Battery management system and method with auxiliary power source

Abstract

For controlling charging and discharging of an electrochemical storage device, a battery management system (BMS), and corresponding method, has battery monitoring and control circuits, input and communication ports, for sensor inputs and for communicating with an external charging power supply. Positive and negative power connections connect an electrochemical storage device and the external charging power supply, with a disconnect switch in at least one of the power connections. The BMS has an input for an external voltage source to provide an alternative voltage source for the battery management system. A selection circuit selectively connects one of the power connections and the external voltage source to the battery monitoring and control circuits, and actuates the disconnect switch.

Description

[0001] Battery Management System and Method with Auxiliary Power Source

[0002] FIELD

[0003] This disclosure relates to a battery management system (BMS) for electrical energy storage devices that may comprise multiple cells and a method of operation and powering of the battery management system.

[0004] BACKGROUND

[0005] The following paragraphs are not an admission that anything discussed in them is prior art or part of the knowledge of persons skilled in the art.

[0006] Electrical energy storage devices may comprise one or more of secondary or rechargeable cells, which may be cells of varying types and varying energy storage capacity. Electrical energy storage devices may also include supercapacitors, which also may be of different types and capacities. The disclosed BMS and method may be suited for any electrical energy storage application, from individual cells to large GigaWatt hour systems used for grid support.

[0007] Examples of secondary batteries are lithium ion (Li-ion) batteries, sodium ion batteries and lead-acid batteries.

[0008] Lithium ion (abbreviated Li-ion) technology is currently amongst the highest performing and is experiencing high market growth in commercial rechargeable batteries. However, even within this sub-category there are still multiple chemistries such as lithium cobalt oxide (LiCoO2), lithium iron phosphate (LFP), lithium manganese oxide (LMO), lithium nickel manganese cobalt oxide (NMC), lithium nickel cobalt aluminum oxide (NCA) and lithium titanate (LTO). Other popular rechargeable cell chemistries include lead acid, Nickel-Cadmium (Ni-Cad) and Nickel metal hydride (NiMH).

[0009] Sodium ion batteries are an example of a newer electrochemical storage technology capable of discharging down to 0 Volts.

[0010] Supercapacitors, also known as electric double-layer capacitors or ultracapacitors, also function as rechargeable electrical energy storage units or cells. Unlike rechargeable batteries, supercapacitors feature low energy densities but have high power capabilities and are therefore typically used to meet short-term high power demands. Supercapacitors are also an example of a storage technology capable of discharging down to zero Volts. The combination or hybridization of rechargeable batteries and supercapacitors may be desirable for applications that require both fast charging and discharging as well as high energy storage capabilities, with maximum battery lifetime. Such applications include, for example, electric and hybrid electric vehicles and stationary energy storage devices required to charge and discharge at high rates.

[0011] Battery management systems for many electrochemical devices rely on the fact that the devices have to be operated between set voltage limits, and that at a prescribed zero state of charge the device will still have a certain amount of charge and voltage available, usually sufficient to meet the low power demands of monitoring and control circuits of the battery management system. Accordingly, no separate power supply is needed for the BMS.

[0012] INTRODUCTION

[0013] The following introduction is intended to introduce the reader to this specification but not to define any invention. One or more inventions may reside in a combination or sub-combination of the apparatus elements or method steps described below or in other parts of this document. The inventor does not waive or disclaim his rights to any invention or inventions disclosed in this specification merely by not describing such other invention or inventions in the claims.

[0014] According to one aspect of the present disclosure there is provided a battery management system comprising: battery monitoring and control circuits; an input port, for receiving sensor inputs from an electrochemical storage device; a communication port, for at least one of: communicating with an external charging power supply for charging the electrochemical storage device; receiving control inputs; and communicating to higher level systems about the states of the electrochemical storage device; positive and negative power connections, for providing, in use, connections between the electrochemical storage device and the external charging power supply and between the electrochemical storage device and a load powered by the electrochemical storage device; a disconnect switch in at least one of the power connections; a connection for an external voltage source, providing, in use, an alternative voltage source for supplying power to at least part of the battery management system; a selection circuit for connection to at least one of the power connections and to the external voltage source, for selectively connecting, in use, one of the power connections and the external voltage source to the battery management system.

