Battery management system, method of operating a battery management system

Abstract

A battery management system (20) comprising : -a battery pack (19) including at least one stack of battery cells (13); -a master electronic control unit (15), said BMS master; -at least two wireless communication device (11), said RF masters, wherein one of the RF masters is active, said active RF master (11a), and the others are passive, said passive RF masters (11b); -a plurality of monitoring devices (12), said BMS slaves, each slave monitoring one cell. wherein an active RF master is configured to calculate and send commands to the BMS slaves, and a passive RF master is configured to only receive data from the BMS slaves, wherein RF masters and BMS slaves being configured to communicate solely using wireless (18) communication link, and BMS master and RF masters being configured to communicate using a wired (16) communication link, in order to exchange data for managing the battery pack.

Description

BATTERY MANAGEMENT SYSTEM, METHOD OF OPERATING A BATTERY MANAGEMENT SYSTEMField of the invention

[0001] The present invention relates to a battery management system and a method of operating this battery management system, more specifically a battery pack implemented for example in an electric or hybrid electric vehicle or in a system for storage energy. Such a system or method improve and optimize the operational performance (e.g. autonomy, lifetime, power, ...), production (e.g. costs, weight, complexity, scarcity of raw materials, ...) or recycling (costs, degree of recycling, complexity, ...) of batteries for various uses such as electric mobility, energy storage, to name only two applications.Background of the invention

[0002] The importance of battery-based power systems within today's markets is well understood by one of skill in the art. These battery systems are replacing traditional power systems in several different markets as products are moving towards more environmentally-friendly and cost- effective power solutions. For example, markets such as the electrical vehicle and home energy markets are experiencing rapid growth as battery powered systems are becoming more dynamic in their ability to store and deliver power to corresponding products. This movement away from traditional power sources (e.g., fossil fuels, coal, etc.) to battery-based power sources is placing higher performance demands on the management of battery cells to ensure proper operation within ever-increasing complex products.

[0003] Many battery-based power systems have a centralized management controller that communicates with multiple battery management integrated circuits. Each of these battery management integrated circuits manages a plurality of battery cells and performs various tasks. For example, a battery management integrated circuit may sense voltage and charge levelson battery cells, may manage charge by bleeding charge or re-charging cells, as well as perform other sensing operations and low-level battery management functions.

[0004] In this context, there is a need for a more robust and dynamic alternative battery management system solution.of the invention

[0005] To this end, the various aspects of the invention relate to a battery management system and a method of operating this battery management system disclosed in the set of claims.

[0006] In particular, this system includes a battery pack including at least one module comprising at least one stack of battery cells, a master electronic control unit (BMS master) and a slave electronic control unit (BMS slave) associated with each battery cell. The method comprises at least exchanging of data between the master electronic control unit (BMS master) and the slave electronic control unit (BMS slave) for managing the battery pack by using solely wireless communication technology.

[0007] Furthermore, RF masters exchange amongst each other their states about their respective roles via the wired communication link.

[0008] The proposed invention presents the following advantages:- retrieve all necessary information(s) or needs to implement all standard and advanced functions of a BMS of battery cells within a synchronization period, in particular less than 100ms, preferably less than 50ms;- having an availability rate of minimum 99.90% on the reception of commands sent by the BMS control system and on reception of information sent by BMS slaves;setting up, with robustness, some parameters for each cell, in particular be 100% sure a command sent by BMS control system is received by BMS slaves.Brief of the

[0009] The present invention will be described subsequently in more detail with reference to the attached drawing, given by way of examples, but in no way limited thereto, in which:- Figure 1 is a schematic representation of a battery management system, according to embodiments of the present invention;- Figure 2 is a schematic representation of a redundancy system for RF communication of a BMS master of the battery management system according to embodiments of the present invention.Detailed description of the invention

