents. Thus, according to this embodiment, the system may be configured so that only current decreases are determined dependent on battery events, while current increases may be dependent on other conditions. 
     According to an embodiment, the master controller is configured to determine the current control signal dependent on a timer signal so that changes of the current control signal is only possible at times given by the timer signal. Advantageously, both increases and decreases in the charging current, are only possible at allowed times or allowed periods of time, so that decreases in the charging current can prioritized over charging current increases 
     According to an embodiment, the slave controller is configured to determine a fully charged condition of one of the battery modules dependent on a comparison of the charging current with a current threshold or to determine the fully charged condition when all cell voltages of the battery cells in the battery module has reached a maximum voltage. 
     According to an embodiment the battery module comprises one of the switches. Advantageously, the switches are comprised by the battery modules, i.e. so that each battery module houses a switch. In case the switches were arranged externally to the battery modules, the switches would have to be dimensioned according to a worst-case scenario of the possible different types (e.g. with different load characteristics) of battery modules that are allowed to be connected, e.g. so that the switches are dimensioned to a maximum charge and discharging current of the battery modules which are allowed to be connected to the connection arrangement. 
     In case of internal switches, the internal switch in each battery module need only be dimensioned to fit the maximum charge and discharge current of the module. 
     According to an embodiment, the switch is controllable to connect or disconnect the battery module from the current source or a load. Advantageously, the switch may be controllable via control signals from the master controller and/or the slave control unit. 
     A battery module according to any of the preceding claims, wherein upon detection of a battery event, the reduction of the charging current is: a first predetermined percentage of the total battery cell capacity of the battery module, or a second predetermined percentage of the maximum charging current of the current source; whichever reduction is the smallest. Thus, when a battery is detected, the charging current is decreased by the smaller of the first predetermined percentage of the total battery cell capacity of the battery module and the second predetermined percentage of the maximum charging current of the current source. 
     In an embodiment, upon no detection of a battery event for a predetermined amount of time, the increase of the charging current is: a third predetermined percentage of the total battery cell capacity of the battery module, or a fourth predetermined percentage of the maximum charging current of the current source; whichever increase is the smallest. Thus, when no battery event is detected for a predetermined amount of time, the charging current is increased by the smaller of the third predetermined percentage of the total battery cell capacity of the battery module and the fourth predetermined percentage of the maximum charging current of the current source. The charging current is thus increased by a third predetermined percentage, e.g. 1%, of the accumulated nominal capacity of all active battery modules or by a fourth predetermined percentage, e.g. 1%, of the charger&#39;s maximum charging current, whatever is smallest. The charge current is typically increased at predefined time intervals, such as every 100 mS, unless an event has occurred. 
     It should be noted that the nominal value of the first, second, third and fourth predetermined percentage may be similar. Thus, for example the first and second predetermined percentages equal e.g. 8%, 7%, 6% or 5%, whilst for example, the third and fourth predetermined percentages equal 1% or 2%. Typically, the third and/or fourth predetermined percentages are smaller than the first and/or second predetermined percentage, since it is advantageous that the reduction of charging current is relatively large, and the increase of the charging current is relatively smaller. The increase and reduction of the charging current upon detection of a battery event and the lack of a battery event, respectively, depends upon both the battery cell capacity within the battery module and the capacity of the current source, since this limits the variations in charging current. 
     A second aspect of the invention relates to a battery system comprising
     a master controller arranged to determine a current control signal for controlling and adjusting a charging current from a current source ( 102 ),   one or more battery modules ( 103 ) comprising battery module terminals ( 122 ),   a connection arrangement ( 121 ) arranged to electrically connect the one or more battery modules ( 103 ) in parallel via the battery module terminals ( 122 ) to enable parallel charging/discharging via individual switches ( 104 ), where each battery module comprises
       at least one battery cell,   a slave control unit configured to monitor a battery condition of the battery module and to determine a battery event based on the battery condition, where the master controller is configured to   
       determine the current control signal dependent on the battery event from any of the battery modules so as to cause a reduction or increase of the charging current, and to determine the current control signal dependent on battery module capacities of the one or more battery modules being connected to the current source or via the individual switches.   

     It should be noted, that the battery system may comprise only one battery module. In this case, the single battery module comprises an active slave control unit as well as an active master controller. 
     Some of the advantages of the battery system are already described in relation to the description of battery modules. In general, it is advantageous that the battery system is adaptive, dynamic and self configurating. The battery system can function with one or a plurality of battery modules, and the number of battery modules need not be known to the battery system prior to charging/discharging. Moreover, the number of battery modules may change during the charging/discharging; for instance, if the battery module containing the active master controller stops functioning or if an extra battery module is added, the battery system will decide on which battery module to subsequently operate as active mater controller. Moreover, newer and older battery modules may be mixed and charged at the same time in the battery system. Thus, the battery system of the invention is a flexible and dynamically adaptive battery system. When two or more battery modules are connected in parallel via the battery system connector, each slave control unit of the battery modules are capable of generating battery conditions which could generate a battery event. Since the current control signal is determined based the battery event from any battery module, the reduction of the current is adapted dependent on any of the parallel charged or discharged battery module. Advantageously, the adjustment of the charging current supplied to the parallel connected battery modules dependent on battery events from any of the battery modules will optimize charge performance of each module. 