[0015] According to another aspect of the present disclosure there is provided a battery management system comprising:

[0016] A battery monitoring and control unit; an input port, for receiving sensor inputs from an electrochemical storage device; a communication port, arranged to communicate with an external charging power supply for charging the electrochemical storage device; a first power connection and a second power connection, for providing, in use, connections between the electrochemical storage device and the external charging power supply or between the electrochemical storage device and a load powered by the electrochemical storage device, wherein one of the first power connection and the second power connection is a positive power connection and the other of the first power connection and the second power connection is a negative power connection; a disconnect switch in the first power connection; a selection circuit arranged to receive power from an external voltage source and arranged to receive power from at least one of the first power connection and the second power connection, the selection circuit further arranged to provide power to at least part of the battery management system from at least one of: the external voltage source and the at least one of the first power connection and the second power connection. According to another aspect of the present disclosure there is provided a method of controlling charging and discharging of an electrochemical storage device, the method comprising: providing a battery management system as defined above, an electrochemical storage device, an external charging power supply connectible to the positive and negative power connections, and an external voltage source connected to the selection circuit; connecting the positive and negative power connections to the electrochemical storage device, and connecting the input port to sensor inputs of the electrochemical storage device; and the selection circuit selectively connecting the external voltage source to power the battery monitoring and control circuits when the voltage of the electrochemical storage device is below a predetermined level.

[0017] Providing a selection circuit with the ability to connect to an external voltage source allows the electrochemical storage device to be charged safely even when its state of charge has dropped to a level at which it can no longer power the battery monitoring and control circuits itself. As will be further discussed later, this may happen either when the electrochemical storage device has discharged to a point at which its output voltage is insufficient to power the battery monitoring and control circuits or when a failure has occurred within the electrochemical storage device removing its ability to power the battery monitoring and control circuits. For a storage device capable of discharging safely down to 0 Volts, such as supercapacitors or sodium ion cells, the former condition may happen regularly, even up to every discharge cycle. While the battery monitoring and control circuits are unpowered, stored historic data from the sensors, which provide information on the state of the electrochemical storage device, are unavailable. Without access to such data, charging the electrochemical storage device is risky as it may be in a dangerous failure state. Accessing the data allows the electrochemical storage device’s history to be evaluated for unsafe conditions so that an informed decision on safe charging can be made. The selection circuit allows the external voltage source to power the battery monitoring and control circuits even when the storage device itself cannot power the battery monitoring and control circuits. However, the selection circuit also allows the battery monitoring and control circuits to be powered by the electrochemical storage device so that an external voltage source need not be permanently connected, or need not be continuously depleted during normal use. The selection circuit can select between the two alternative power sources as required.

[0018] The selection circuit may be separate from the battery monitoring and control circuits.

[0019] The selection circuit may be integral with the battery monitoring and control circuits.

[0020] The selection circuit may comprise two diodes, one diode providing a connection to one of the power connections and the other diode providing a connection to the external voltage source, the diodes being configured so that, whichever of the said one of the power connections and the external voltage source provides the highest voltage powers the battery monitoring and control circuits.

[0021] The external voltage source may be connected to the battery monitoring and control circuits through the selection circuit.

[0022] The external voltage source may be connected to the battery monitoring and control circuits through the communication port.

[0023] In some examples, the negative power connection and the battery monitoring and control circuits are connected to a common voltage potential; the selection circuit is connected between the positive power connection and the battery monitoring and control circuits; and the disconnect switch is provided in the positive power connection.

[0024] In some examples, the disconnect switch is arranged to be actuated by the battery monitoring and control unit.

[0025] In some examples, the communication port is arranged to receive control inputs.

[0026] In some examples, the selection circuit is arranged to provide power to the battery monitoring and control unit.

[0027] Further aspects of this disclosure include a battery management system, in any of the forms discussed above, in combination with an electrochemical storage device.

[0028] The electrochemical storage device may comprise a lithium ion battery.

[0029] The electrochemical storage device may comprise a sodium ion battery.

[0030] The electrochemical storage device may comprise a supercapacitor. The methods discussed above may comprise providing an electrochemical storage device capable of repeated discharge to zero volts, and the method may comprise: receiving the electrochemical storage device as manufactured and uncharged; the selection circuit selecting the external voltage source to provide power to the battery monitoring and control circuits; and charging the electrochemical storage device.

[0031] In some examples, the battery monitoring and control unit controls the charging step.