[0010] With reference to Figures 1 - 2, a battery management system 20 also known under the acronym “BMS” comprises a battery pack comprising several modules, including cells 13 in series. Inside a module, the cells 13 are mechanically grouped by ten, this group is called stack. The battery pack 19 can comprise one hundred cells (for example limit is one hundred and twenty by the current protocol). For example, in terms of physical / mechanical view, it is (could be) divided in modules, for instance in a car, a module of sixty cells (or six stacks) and another of forty cells (or four stacks) which are not physically located at the same place. But from the point of view of the electronic control unit, said BMS Master (logical view), it is a pack of n cells (one hundred in our car). Nevertheless, the mechanical introduces some limitations in terms of energy transfer between cells.

[0011] The battery management system 20 comprises an electronic control system 14 and an electronic wireless system 10.

[0012] The electronic control system 14, said BMS control system, implements in a non-limiting and non-exhaustive manner:- all algorithms relating to management of the battery pack 19;- the wireless communication with the battery pack 19;- the wired communication 21 with the central unit of a vehicle.

[0013] The electronic wireless system 10 implements in a non-limiting and non-exhaustive manner:- all algorithms relating to management of a battery cell 13;- the wireless communication between the BMS control system 14 and the battery pack 19.

[0014] The electronic control system 14 comprises a central unit named BMS master 15. BMS master 15 implements all algorithms relating to management and functional safety (e.g. relays state) of the battery pack 19 besides handing the sensors (e.g. battery pack current, voltage), actuators (e.g. open / closing relays) of the battery pack 19 and communication 21 to the central unit vehicle.

[0015] The electronic control system 14 comprises a RF master 11 . A RF master implements the wireless communication with the BMS slaves 12 of the battery pack 19, and as it will be described hereafter ensures the redundancy of the wireless communication 18.

[0016] The electronic wireless system 10 comprises a plurality of electronic control unit 12 also named BMS slave, wherein each BMS slave 12 is associated with one battery cell 13.

[0017] According to an architectural setup, a central unit of a vehicle communicates with the central unit 14, also called BMS control system, of a battery management system (BMS) 20. BMS control system 14 is being in communication (sending commands I receiving information) with the batteries 19 via a wireless communication link 18. Wireless communication is being facilitated by electronic control units or monitoring devices (also called BMSslaves) 12 associated with each battery cell 13. BMS control system 14 comprises an electronic control unit (also called BMS master) 15 that communicates with the central unit of a vehicle via a wired communication link 21.

[0018] According to figure 1 , BMS unit 14 is an intelligent main electronic control unit also called “master electronic control unit”. Each BMS slave 12 is associated with each battery cell 13, this BMS slave 12 being an intelligent electronic control unit, also called “slave electronic control unit”, including embedded software. The standard functionalities of a BMS slave 12 (e.g. command of the relays, measures of battery pack voltage and current) and the advanced features e.g. static cell balancing by current bypass 1 , dynamic cell balancing by energy transfer between cells 2, cell internal impedance measurement 5, dendrites reduction by cell heating 3 are handled over a wireless communication link with each cell 4.

[0019] In a preferred embodiment of this invention, this battery management system 20 is dedicated to the automotive industry. Each battery cell - basic building unit of an EV battery pack - has its own control unit, named monitoring device, named also BMS slave. It monitors health, current, voltage, temperature, and charge control. These BMS slaves communicate with and are driven by a central unit named BMS master 15 which is based on a primary, said active 11a, and a secondary, said passive 11 b, masters for redundancy.

[0020] According to figure 2, this battery management system 20 comprises:- an electronic control system, said BMS control system, 14 comprising: a master electronic control unit 15, said BMS master, and at least two wireless communication device 11 , said RF master, wherein one of the RF masters 11 is active, said active RF master 11 a, and the others are passive, said passive RF masters 11 b;a plurality of monitoring devices 12, said BMS slaves, each slave monitoring one battery cell.