     Advantageously, since the battery modules are charged in parallel, with a suitable charging power, it may be possible to improve the charging speed compared with a system where battery modules are charged sequentially one by one, due to the limited maximal charging current of a single battery module. The improved charging speed may be achieved while reliability and safety are maintained since the charging current is adjustable and dependent on any battery event. 
     According to an embodiment, each of the battery modules comprises a digital processor which is configurable to operate as the master controller. 
     Advantageously, the processor used for operating the slave control units may also operate the master controller. 
     According to an embodiment, the configuration to operate as the master controller is determined dependent on individual data stored by each of the battery modules. 
     According to an embodiment, the battery system comprises a register which stores identification data obtained from each of the battery modules and wherein the master controller is configured to store charging data in the register indicating a fully charged and/or discharge condition of the battery modules. 
     According to an embodiment, the battery system comprises a communication function, such as a CAN bus, arranged to communicate information, such as the battery event, battery identification or status, from the slave control unit to the master controller and to communicate the current control signal to the current source. 
     According to an embodiment, the master controller is configured to request battery modules individually to connect to the current source dependent on battery module voltages obtained from the one or more battery modules, where the battery module voltage is a voltage over the series connected battery cells. Advantageously, by selectively charging one or more battery modules dependent on their battery module voltages, the battery module voltages of all battery modules can be equalized before all battery modules are electrically connected in parallel. For example, during an initial charging process where battery modules may be charged individually or in groups dependent on the battery module voltages in order to equalize battery module voltages among the connected battery modules. For example, the battery modules with the lowest module voltage is connected to the current source first. The other modules, i.e. the battery modules which are not connected to begin with, are connected in parallel with the first-connected modules automatically when the modules voltages of the initially connected modules reach the voltage level of modules with higher module voltages. 
     According to an embodiment of the battery system, upon detection of a battery event, the reduction of the charging current is: a fifth predetermined percentage of the total battery cell capacity of the battery modules of the battery system, or a sixth predetermined percentage of the maximum charging current of the current source; whichever reduction is the smallest. 
     According to an embodiment of the battery system, upon no detection of a battery event for a predetermined amount of time, the charging current is increased: a seventh predetermined percentage of the total battery cell capacity of the battery modules of the battery system, and an eighth predetermined percentage of the maximum charging current of the current source; whichever increase is the smallest. 
     It should be noted, that the term “total actual battery capacity” is meant to denote the battery capacity of the active battery modules  103  of the battery system  100 . An “active” battery module  103  is a battery module  103  having a closed switch  104 . A battery module having battery cells in the process of being charged is thus an active battery module. 
     A third aspect of the invention relates to a battery powered apparatus, such as a floor cleaning machine, comprising the battery system of the second aspect and a load, such as an electrical motor drive, where the apparatus including the load is arranged to be powered by the battery system. 
     A fourth aspect of the invention relates to a battery-charger system comprising the battery system of the second aspect and the current source. 
     A further aspect of the invention relates to a method for charging a battery system, where the battery system comprises
     a master controller arranged to determine a current control signal for controlling and adjusting a charging current from a current source,   one or more battery modules comprising battery module terminals,   a connection arrangement arranged to electrically connect the one or more battery modules in parallel via the battery module terminals to enable parallel charging/discharging via individual switches, where each battery module comprises
       at least one battery cell,   a slave control unit configured to monitor a battery condition of the battery module and to determine a battery event based on the battery condition, where the method comprises:   
       determining the current control signal dependent on the battery event from any of the battery modules so as to cause a reduction or increase of the charging current, and   determining the current control signal dependent on battery module capacities of the one or more battery modules being connected to the current source or via the individual switches.   

     In general, the various aspects and embodiments of the invention may be combined and coupled in any way possible within the scope of the invention. These and other aspects, features and/or advantages of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the invention will be described, by way of example only, with reference to the drawings, in which 
         FIG. 1A  shows a battery-charger system comprising a battery system with two battery modules, 
         FIG. 1B  shows a battery-charger system comprising a battery system with only one battery module, 
         FIG. 2A  shows the maximal charging current of a battery module as a function of temperature, 
         FIG. 2B  shows voltage and current as a function of time in a charging process, 
         FIG. 3  shows an example of an event-controlled charging process with two battery modules, 
         FIG. 4  provides an overview of some battery events, and 
         FIG. 5  is a flow diagram over a method to determine the master controller. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
       FIG. 1A and 1B  show a battery-charger system  180  comprising a battery system  100  and a current source  102  arranged to supply a charging current to one or more battery modules  103  of the battery system  100 . The current source  102  is controllable to adjust the charging current dependent on the current control signal  151  from a master controller  101 . The current source  102  is connectable to the battery system  100  via terminals  193 . However, it should be noted that the battery system  100  may be arranged to be connected substantially continuously to the current source  102 , e.g. when the current source  102  is part of an onboard charger. In such cases, the terminals  193  may be expendable. 