[0032] When the electrochemical storage device is sufficiently charged, the selection circuit may select the electrochemical storage device as a source of power for the battery monitoring and control circuits.

[0033] BRIEF DESCRIPTION OF THE DRAWINGS

[0034] For a better understanding of the present invention and to show more clearly how it may be carried into effect, reference will now be made, by way of example, to the accompanying drawings which show, by way of example, the present invention and in which:

[0035] Figure 1 is a schematic of a battery, a battery management system and a selection circuit in accordance with one aspect of the present disclosure;

[0036] Figure 2 is a schematic of a battery, a battery management system and a selection circuit in accordance with a second aspect of the present disclosure;

[0037] Figure 3 is an example of a Vmonitoring selection circuit; and

[0038] Figure 4 is another example of a Vmonitoring selection circuit.

[0039] DETAILED DESCRIPTION

[0040] Various apparatuses or methods will be described below to provide an example of an embodiment of each claimed invention. No embodiment described below limits any claimed invention and any claimed invention may cover apparatuses or methods that are not described below. The claimed inventions are not limited to apparatuses or methods having all of the features of any one apparatus or method described below or to features common to multiple or all of the apparatuses described below. It is possible that an apparatus or method described below is not an embodiment of any claimed invention. The applicants, inventors and owners reserve all rights in any invention disclosed in an apparatus or method described below that is not claimed in this document and do not abandon, disclaim or dedicate to the public any such invention by its disclosure in this document.

[0041] In general, for any energy storage medium, care must be taken to ensure that the stored energy is not released in an uncontrolled manner. Furthermore, the energy storage devices (e.g. electrochemical cells) must be maintained within their designed operating and storage parameters to avoid rapid degradation of their ability to store energy.

[0042] In modern electrochemical battery systems, electronic components and integrated circuits are used to monitor the parameters of electrochemical cells and ensure that their limits are not exceeded. In cases where these parameters are exceeded, these circuits can take control actions to prevent damage to the electrochemical energy storage devices. In this disclosure, the circuits will all be referred to as the Battery Monitoring and Control Circuits (or equivalently as the battery monitoring and control unit).

[0043] For many types of electrochemical cells it is necessary to ensure that certain parameters, particularly the state of charge, remain within set limits. For example, Lithium ion cells during manufacture are subject to a formation step, when the cells are charged for the first time, during which a solid-electrolyte-interphase layer is formed. Once this layer is formed the cells are working cells that can be charged and discharged, but their state of charge must be maintained within set limits, e.g., between 2.8 volts (0% state of charge) and 4.2 volts (100% state of charge). If the cells are subject to excessive discharge, taking their voltage below 2 volts, then various degradation processes can occur in the cells and possible internal short circuits.

[0044] Other types of cells, such as sodium ion cells are not constrained in the same way. They can be left in a zero voltage state after manufacture and are often shipped in a fully discharged or zero volt state. In use, it is permissible to discharge sodium cells to zero volts and this can be done repeatedly. However, the state of the art battery management systems for battery systems require electrical power to operate, and usually they rely on the electrochemical cells they are connected to for this electrical power. For many types of electrochemical cells, e.g. lithium ion cells operated in the permitted range, when an indication is provided to a user that the electrochemical energy storage devices are fully discharged, there is still sufficient residual electrochemical energy available to power the battery management system; for most electrochemical cell types, there will be power available for a reasonable time practically, as monitoring the state of charge of a battery requires only a small amount of power as compared to that provided by the battery or electrochemical cells for their primary purpose. It is necessary to monitor the state of charge for secondary (rechargeable) electrochemical energy storage devices so that this information can be communicated with a charger, which can then be operated according to the actual state of charge.

[0045] To date, powering the battery management system from the electrochemical cells it is connected to has been adequate. However, there are at least two scenarios where relying on the electrochemical cells to power the battery management system is inadequate, namely:

[0046] (a) forensic analysis of electrochemical energy storage devices that have failed; and

[0047] (b) new electrochemical energy storage technologies, e.g. sodium ion cells, that are able to discharge down to 0 V.

[0048] The following subsections will discuss each scenario in detail.

[0049] Many battery management systems for electrochemical energy storage devices will record historical data about the cells they are connected to in order to inform the user about the performance of the cells. For example, the monitoring and control circuits will estimate the useable capacity of the cells. The circuits are also capable of recording historical parametric data from the cells such as voltage, current, temperature, etc. The battery management systems are and can be designed to maintain this historical data even in the absence of power, e.g. in a non-volatile memory.