[0021] This BMS master 15 has a redundant system for RF communication by comprising a primary, or active 11 a, and a secondary, or passive 11 b, masters as it is visible in figure 2. In fact, this approach implements a full redundant system, where the complete BMS unit or RF master 11 is backed up: a primary and a secondary RF master 11 , 11 a, 11 b. As said before, a BMS unit is logically composed of a BMS master 15, a RF master 11 , and N BMS slaves 12. The approach we have retained, in order to reduce the cost too, is a full redundant backup only of the wireless or RF part. There is only one BMS master 15 but inside it, there is an active 11 a and passive 11 b RF masters which are used for the communication the BMS slaves 12. The two RF masters 11 a, 11 b are communicating with the BMS master 15. In the rest of the document, the primary master 11a as to be replaced by the active RF master 11 a. The secondary master 11 b as to be replaced by the passive RF master 11 b.

[0022] The active RF master 11 a calculates and sends commands, the passive one only receives. Active RF master 11 a sends and receives from BMS 15 to slaves 12 and vice-versa. The passive RF master 11 b receives everything as the active RF master 11 a but sends nothing to BMS slaves 12, or, according to one embodiment to the BMS Master 15.

[0023] According to one embodiment, in this BMS master 15, the primary master, or active RF master 11 a, also called “primary master electronic control unit” comprises a wireless communication module which is in an active mode. The secondary master, or passive RF master 11 b, also called “secondary master electronic control unit” comprises a wireless communication module which is in a passive mode. In this context, the primary master 11 a is in an active operating mode and the secondary master 11 b is in a passive operating mode. For example, the wireless communication module of secondary master electronic control unit is able to switch in theactive mode when the primary master electronic control unit is faulty. In particular, active RF master 11 a and passive RF master 11 b can switch roles or assignment, namely active role or passive role, when the current active master 11 a is faulty. Roles can be switched by RF masters 11 , 11a, 11 b themselves or by BMS master 15.

[0024] Redundancy is the ability to replace the active 11 a role of RF master 11 by one of the passive 11 b RF masters 11 , in case of the malfunction of the active RF master 11 . This results in increasing reliability of the system.

[0025] E.g. If active RF master 11 becomes malfunctioning (e.g. no more sending commands I requests to BMS slaves 12), and BMS master 15 recognizes the problem, BMS master 15 would request one of the passive RF masters 11 to take the role of the active RF master 11 .

[0026] In case both active and passive RF masters 11 are functioning, the redundancy results in increasement the availability of the information sent back by BMS slaves 12.

[0027] E.g. While we get back information from all BMS slaves 12 by active RF master 11 , we may have missed one (information from) BMS slave 12 e.g. “address 5”. But this BMS slave 12 (address 5) could be received by passive RF master 11 (if BMS slave 12 with address 5 is working). All RF masters 11 then transmit the information to BMS master 15 and it will merge the data received from all RF masters 11 . In this manner the probability of losing information is reduced.

[0028] Such an architecture is capable of handling (sending commands I request to and receiving information from) at least one hundred cells within synchronization period (20 ms). If more than one hundred cells are required, the time for communication will increase. However, the electronic wireless system 10 can be duplicated to double (or triple and so on) the number of BMS slaves 12 (e.g. 800V respectively up to 5200V) battery pack) with thesame performance, i.e. up one thousand three hundred cells information can be communicated within the synchronization period (20 ms).

[0029] Hereafter, according to one embodiment, functionalities of each main element are described in details:

[0030] BMS master 15 :- receives and checks commands I requests from central unit of the vehicle CUV via a proprietary protocol on the CAN network (wired communication link 21 );- updates the behaviour of battery related algorithms (e.g. cells balancing via bypass or energy transfer, charging of the battery) and other algorithms (e.g. command I state of actuators, functional safety);- encodes the commands I requests and send these to all RF masters 11 ;- receives all data I replies from all RF masters 11 and all BMS slaves 12. Due to redundancy, each RF master 11 is sending back all the data I replies it receives from all the BMS slaves 12. BMS master 15 must merge all data I replies of the BMS slaves 12;- checks, and compute statistics (in that order) of data I reply from each RF master 11 and BMS slave 12 (depending on the command);- extended checks of all the data (e.g. if the measurement is within a range, consistency check between a measure or set of measures or data and command);- encodes data (e.g. as previously) for extended monitoring purposes;- send back information required by the vehicle proprietary protocol (CAN);- updates its own device firmware (DFU) and those of RF masters 11 and, over the air (OTA), BMS slaves 12, all received from the vehicle central unit or an external service tool.