     Even though  FIG. 1A and 1B  also show a load  190 , it should be noted that the load  190  connected via load terminals  192  to the battery-charger-system  180  or to the battery system  100  is not part of the battery-charger system  180  itself. Instead, the battery charger system  180  and the load  190  with an optional switch  191  forms a battery powered apparatus, where the apparatus is arranged to be powered by the battery system  100 . 
     Each battery module  103  comprises one or more battery cells  105  which are arranged in series. The connected battery cells  105  of a battery module constitutes a core-pack  107 . 
     A function of the master controller  101 , which is comprised by the battery system  100 , is to determine the current control signal  151 , which could be implemented in the communication bus  181 , for controlling and adjusting a charging current from the current source. The current control signal  151  may be a digital or analogue control signal. For example, the current control signal  151  may be in format which is compatible with a communication bus format such as a CAN bus format. The current control signal may contain information, e.g. a digital or analogue value, which directly specifies the desired charging current, or the current control signal may indirectly specify the desired charging current, e.g. by specifying a change in the charging current or by including information which is translated by the current source, e.g. via a predetermined look-up table, into the desired charging current. 
     The current source  102  may be an electronically controllable power supply which can deliver a DC current according to the current control signal  151 . The voltage amplitude at the output of the current source  102  may be controlled to a desired voltage level, e.g. a constant or substantially constant voltage. The current may be controllable e.g. in a range from zero or substantially zero to 735 A, such as up to 1000 A, for a system with up to 25 battery modules. 
     The battery system  100  comprises a connection arrangement  121 , principally illustrated in  FIGS. 1A and 1B , arranged to establish electrical connection with battery module terminals  122  of the battery modules so that the input/output current terminals of the battery modules  103  are parallel connected with the current supply terminals of the current source  102 . 
     In addition to the electrical connectors, the connection arrangement  121  may comprise mechanical structures such as guides to ensure that battery modules are not connected with reverse polarity. Other mechanical connections of the battery modules are possible such as bolted connections. 
     The parallel connection between the battery modules may be established via a power bus  125  which connects all connection arrangements  121  in parallel with the current source  102  and with the load  190  or the load terminals  192  of the battery system  100 . 
     In an example the battery module terminals  122  may be connection terminals such as threaded terminals which are detachably connectable with corresponding connection terminals of the connection arrangement  121 . The connection arrangement  121  may comprises connection wires which establish the electrical connection from the current source  102  to the first battery module, from the first battery module to the second battery module, etc. For example, the connection arrangement  121  may comprise a plurality of connection wires, where each of them connects one terminal for the first battery module to a terminal of the second battery module. Other connection wires connect from the output terminals of the current source to the terminals of the first battery module. In this example, the power bus  125  comprises the connection wires arranged between the battery modules and the current source. 
     In another example, the battery modules  103  are arranged to be detachably connected with the connection arrangement  121  via the battery module terminals  122 . For example, the connection arrangement  121  may comprise an electrical rail system to which the battery module terminals are connectable. 
     Individually controllable switches  104  are provided in the electrical connection between the current source  102  and the battery cells  105  in order to disconnect/connect the battery cells  105  from/to the current source  102  or the load unit  190 . 
     The load  190  may be any electrical consumer of a battery powered apparatus such as floor cleaning device. The load  190  may be connected/disconnected from the battery system  100  via an optional switch  191 . For example, the load  190  may comprise electric motors, pumps, etc. of the battery powered apparatus. 
     The battery system  100  may comprise a communication bus  181  configured according to standards such CAN, I2C, SPI, RS232 or other. The communication bus connects the current source  102  and the battery modules to enable transmission of control signals, such as the current control signal  151 , and other signals such as battery event signals. 
     The communication bus  181  may further comprise a battery mode control function  182  arranged to activate the battery modules from a powered down mode where switches  104  are open to a powered mode where switches  104  are closed. The battery mode control function  182  or other control function of the communication bus  181 , may further be arranged to control the optional switch  191  to open when a charging process is initiated, and to close when the load  190  of the battery powered apparatus is to be powered by the battery modules. As shown in  FIGS. 1A and 1B  by dashed lines, the communication bus  181  may be arranged to be in communication with the load  190 . 
     The individually controllable switches  104  may be comprised by the battery modules  103  so that each battery module comprises a controllable switch  104 . Alternatively, the switches  104  may be externally located switches, i.e. arranged in series with the electrical connection between each battery module  103  and the current source  102  to individually connect/disconnect the battery modules to/from the current source  102 . 