[0050] When there is no residual energy available, e.g. electrochemical energy storage devices have failed or the electrochemical energy storage devices are those capable of discharge to 0 volts, there is no power for the battery management system. Without such power, it is impossible to read any historical data from the battery monitoring and control circuits (e.g. from the non-volatile memory) to help determine the root cause of the failure of the electrochemical energy storage devices or the actual current state of electrochemical energy storage devices capable of discharge to 0 volts.

[0051] It is desirable to enable the retrieval of this historical data for forensic analysis without needing to disconnect the BMS from the cells. It is also desirable to enable the retrieval of information about the cells in their current state. Doing so requires providing power to the battery monitoring and control circuits.

[0052] Figure 1 shows one aspect of the present disclosure. In Figure 1 , a battery 10, comprises a plurality of electrochemical cells. The battery 10 has sensor inputs or connections 16, for providing, e.g., data on voltage, temperature, etc.

[0053] A battery monitoring system (BMS) is indicated schematically at 40 and includes battery monitoring and control circuits 20, connected to the sensor inputs 16 via an input port 17. The BMS is provided with a negative power connection 12 and a positive power connection 14, the negative connection 12 also being connected to a common voltage potential, as indicated at 18. A positive terminal of the battery 10 is connected to the positive power connection 14 through a switch 28 and a negative terminal of the battery 10 is connected to the negative power connection 12. The positive and negative power connections provide bi-directional power flow between the electrochemical storage device and a source or load. In some use cases only a source is connected (for charging). In some use cases only a load is connected (discharging). In other use cases, connections may be maintained to both a source and a load. The battery monitoring and control circuits 20 have a communication port 22, for relaying sensor data. The communication port may be used only for sending data out (i.e. unidirectional communication), but in some examples can also be used to receive control signals, e.g. to regulate the charging and discharging of the battery 10 (bidirectional communication).

[0054] A Vmonitoring selection circuit 24 is connected between an external voltage source 26, shown connected to the common voltage potential, and the battery monitoring and control circuits 20. The Vmonitoring selection circuit 24 is also connected to the positive power connection 14 and to the common voltage potential. The Vmonitoring selection circuit 24 is capable of selecting either the voltage available from the positive power connection 14 or the voltage from the external voltage source 26, to supply power to the battery monitoring and control circuits 20. The external voltage source 26 can be any suitable voltage source, e.g. a battery, a hydrogen fuel cell, a connection to an electrical grid, other electrical power supply, etc.

[0055] A disconnect switch 28 is provided in the positive power connection, and is controlled by the battery monitoring and control circuits 20, which will open the switch 28 in order to protect the battery 10 from damage via an external current or voltage, e.g. if an external current or voltage may drive the voltage or current of the battery outside of set minimum and maximum values the disconnect switch 28 will be opened. For some applications, it may be desirable to provide a second disconnect switch in the negative power connection 12. In some applications a single disconnect switch in the negative power connection 12 may be sufficient on its own (i.e. without also requiring a disconnect switch in the positive power connection).

[0056] Thus, the battery monitoring and control circuits 20 are powered by Vmonitoring. Vmonitoring is supplied by the Vmonitoring selection circuit 24, and is able to change automatically (or in a controlled manner) between the two input sources, Vbat from the positive power connection 14 and Vextemai from the source 26. The Vmonitoring selection circuit 24 does not allow power to flow from Vextemai to Vbat and vice versa. This feature ensures that if the electrochemical cells are at 0 V or are failed in a short circuit manner, Vmonitoring is not also short-circuited or tied to 0 V by Vbat.

[0057] Figure 2 shows another aspect of the present disclosure. Elements similar to Figure 1 are given the same reference numeral. Thus, a battery 10 is connected to a negative power connection 12 and a positive power connections 14, the negative connection 12 also being connected to a common voltage potential, as indicated at 18. The overall BMS is here indicated schematically at 42.

[0058] Battery sensor inputs or connections 16 are connected to an input port 17 of battery monitoring and control circuits 30, which have a communication port 32. A Vmonitoring selection circuit 34 is integrated into the battery monitoring and control circuits 30. An external voltage source 36 supplies power through an external communication bus to the communication port 32 and thereby supplies power to the Vmonitoring selection circuit.