[0031] RF master 11 :- receives and checks commands I requests from BMS master 15, sends back information regarding its own measures and states (e.g. power supply, CPU temperature, errors) to BMS master 15;- processes and / or forwards commands / requests to BMS slaves 12;- receives, checks, and compute statistics (in that order) of data I reply from each BMS slaves 12 (depending on the command I request);- forwards data I reply from each BMS slaves 12 to BMS master 15;- updates its own device firmware (DFU) and those, over the air (OTA) of BMS slaves 12, all received from BMS master 15.

[0032] BMS slave 12 :- receives, checks, compute statistics (in that order) commands I request from active RF master 11 ;- processes commands (e.g. cell management (we will describe these in the protocol);- collects information regarding its own state (e.g. cell voltage I current / temperature, errors (such as overcurrent, undercurrent, overvoltage, undervoltage, RF wireless communication) CPU temperature);- sends back above information to all RF masters 11 ;- update its own device firmware (DFU) received from BMS master 15 via all RF masters 11 wirelessly. Due to limitations of standard and robust wireless protocol used for the over the air (OTA) device firmware update (DFU), even active RF master 11 and passive RF masters 11 are exchanging data with BMS slaves 12. Each RF master 11 is communicating with 50 BMS slaves 12 (e.g. active RF master 11 is responsible to update BMS slaves 12 with addresses 1 to 50 and passive RF master 11 is responsible to update BMS slaves 12 with addresses 51 to 100). In that case, all RF masters 11 and their associated BMS slaves 12 are communicating over different radio channels. These specifications concern only the over the air (OTA) device firmware update (DFU)) as another protocol (wireless standard) than which is described in this document is used.

[0033] The primary master 11 a or the secondary master 11 b comprises a main system that will be connected to:- a central control system like for example an EMU (acronym for “Electronic Monitoring Unit”) and / or ECUs (acronym for “Electronic Control Units”) in a vehicle via a communication system like a CAN (acronym for Controller Area Network);- a BMS slave via RF communication (acronym for Radio frequency communication).

[0034] The primary master 11 a or the secondary master 11 b comprises:- CPU Core 1 : communication interface; management of digital and analogic I / O locally and remotely (via I2 C bus) of the BMS master;- CPU Core 2: RF communication interface with each BMS slaves 12 ; cell data management and algorithmic (static and dynamic balancing, heating, impedance measurement);- CPU Core 3: Ethernet communication and diagnostic interface.

[0035] The primary master 11a or the secondary master 11 b also comprises a master radio. This master radio is a RF transceiver of this BMS master. This master radio is able to manage the protocol and wireless communication with the BMS slaves 12. In all cases, the master radio is redundant (primary and secondary master).

[0036] In this configuration, these primary 11 a and secondary masters 11 b both receive, via RF communication, the messages from the BMS slaves. Optionally, via CAN, they also receive instructions from the central control system like an EMU. These primary and secondary masters have the same technical characteristics. In this context they execute the same algorithms based on the same instructions and data from their environment, and therefore produce the same results.