     Due to the parallel connected battery modules  103 , the battery modules which are connected via switches  104  can be charged or discharged in parallel. 
     Each of the battery modules comprises at least one battery cell such as Nickel-cadmium battery cells, Lithium-ion battery cells, nickel metal hydride battery cells. The battery cells may be series connected to establish a sufficiently high voltage. 
     The battery modules may be configured with active or passive balancing such as a balancing circuit (not shown), which can be switched in, in parallel with each battery cell, when the battery cell reaches a fully charged level such as a predetermined voltage level or charge status. The purpose of the balancing circuit is to keep the individual battery cells in balance. 
     Each of the battery module comprises a slave control unit  106  configured to monitor a battery condition of the battery module. Examples of battery conditions includes cell voltages of individual battery cells  105 , battery module voltages across all battery cells of a battery module, module charging currents flowing into individual battery modules  103 , battery module capacities and temperatures of the modules under charge and discharge. 
     Both the slave control unit  106  and the master controller  101  may be configured to control the switches  104 . 
     The slave control unit  106  is further arranged to determine battery events based on the battery condition. 
     Battery events comprises voltage events of the battery cells which are generated by the slave control unit  106  when individual cell voltages reach a voltage threshold, Vmax, which is reached when the cell is considered fully charged. When all battery cells  105  of a battery module  103  have reached Vmax, the battery module is considered fully charged, and a “Fully Charged” battery event Mchar is generated. The fully charged condition may further be conditioned in that the charge current to the module is below a predetermined level. A battery module may be considered fully charged in other situations, as described in connection with  FIG. 3 , where a “Fully Charged” battery event is similarly generated. Thus, such battery events may be used to signal that a battery cell  105  and/or a battery module  103  is fully charged. 
     Other battery events may be generated dependent on battery module temperatures. As shown in  FIG. 2A , the maximal charging current  201  of a battery module  103  depends on the battery module temperature  202 . For example, as illustrated, maximal charging currents I 1 , I 2  and I 3  apply for temperatures in the temperature intervals T 1 -T 2 , T 2 -T 3  and T 3 -T 4 , respectively. Accordingly, a battery event may be generated when the temperature is within the ranges T 1 -T 2  and T 3 -T 4 , in order to set a maximal charging current according to the temperature, or to disconnect the battery module from the power bus  125  if the temperature is outside the allowed temperature range, e.g. if the temperature is above T 4  or below T 1 . 
     The battery events may be determined by the slave control unit, although some battery events may be determined by the master controller based on information from one or more of the slave control units. 
     If a slave control unit  106  detects that a temperature of its battery cells  105  are above a maximum temperature, such as 50 degrees, the slave control unit may disconnect the battery module from the power bus  125  to avoid damages. The slave control unit may further send a “high-temperature” message to the master controller  101  which send switch control signals to other battery modules so as to disconnect these battery modules from the power bus. 
     Other battery events may be generated when the module charging current exceeds a maximum current, e.g. if the module charging current exceeds the maximal charging current  201  as specified for a given temperature range of the battery module temperature  202 . 
     In general, a battery event may be generated by the slave control unit from any of the battery modules in order to generate a current control signal which controls the current source  102  to decrease the charging current. In other situations, a battery event may be generated by the master controller  101  based on information from the slave control units  106 . 
     The slave control unit  106  may be implemented as software code designed to carry out the functions of the slave control unit and to be executed by a digital processor comprised each of the battery modules  103 . 
     In general, a single master controller  101  is needed by the battery system  100 . Each of the battery modules  103  may be configured to establish the master controller. For example, the master controller  101  may be implemented as software code designed to carry out the functions of the master controller  101  unit and to be executed by a digital processor comprised each of the battery modules  103 , such as the digital processor which runs the slave control unit  106 . 
     Alternatively, the battery system  100  such as a housing of the battery system  100  may comprise a digital processor or other electronic circuit configured to embody the master controller  101 , e.g. via a digital processor arranged to run software code designed to carry out the functions of the master controller  101 . 
     In case the master control unit  101  is comprised by one of the battery modules  103 , the configuration of the battery module  101  to operate as the master controller may be determined dependent on individual data stored by each of the battery modules  103 . Such individual data may include a date, such as the production date, of the battery module  103 , fault conditions stored by the module, a serial number of the battery module, the actual charging capacity, number of charge/discharge cycles and other charging data of the battery module. In this way, a single battery module can always be pointed out to be responsible to carry out the master controller function. 