[0059] A disconnect switch 38, as for switch 28 in Fig. 1 , is provided in the positive power connection 14, and is controlled by the battery monitoring and control circuits 30.

[0060] In both aspects of the selection circuit, in one mode of operation, the selection circuit 24, 34, will supply power to the battery monitoring and control circuits 20, 30 from the battery 10. If the voltage of the battery 10 drops below a permitted minimum for normal operation, the battery monitoring and control circuits 20, 30 will open the switch 28, 38 to protect the battery from further discharge. At this time, the battery monitoring and control circuits 20, 30 will continue to be powered by the battery 10. If the battery voltage drops further, then the Vmonitoring selection circuit 24, 34 will switch to another mode of operation, drawing power from the external voltage source 26, 36, if one is available. If no such external power source is available in this mode of operation, the battery monitoring and control circuits 20, 30 will default to accepting power from an external voltage source once it becomes available.

[0061] Thus, the BMS 40, 42 can be provided with two distinct voltage levels to effect switching and control. Below a first voltage level, a minimum voltage for safe operation of the battery 10, the switch 28, 38 will be opened to protect the battery 10 from further discharge into an attached load. As detailed, the BMS 40, 42 and its battery monitoring and control circuits may continue to draw power from the battery 10 as this should represent only a small current draw. The connection of the monitoring and control circuits 20, 30 to the power connection 14 is located between the switch 28, 38 and the connection to the battery, i.e. the opening of the switch 28, 38 severs the connection of the battery 10 to the load, but does not sever the connection of the battery 10 to the monitoring and control circuits 20, 30. If the voltage of the battery 10 falls further below a second set level, lower than the first voltage level, then the BMS 40, 42 (or at least the battery monitoring and control circuits 20, 30) is disconnected from the battery 10 and may be connected to the external voltage source 26, 36, when available.

[0062] In an alternative implementation, when the external voltage source 26, 36 is available, the external voltage source 26, 36 can be set as the default source for the BMS 40, 42. Where the external source 26, 36 is in effect limitless, e.g. it is a connection to an electricity distribution grid, there may be no need to switch to using the battery 10 as a power source. Where the external source 26, 36 is itself of limited capacity, e.g. is a rechargeable or non-rechargeable battery, then provision can be made for switching to the battery 10 as the power source for the BMS 40, 42. Such switching may be effected manually or may be effected by an algorithm. E.g., if the algorithm detects that the battery 10 is being charged or that the battery 10 has a certain prescribed charge state, the battery 10 can be selected as the power source.

[0063] In the case of a battery that is fully discharged, in the sense of having zero volts at its output, e.g. in the case of a fully discharged sodium ion battery, the Vmonitoring selection circuit 24, 34 will select the external voltage source to power the BMS 20, 30. If desired by the application, the Vmonitoring selection circuit 24, 34 will select Vbat once there is sufficient voltage available at the battery 10 so as to minimise drain of the external voltage source 26, 36.

[0064] Referring to Figure 3, there is shown an implementation of a Vmonitoring selection circuit, here indicated at 44, which comprises a pair of diodes 46. As shown, one of the diodes 46 is connected to Vbat, i.e. to the positive power connection 14, and the other of the diodes 46 is connected to Vextemai, i.e. to the external voltage source 26 or 36. The diodes 46 are configured so that whichever of the voltage sources Vbat and V external has the highest voltage, that voltage powers the battery monitoring and control circuits 20, 30.

[0065] Figure 4 shows an alternative implementation of a Vmonitoring selection circuit 44. This implementation is similar to the Figure 3 implementation, but with the addition of a controllable switching device 50 (here a MOSFET) in each power connection (i.e. in series with the diodes 46). Each MOSFET 50 has a control signal applied to its gate to determine whether it is ON (conducting) or OFF (non-conducting). For example, if the input signal “Disable Vextemai” is disconnected then the gate voltage of the FET 50 is Vexternal. As this is a p-type MOSFET it is ON in this arrangement. This may be taken as the default arrangement. On the other hand if the “Disable Vextemai” signal is connected to ground (or low) then the MOSFET 50 will be turned OFF, disconnecting the Vextemai voltage source. Thus the BMS 40, 42 (or indeed a separate controller) can control which voltage source is used to power the monitoring and control circuits 20, 30 by suitable control of the inputs “Disable Vbat” and “Disable Vextemai”. This arrangement may be useful for example where the BMS 40, 42 becomes aware of an external voltage source that can be used in preference to the battery 10. The BMS can the disconnect Vbat from the selection circuit 44, thereby forcing the monitoring and control circuits 20, 30 to be powered from the external voltage source.