[0037] However, at any given time, only one of them is active 11a in sending, via RF communication, instructions to the BMS slaves 12. In other words, at any given time, the master which is in an active operating mode, here the primary master 11 a, is able to receive and / or transmit data from / to at least one of the BMS slaves 12. When the master which is in a passive operating mode, here the secondary master 11 b, is only able to receive the same data as the primary master 11 a from at least one BMS slave 12. Optionally, via CAN, only one of the two transmits information to the EMU. According to one embodiment, an interface, other than CAN and RF, exists between the primary master and secondary master to identify which of the two is in the active operating mode and to allow, if necessary, switching from passive to active operating mode, respectively from active to passive operating mode.

[0038] It can be noted that depending on the technical options chosen, to implement redundancy (primary and secondary masters), one can either :- duplicate the BMS master that is to say the primary master 11 a is duplicated in the secondary master 11 b ; in which case full redundancy is introduced (see also for the CPU Core 1 functions) with respect to the EMU;- duplicate only (and at least) the master radio that is to say that the master radio of the primary master 11 a is duplicated in the master radio of the secondary master 11 b; in which case the redundancy applies only to the RF communication part of the BMS.

[0039] The primary 11 a and secondary 11 b masters are able to communicate periodically, via RF communication (acronym for Radio frequency communication), with each BMS slave 12, every 20 ms thanks to a proprietary communication protocol. This implies that every 20 ms, the master which is in an active operating mode, here the primary master 11 a, gives its (common) instructions to the BMS slaves 12 and receives, from each BMS slave 12, a set of cell-specific information(s).

[0040] In this context, the communication channel between the BMS master 15, namely RF masters 11 and all BMS slaves 12 is wireless. The primary 11 a or the secondary 11 b master is able to use a radio transceiver, here the master radio, to generate the radio signals and manage the protocol. Preferably, this communication channel operates in the 2.4 GHz base band (2400 - 2420 MHz), for example based on the basic RF stack of Bluetooth® I ZigBee® (IEEE 802.15) and Wi-Fi® (IEEE 802.11 ) communications with the following configuration: 2-GFSK modulation, 1 MHz bandwidth, 20 channels, 2 Mbps data rate and fixed user payload of 6 bytes. The developed protocol does not follow the IEEE 802.15. xx or IEEE 802.11.xx standards. (Standard protocol was suggested to increase to increase reliability, we prefer to be fast over robustness for data collection from the cells - 20 ms.). This channelcommunication is composed of several wired transmission lines (present on each module or battery stack panel) and couplers that allow the BMS slaves to connect to a local transmission line on the panel.

[0041] In another embodiments, the primary master 11 a or the secondary 11 b master is also able to communicate periodically with the central control system (for example the EMU) via CAN, and that every 10 ms depending on the application. For example, the primary 11a or secondary master 11 b can communicate with this central control system.

[0042] Regarding the BMS slaves 12, they can each manage one battery cell, as well as wireless communication technology with the master which is in an active operating mode, here the primary master, via the master radio.

[0043] In this configuration, the wireless communication between the primary or the secondary masters 11 , 11 a, 11 b, and all the BMS slaves 12 can be managed by a proprietary protocol based on a "market" RF communication stack, configurable and implemented in an MCUs (acronym for microcontroller unit) comprise in the primary and secondary masters and the BMS slaves. These MCUs are dedicated to low power RF communication. This RF stack can be here a Bluetooth® stack which is a software that is an implementation of the Bluetooth® protocol stack.

[0044] According to one embodiment, the primary and secondary masters 11 , 11 a, 11 b, and the BMS slaves 12 maintain tables of error counters and nature of the error (reception, timeout, CRC, transmission). It can be noted that BMS slave 12 can also be marked as inactive.

[0045] Furthermore, in order to minimize the energy consumption of the BMS slaves 12, the RF master 11 , 11 a, 11 b which is in an active operating mode, here the primary master 11a, controls the sleep and wake-up of the BMS slaves 12.