     The battery system  100  may further comprise a register  170 ,  170 ′, e.g. a digital memory, which stores identification data obtained from each of the battery modules. For example, the master controller  101  may be configured to store charging data in the register  170 ,  170 ′ indicating which of the battery modules has reached the fully charged battery module condition/event Mchar, a fully discharged battery module condition Mdis and other conditions such as over- and under-temperature conditions and defect conditions. A common register  170 ′ may be comprised by the battery system which is accessible for reading and writing by the master controller  103 . Alternatively, one or any of the battery modules may have the register  170  implemented in a memory of the battery module. Advantageously, if a battery module  103  is configured to implement the master controller  101 , that battery module may additionally implement the register  170 . 
     The battery system  100  may be configured with a communication function arranged to communicate information from the slave control units  106  to the master controller  101 , such as battery event information, from the master controller  101  to the slave control units  106 , such as switch control signals to operate the switches  104 , and from the master controller to the current source  102 , such as the current control signal  151 . 
       FIG. 2B  shows a charging process where curve  211  is the voltage across the power bus  125 , i.e. substantially the voltage across the series connected battery cells, and where curve  212  is the current supplied by the current source  102 . The time from t 0  to t 1  is an initial charging period where the current  212  is constant or substantially constant and where the voltage increases from an initial voltage at t 0  to a maximum voltage at t 1 . The time from t 1  to t 2  is the final charging process which is described in detail in connection with  FIG. 3 . 
     The initial charging period may start with determining which of the battery modules  103  should be configured to operate as the master controller  101 , in case two or more of the battery modules are configurable to operate as master controller. 
     The master controller may update the register  170 ,  170 ′ with data from the battery modules, such as serial number or other identification data of the battery modules, the nominal capacities, defect condition data indicating if a module is defect, over- and under-temperature conditions in case any of the battery modules  101 —or any of the modules which are not fully charged or defect—satisfies such over- and under-temperature conditions (cf.  FIG. 2A ), and charging data indicating if a battery module is fully charge or fully discharged. 
     The master controller may determine that all battery modules are disconnected from the power bus  125 , if any of the battery modules has an over- and under-temperature condition. 
     The master controller may be set to a wait state, waiting for a “ready message” from the slave control unit  106  of the battery module affected by the over- and under-temperature condition, so that charging can be continued when the temperature returns to the allowed temperature range. 
     The master controller may be configured to obtain battery module voltages from each of the battery modules or any of the modules which are not fully charged or defect. The battery module voltage is the voltage measured over all battery cells of a battery module, i.e. over the core-pack  107 . In order to equalize battery module voltages among the battery modules, the master controller may be configured to request that the one or more battery modules having the lowest battery module voltages or having battery module voltages below a certain minimum voltage limit, to connect to the power bus  125  via the switches  104 . The connection request may be a in the form of a connection request signal which may include identification data of the battery modules which should connect, where the connection request signal may be transmitted via the communication bus  181 . 
     The connection request may directly control the switches to connect/disconnect, or the slave control unit  106  of the relevant battery modules may control the switches dependent on the connection request. 
     Accordingly, the master controller may be configured to request battery modules individually to connect to the power bus  125  dependent on battery module voltages obtained from one or more the battery modules. 
       FIG. 3  shows an example of an event-controlled charging process and how the current control signal is determined dependent on the battery event so as to cause a reduction or increase of the charging current. The abscissa axis shows the charging time and the ordinate axis shows the charging current in amperes. 
     The  FIG. 3  example is based on charging two battery modules  103  with a 1200 Watt current source  102  after the initial charging process. The current source  102  has a maximum output current, here a maximum of 36 Ampere, and is indicated by line  402 . Each of the battery modules has a nominal capacity of 44800 mAh. 
     The first battery module  103  is named M 1  and the ten battery cells of battery module M 1  are named M 1 C 1 -M 1 C 10 . The second battery module  103  is named M 2  and the ten battery cells of battery module M 2  are named M 2 C 1 -M 2 C 10 . 
     Line  403  indicates the maximal charging current  201  of each of the battery modules M 1 , M 2  for a normal temperature range, e.g. in the range from 10 to 40 degrees Celsius. 
     Curve  401  is the charging current supplied by the current source  102 . Since the charging current is generated in response to the current control signal, the current control signal could be represented by curve  401 , particularly when the current control signal is proportional with the desired charging current. 
     The master controller is configured to determine the current control signal dependent on a timer signal so that changes of the current control signal is only possible at times given by the timer signal. 
     Curve  404  is a fully charged current level which defines when a given battery module is considered fully charged. That is, when the charging current for a given battery module  103  decreases below the fully charged current level  404 , when the charging current has been below the fully charged current level  404  for a period of time or when a time-average of the charging current obtained over a period of time is below the fully charged current level  404 , the battery module can be considered fully charged. The fully charged current level  404  may be determined as a fraction of the battery module capacity, such as 1/20 of the battery capacity, e.g. the battery capacity specified by the manufacture&#39;s datasheet. 