[0066] The monitoring and control circuits may be implemented with discrete components, an integrated circuit, or a combination of the two. For off the shelf monitoring and control circuits, the Vmonitoring selection circuit may be implemented as a discrete subcircuit. However, the Vmonitoring selection circuit may also be integrated into an integrated circuit as part of the battery monitoring and control circuits.

Claims

CLAIMS:

1. A battery management system comprising: a battery monitoring and control unit; an input port, for receiving sensor inputs from an electrochemical storage device; a communication port, arranged to communicate with an external charging power supply for charging the electrochemical storage device; a first power connection and a second power connection, for providing, in use, connections between the electrochemical storage device and the external charging power supply or between the electrochemical storage device and a load powered by the electrochemical storage device, wherein one of the first power connection and the second power connection is a positive power connection and the other of the first power connection and the second power connection is a negative power connection; a disconnect switch in the first power connection; a selection circuit arranged to receive power from an external voltage source and arranged to receive power from at least one of the first power connection and the second power connection, the selection circuit further arranged to provide power to at least part of the battery management system from at least one of: the external voltage source and the at least one of the first power connection and the second power connection.

2. A battery management system as claimed in claim 1 , wherein the selection circuit is separate from the battery monitoring and control unit.

3. A battery management system as claimed in claim 1 , wherein the selection circuit is integral with the battery monitoring and control unit.

4. A battery management system as claimed in claim 1 , 2 or 3, wherein the selection circuit comprises two diodes, one diode providing a connection to one of the power connections and the other diode providing a connection to the external voltage source, the diodes being configured so that, whichever of the said one of the powerconnections and the external voltage source provides the highest voltage powers the battery monitoring and control unit.

5. A battery management system as claimed in any preceding claim, wherein the external voltage source is connected to the battery monitoring and control unit through the selection circuit.

6. A battery management system as claimed in 1 , 2, 3 or 4, wherein the external voltage source is connected to the battery monitoring and control unit through the communication port.

7. A battery management system as claimed in any preceding claim, wherein the negative power connection and the battery monitoring and control unit are connected to a common voltage potential, wherein the selection circuit is connected between the positive power connection and the battery monitoring and control unit, and the disconnect switch is provided in the positive power connection.

8. A battery management system as claimed in any preceding claim, wherein the disconnect switch is arranged to be actuated by the battery monitoring and control unit.

9. A battery management system as claimed in any preceding claim, wherein the communication port is arranged to receive control inputs.

10. A battery management system as claimed in any preceding claim, wherein the selection circuit is arranged to provide power to the battery monitoring and control unit.

11. A battery management system as claimed in any preceding claim, in combination with an electrochemical storage device.

12. A battery management system as claimed in claim 11 , wherein the electrochemical storage device comprises a lithium ion battery.

13. A battery management system as claimed in claim 11 , wherein the electrochemical storage device comprises a sodium ion battery.

14. A method of controlling charging and discharging of an electrochemical storage device, the method comprising:a. providing a battery management system as claimed in any one of claims 1 to 10, an electrochemical storage device, an external charging power supply connectible to the positive and negative power connections, and an external voltage source connected to the selection circuit; b. connecting the positive and negative power connections to the electrochemical storage device, and connecting the input port to sensor inputs of the electrochemical storage device; c. the selection circuit selectively connecting the external voltage source to power the battery monitoring and control unit when the voltage of the electrochemical storage device is below a predetermined level.

15. A method as claimed in claim 14, comprising providing an electrochemical storage device capable of repeated discharge to zero volts, the method comprising: receiving the electrochemical storage device as manufactured and uncharged; the selection circuit selecting the external voltage source to provide power to the battery monitoring and control unit; and charging the electrochemical storage device.

16. A method as claimed in claim 15, wherein the battery monitoring and control unit controls the charging step.

17. A method as claimed in claim 15 or 16, wherein, when the electrochemical storage device is sufficiently charged, the selection circuit selects the electrochemical storage device as a source of power for the battery monitoring and control unit.

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