[0046] Regarding the control of the sleep :- the primary master 11 a receives the standby command from the central control system (for example the EMU);- before going to sleep itself, it sends a unicast request (SETUP::SLEEP) to each BMS slave 12;- each BMS slave 12 confirms in its response (INFO::SLEEP) that it has received the request and then goes into sleep mode;- the primary master repeats the request n times if it does not receive any acknowledgement from one or more BMS slaves; after n unsuccessful repetitions, it considers that the BMS slave 12 will put itself on standby (see next point);- after n minutes without any communication from the primary master 11a (periodic request every 20 ms, unicast messages), the BMS slave 12 switches to standby;- after confirmation from all BMS slaves 12, or n unsuccessful repetitions with potential BMS slaves 12 not responding, the primary master 11 a acknowledges the end of the process, via CAN, to the EMU; the latter can then command the battery management system to be powered OFF;- the consumption per BMS slave in standby is about 50-60 nA, i.e., a total consumption of 0.5-0.6 mA for the whole battery management system (the primary masted 1 a is triggered and therefore its consumption is zero).

[0047] Concerning the control of the wake-up :- the RF MCU used in the BMS slaves 12 has different sleep levels, the deepest of which (i.e. the one with the lowest power consumption) allows to be woken up by a specific RF pattern (OOK packet) at a specific rate (carrier 2.45 MHz I rate 1 Kbps I ASK modulation);- the primary master 11 a uses the same RF MCU in its Master Radio part in order to generate an RF pattern compatible with the BMS slave 12 for its alarm clock;- the reception of this RF pattern allows to wake up all the BMS slave 12;- the BMS master 15 then sends a unicast request (SETUP:: WAKE-UP) to each BMS slave;- each BMS slave 12 confirms in its response (INFO::WAKE-UP) the good reception of the request to indicate that it is awake and ready to receive other commands;- the primary master 11a repeats the RF wake-up pattern n times if it does not receive any acknowledgement from one or more BMS slaves 12; after n unsuccessful repetitions, it considers that the BMS slave 12 has a problem and will inform, via CAN, the central control system (for example the EMU);- after n minutes without any communication from the primary master 11a (periodic request every 20 ms, unicast messages), the BMS slave 12 goes into standby;- the primary master 11a acknowledges the end of the process, via CAN, to the EMU; the latter can then continue its process.

[0048] In order to achieve a time of 20ms between two consecutives transmissions of a message between primary master 11 a and the BMS slaves 12, the following time constraints must be met :- duration of transmission of the message between the primary master 11a and the BMS slave 12 must be inferior to 6.5 ps;- duration of jitter synchro among all BMS slaves 12 must be inferior to 0.3 ps;- BMS slave 12 transmit duration must be inferior to 175 ps;primary master 11 a transmit duration must be inferior to 205 ps;BMS slave 12 acquisition time must be inferior to 65 ps.

[0049] The invention also relates to a method of operating a battery management system 20 including the battery pack comprising at least one module including the at least one stack of battery cells 13, a master electronic control unit 14 and a slave electronic control unit 12 associated with each battery cell 13, the method comprising exchanges of data between the master electronic control unit and the slave electronic control unit for managing the battery pack by using solely wireless communication technology.

[0050] In these method and system, a real-time identification of the voltage differences between each cell is performed and analysed. The BMS Master executes an algorithm to transfer energy from the cells with the highest voltages to those with the lowest. This algorithm adapts its instructions for switching on / off the energy transfer to each BMS Slave according to the variations of the voltage topology, respectively in order to minimize the voltage differences between cells while minimizing the energy transfer losses. Moreover, during operation or before charging, bring the cells to a temperature range that ensures better efficiency of the battery (discharge or charge). This also reduces the impact of dendrite accumulation (branchlike protuberances that appear on the surface of lithium metal anodes and can eventually produce a short circuit). Furthermore, measuring the voltage of a cell does not give full information on its capacity and is not an indication of aging. The knowledge at any time of the internal resistance I impedance of each cell allows to identify its state of health I aging. This is a complex process, mixing current injection measurements at different frequencies with standard measurements, and as such should be mainly performed during or at the end of every n charging cycles (e.g. 10 cycles) and not during operation. The data collected in this way is mainly used to quantify the SOH and adjust the actual capacity, and in correlation with historical data, for maintenance and diagnosis in operational mode.