     In this example, the master controller  101  only determines the current control signal or changes in the current control signal at specific times, here every 100 ms. Therefore, changes in the charging current  401  is only allowed after the lapse of a certain time interval such as the 100 ms time interval. The specific times or allowed times where a change of the current control signal or charging current is allowed, may include a certain tolerance time interval wherein the current control signal or the charging current is allowed to be determined, e.g. in response to a battery event. These allowed times or allowed time intervals are indicated with reference  410 . 
     Accordingly, any battery event from any of the battery modules generated between the specific times, i.e. within the time intervals such as the 100 ms time intervals, may be disregarded. 
     That is, only battery events, such as only one battery event, from any of the battery modules is accepted when the event occurs at the specific times, i.e. allowed times, or within the tolerance time interval of the allowed times. 
     The master controller may be configured to only read the battery event when a timer signal signals an allowed time. The battery event could be transmitted as a battery event signal transmitted via the communication bus  181  such as the aforementioned CAN bus and prioritized over other signals on the bus to avoid delays. Accordingly, the master controller  101  is configured to determine the current control signal dependent on the timer signal so that changes of the current control signal is only possible at times given by the timer signal. 
     The charging current  401  at zero point of charging time, i.e. the point at the crossing between the coordinate axes, is the charging current as obtained after the initial charging process in  FIG. 2B , i.e. after t 1  in  FIG. 2B  when charge process is changed from constant current to constant voltage  211 . Thus, the curve  401  represents a constant voltage phase. 
     A first battery event happens because the 3rd battery cell  105 , M 1 C 3  of module M 1  reach the voltage threshold Vmax because M 1 C 3  has become fully charged. 
     The slave control unit  106  of battery module M 1  sets the balancing resistor on the 3rd battery cell and sends the Vmax battery event, e.g. via a communication bus. 
     On basis of the Vmax battery event, which is received by the master controller  101 , the master controller determines the current control signal to cause a reduction the charging current due to the reduction of the required charging current. The current control signal causes a reduction of the charging current  401 . 
     The magnitude of the reduction or increase of the charging current  401  may be determined based on predetermined rules. For example, the reduction or increase of the charging current  401  may be determined in dependence of battery module capacities of the battery modules which is currently being charged, i.e. capacities of battery modules which are not being charged, e.g. since they have reached a fully charged state, are disregarded. 
     In the example in  FIG. 3 , the reduction and increase is determined as predetermined percentages of the actual battery capacities, here the reduction of the charging current is given as 5% of the total actual battery capacity and the increase of charging current is 1% of the total actual battery capacity. The battery module capacities may be the nominal battery module capacities or other measure of the battery module capacity. However, magnitude of the reduction or increase may be limited by to a percentage of the maximum charging current of the current source  102 . 
     Thus, upon detection of a battery event, the reduction of the charging current may be the smallest of: a fifth predetermined percentage of the total battery cell capacity of the battery modules of the battery system, and a sixth predetermined percentage of the maximum charging current of the current source ( 102 ). Similarly, upon no detection of a battery event for a predetermined amount of time, such as e.g. 100 ms, the charging current is increased by the smallest of: a seventh predetermined percentage of the total battery cell capacity of the battery modules of the battery system, and an eighth predetermined percentage of the maximum charging current of the current source ( 102 ). As an example only, upon detection of a battery event, the reduction in the charging current is 5% of the total actual battery capacity, but not higher than 5% of the maximum charging capacity of the current source. As an example only, upon no detection of a battery event for e.g. 100 ms, an increase in the charging current is 1% of the battery capacity, but not higher than 1% of the maximum charging capacity of the current source. 
     The determined current control signal is read by the current source  102  which reduces the charging current  401  according to the current control signal. The current control signal may specify the absolute current value or a relative change. In case the current source  102  is configured to determine the charging current on basis of the battery module capacities and e.g. a predetermined percentage change, the current control signal could simply indicate a desired increase or reduction of the charging current  401 . 
     Since each of the battery modules has a nominal capacity of 44800 mAh, the charging current is reduced by 4.48 A, corresponding to 5% of the total capacity of 2×44800 mAh. 
     In the  FIG. 3  example, the master controller  101  generates the current control signal so as to cause an increase of the charging current automatically every 100 ms, in general after a certain time interval has lapsed, e.g. dependent on the timer signal. Thus, the master controller  101  automatically sends an “increase” current instruction to the current source periodically at specific times, such as every 100 ms. 
     The charge current is increased by e.g. 1% of the nominal capacity of the connected modules M 1  and M 2 , equal to 0.896 Ampere, corresponding to 1% of the total capacity of 2×44800 mAh. 
     Since a reduction in the charging current may be important in order to avoid too high charging current which could damage a battery module  103 , battery events which would cause a reduction in the charging current may be prioritized over the aforementioned automatic increases of the charging current  401 . Thus, in case the master controller  101  at the same time, e.g. at the same “allowed time”, would generate both an automatic increase of the charging current and also receives a battery event for decreasing the charging current  401 , the battery event for decreasing the charging current would be prioritized over the automatic increase of the charging current. 
     Thus, the master controller may be configured to determine the current control signal  151  so as to cause the increase of the charging current, e.g. an automatic increase of the charging current, only in the absence of any battery event for decreasing the charging current. 
     A second battery event happens when the M 1 C 7  battery cell reaches Vmax. The slave control unit  106  in module  1  sets the balancing resistor on the 7th cell and the slave control unit sends a battery event signal Vmax. 
     The different or distinguishable battery event signals may be generated for different kinds of battery events, i.e. a specific and distinguishable battery event may be generated by the slave control units  106  when a battery cell voltage reaches the fully charged cell-voltage Vmax. Alternatively, the same, i.e. a common battery event signal, may be generated for different kinds of battery events. The latter alternative is feasibly when different battery events should generate the same reduction of the charging current, e.g. the same percentage reduction dependent on the battery capacity. 
     The generation of battery events due to battery cells reaching Vmax is continued and the balancing resistors are been set on several cells in both battery modules M 1  and M 2 . 
     A new type of a battery event is generated after all battery cells of a battery module  103  have reached the voltage threshold Vmax. In  FIG. 3 , the last battery cell M 1 C 8  of battery module  1  is fully charged at the instance indicated with letter A. Since this is the last battery cell which reaches Vmax, a fully charged battery module event Mchar is generated. 
     The fully charged battery module event Mchar may be generated by the slave control unit  106  of the battery module which has become fully charged, or by the master controller  103  in response to receiving a “fully charged” message from the slave control unit  106 . 
     In addition to sending the fully charged battery module event Mchar to the master controller  101 , this event signal or a separate fully charged battery module message is sent and registered in the register  170  or  170 ′ so that the register  170 ,  170 ′ stores updated information on which of the battery modules are fully charged. 
     In response to registering the module M 1  as fully charged, the master controller  101  may send an instruction to the battery module M 1  to disconnect the power terminals from the power bus  125  via switch  104 . Furthermore, in response to the disconnect instruction, the slave control unit  106  may ensure that all balancing resistors are released and that battery module M 1  enters a standby mode. 
     Since battery module  1  is disconnected, the charging current  401  is too high for the remaining battery module M 2 . Accordingly, a reduction of the charging current  401  is needed. This may be achieved by configuring each of the slave control units  106  to monitor the battery module charging current flowing into the battery modules via the power terminals. Thus, the battery condition determined by the slave control unit of battery module M 1  may indicate a too high charging current if the battery module charging current is greater than a maximum current  201  specified for the battery module M 1 . The slave control unit of module M 1  may send a battery condition indicating the too high charging current to the master controller  101 , e.g. via the communication bus  181 , and in response the master controller generates a maximum battery module charging current event MaxI indicating that the battery module charging current exceeds the maximum current  201 . Alternatively, the slave control unit of a battery module generates the maximum battery module charging current event MaxI in response to determining that the measured charging current exceeds the maximum charging current  201 . 
     In response to the maximum battery module charging current event MaxI, the master controller  101  determines the current control signal  151 , e.g. dependent on battery module capacities, according to methods which are equivalent with methods for determining the current control signal  151  in response to the Vmax maximum cell voltage event. 
     As shown at the first arrow in  FIG. 3  with reference names Max. Current, the charging current  401  is decreased, e.g. by 5% of the nominal capacity of the remaining connected modules (here connected module M 2 ), equal to 2.24 A, corresponding to 5% of the remaining total capacity of 1×44800 mAh. 
     However, at the next allowed time  410 , the charging current  401 , or the fraction of the charging current flowing into module M 2  (or flowing into other modules in case two or more battery modules are still connected), is still above the maximum current. Therefore, as shown at the second arrow named “Max. Current”, in response to a second generated maximum battery module charging current event MaxI, the charging current  401  is reduced again. 
     The generation of maximum battery module charging current events MaxI is continued, here a total of four times, until the charging current flowing into battery module M 2  is below the maximum charging current. In  FIG. 3 , the maximum charging current  201  of module M 2  is indicated by line  403  and it is seen that after the fourth current decrease, the charging current  401  has decreased below the maximum current level  403 . 
     Now, since the slave control unit  106  of battery module M 2  determines that the charging current flowing into the battery module M 2  is below the maximum current  201 ,  403 , the charging current is automatically increased at the next allowed time  410  based on the current control signal generated by the master controller  101  since no battery events for reductions in the charging current are generated. 
     After this increase in the charging current, here a 1% increase, the maximum current  201 ,  403  of battery module  2  is exceeded again, and another current decrease, here a 5% decrease, is generated in response. 
     The multiple battery events Vmax indicated with a total of five errors named M 2 C 7  is due to an out-of-balance error where the M 2 C 7  cell reaches the voltage threshold Vmax five times, but where the balancing resistor is set for the M 2 C 7  cell the first time the voltage threshold Vmax is reached. 