[0051] Thus, such system and method permit to reduce the cost, weight, and connectivity issues (risk of bad contacts), the idea is to replace the wired communication (mainly CAN bus) between the BMS Master and all the BMS Slaves by a radio wave transmission. In order to minimize the transmission power (thus the energy used), the disturbing radiations and the transmission errors, the idea is to use a radiating coaxial cable, connected to the BMS Master and running as close as possible to the BMS Slaves, which are provided with an antenna in order to homogenize the reception levels between all the BMS Slaves. An alternative to be evaluated would be to replace the radiating coaxial cable with a copper footprint on a PCB.

Claims

CLAIMS1 . A battery management system (20) comprising:- a battery pack (19) including at least one module comprising at least one stack of battery cells (13);- an electronic control system (14) comprising :- a master electronic control unit (15), said BMS master;- at least two wireless communication device (11 , 11a, 11 b), said RF master, wherein one of the RF masters (11 , 11 a, 11 b) is active, said active RF master (11a), and the others are passive, said passive RF masters (11 b);- a plurality of monitoring devices (12), said BMS slaves (12), each slave monitoring one cell; wherein an active RF master (11 a) is configured to calculate and send commands to the BMS slaves (12), and a passive RF master (11 b) is configured to only receive data from the BMS slaves (12), wherein RF masters (11 , 11 a, 11 b) and BMS slaves (12) being configured to communicate solely using wireless (18) communication link, and BMS master (15) and RF masters (11 ) being configured to communicate using a wired (16) communication link, in order to exchange data for managing the battery pack (19).

2. Battery management system (20) according to the preceding claim, wherein the at least two RF masters (11 , 11a, 11 b) are configured to switch roles between active and passive.

3. Battery management system (20) according to claim 1 or 2, wherein the master electronic control unit (15) is configured to assign the roles active or passive.

4. Battery management system (20) according to one of the preceding claims, wherein the at least two RF masters (11 , 11a, 11 b) are configured to assign themselves the roles active or passive.

5. Battery management system (20) according to one of the preceding claims, wherein active RF master (11a) and passive RF masters (11b) communicate with each other via a wired communication link (17).

6. Battery management system (20) according to one of the preceding claims, wherein active RF master (11a) is configured to :- -receives commands I requests from BMS master (15),- - forwards commands I requests to BMS slaves (12),- - receives back information from BMS slaves (12),- - sends back information to BMS master (15).

7. Battery management system (20) according to one of the preceding claims, wherein passive RF master (11a) is configured to :- - receives commands / requests from BMS master (15),- - does not forward commands I requests to BMS slaves (12),- - receives back information from BMS slaves (12),- - sends back information to BMS master (15).

8. Electrical vehicle comprising the battery management system (20) according to the preceding claims.

9. A method of operating a battery management system according to one of the preceding claims, the method comprising exchanges of data :- send commands I requests from BMS master (15) to active RF master (11a) and passive RF master (11 b) via wired communication link,- - forward commands I requests from only active RF master (11a) to BMS slaves (12) via wireless communication link,- - reply information from BMS slaves (12) to both active RF master (11a) and passive RF master (11 b) via wireless communication link,- - send information from both active RF master (11a) and passive RF master (11 b) to BMS master (15) via wired communication link.

10. The method according to preceding claim, wherein active and passive roles of RF masters (11 , 11a, 11 b) are switched by BMS master (15) or the RF master (11 , 11a, 11 b) themselves.

11. The method according to preceding claim, wherein active and passive roles of RF masters (11 , 11a, 11 b) are switched when the active RF master (11a) is faulty.

12. The method according to claim 9, wherein only active RF master (11a) send command or request to all BMS slaves (12).

Images