     Similarly, battery cells M 2 C 4  and M 2 C 2  reaches the voltage threshold Vmax a total of 5 times each. 
     The fully charged current level  404  is determined as 1/20 of the battery capacity of battery module M 2 . The charging current decreases below the fully charged current level  404  while battery module M 2 C 2  continues causing generation of Vmax battery events, while battery cells M 2 C 1 , M 2 C 3  and M 2 C 6  have not reached the threshold voltage Vmax indicating a fully charged level of the cells. 
     However, since the average value of the charging current  401  during a given period, here 1000 ms, has been lower than the fully charged current level  404 , module M 2  is considered fully charged. 
     Accordingly, a second condition for considering a battery module fully charged is obtained dependent on a comparison of the charging current  401 , i.e. the charging current flowing into a given battery module  103 , with a current threshold corresponding to the fully charged current level  404 , such as a current threshold determined dependent on a battery module capacity of said battery module  103 . As illustrated in the specific example, the charging current compared with the fully charged current level  404  may be determined as a time-averaged charging current obtained over a given period. 
     Thus, the battery condition here comprises a situation where the average value of the charging current  401  or a time-average thereof has been lower than the current level  404  for a given period of time. When this battery condition is fulfilled, the slave control unit  106  generates a fully charged message which is received by the master controller which generates a fully charged battery event Mchar. In response, the master controller generates a current control signal causing a reduction of the charging current similar to the previously described fully charged battery event Mchar. However, if it is the last battery module of the plurality of battery modules which has become fully charged, the master controller may generate a current control signal which sets the charging current to a final low current which may be used for powering the e.g. a controller, while all battery modules are disconnected via the switches  104 . 
     The battery capacity used for determining a fully charged condition when the charging current is below a fraction of the capacity, or used for determining the increases/decreases of the charging current may be the nominal capacity, an actual capacity which may be determined as a function of e.g. charging/discharging cycles, and other measures of the battery. 
     It should be noted that  FIG. 3  shows an example of an event-controlled charging process with two battery modules, only, for reasons of simplicity. However, the concept of the event-controlled charging process is similar for battery systems with any practical number of battery modules. 
       FIG. 4  provides an overview of some battery events. Other events include over-current during charge and discharge, short-circuit during charge and discharge, high temperature during charge, low temperature during charge, over-temperature during charge and discharge, under-temperature during charge and discharge and defect battery module. 
     During discharging, when one or more of the battery modules are connected to the load  190  via the closed switch  191  and one or more of switches  104 . Thus, two or more battery modules may be discharged in parallel. The master controller  106  may update the register  170  or  170 ′ during the discharging, e.g. information on the charging status such as when a battery module is fully discharged. 
       FIG. 5  is a flow diagram over a method  500  to determine which of the battery modules should be configured to operate as the master controller, e.g. during charging of the battery system. During step  501 ,  510 ,  511 ,  512  and  514 , all the master controllers  101  of the battery modules  103  are active. The method  500  starts in step  501  and continues to step  510 , wherein it is determined whether the battery system  100  includes a single battery module  103  or more than one battery module  103 . In the case, where step  510  indicates that the battery system  100  does not include more than one battery module, the flow continues to step  511  wherein it is determined that the single battery module  103  becomes the master controller. In this case, both the master controller  101  and the slave control unit  105  of the single battery module  105  become active. Subsequently, the flow ends in step  520 . 
     If step  510  results in the determination, that the battery system  100  comprises more than one battery module  103 , the flow continues to step  512  wherein individual data from the different battery modules  103  are compared. Such individual data are e.g. a date, such as the production date, of the battery modules  103 , fault conditions stored by the battery modules  103 , a serial number of the battery modules  103 , the actual charging capacity, number of charge/discharge cycles and other charging data of the battery module  103 . For example, the decision on the battery module  103  to be master module is the battery module having the newest production date or the highest serial number. In this way, a single battery module can always be pointed out to be responsible to carry out the master controller function, in step  514 . Subsequently, only the master controller of the single battery module appointed to be the battery module also responsible to carry out the master controller function is active, whilst the master controller of the remaining battery modules, if any, are passive. However, the slave control unit  106  of the battery modules  103  with a passive master controller will remain active. The method ends in step  520 . 
     Although the present invention has been described in connection with the specified embodiments, it should not be construed as being in any way limited to the presented examples. The scope of the present invention is to be interpreted in the light of the accompanying claim set. In the context of the claims, the terms “comprising” or “comprises” do not exclude other possible elements or steps. Also, the mentioning of references such as “a” or “an” etc. should not be construed as excluding a plurality. The use of reference signs in the claims with respect to elements indicated in the figures shall also not be construed as limiting the scope of the invention. Furthermore, individual features mentioned in different claims, may possibly be advantageously combined, and the mentioning of these features in different claims does not exclude that a combination of features is not possible and advantageous.