WO2011158051A1

WO2011158051A1 - System and method for charge balancing and/or charging electrical energy-storing units

Info

Publication number: WO2011158051A1
Application number: PCT/HU2011/000055
Authority: WO
Inventors: Ferenc Stangl; József MARINKA-TÓTH; János Kincses
Original assignee: Ferenc Stangl; Marinka-Toth Jozsef; Kincses Janos
Priority date: 2010-06-14
Filing date: 2011-06-14
Publication date: 2011-12-22
Also published as: HUP1000311A2; HUP1000311D0

Abstract

The invention on one hand relates to a system for the charge balancing and/or charging of electrical energy- storing units (10) connected in series, which system comprises centrally controlled energy- control units (12) interconnected between the poles of the individual energy- storing units (10), which energy- control units (12) are provided with a control circuit and an energy- transfer circuit. The energy transmitting circuits are connected to a feed conductor (22) attached to the alternating current energy source, and comprise an energy- transfer capacitive element (24) being connected to the feed conductor (22) and enabling energy flow between the alternating current energy source and the given energy- storing unit (10), as well as a switching means suitable for realizing selective energy transfer being operated by the control circuit (40).

Description

- A -

SYSTEM AND METHOD FOR CHARGE BALANCING AND/OR CHARGING ELECTRICAL ENERGY-STORING UNITS

TECHNICAL FIELD

The invention relates to a system and a method for charge balancing and/or charging electrical energy-storing units coupled in series. The invention especially relates to a system and a method for energy management of numerous energy- storing units, wherein the individual energy-storing units are monitored separately and the charging and/or charge balancing of them is controlled in a selective manner.

BACKGROUND ART

Prior art rechargeable batteries are made up of numerous cells and are provided with an internal electronics in order to ensure the required power as well as to meet the safety requirements. The individual cells are connected in series so as to provide higher battery voltage for high-voltage applications, such as computers or vehicles. Metal hybrid cells, e.g. nickel-metal-hybrid NiMH, as well as lithium based, e.g. Li-ion cells may damage or even explode if they are out of the designated charging range. The internal electronics are therefore primarily used to ensure safe voltage, current and temperature values.

Further examples for the application of internal electronics in the batteries are firstly for the measurement of the charge level of cells - e.g. for establishing the remaining operating or charging time - and secondly for charging or discharging process management and signaling thereof to a central control.

As is commonly known in the field, voltages of the individual cells do not necessarily remain balanced (equal) in the case of serially coupled cells or cell groups. In such batteries, overvoltage of certain individual cells compared to others may occur, whilst other cells may have lower voltages. Therefore the batteries may additionally contain circuits to monitor the individual cells, by way of example, for the purposes of shorting out irregular cells or balancing voltages between cells. Systems and methods for charge balancing serially coupled energy-storing units, such as battery cells are disclosed e.g. in US 5,631 ,534, US 5,710,504, US 5,814,970, US 5,982,143, US 2005/0140335 A1 , US 6,404,166 B1 , US 2008/0185994 A1 , US 2009/0146610 A1 and US 2010/0052615 A1.

Prior art solutions generally realize passive balancing, i.e. deflecting excess energy through a resistance, as it can be implemented in a relatively simple and cost- effective manner. According to the prior art technology, there is no such active balancing solution (selective feed of cells and units, selective energy transmission among them), which could provide high efficiency.

US 7,615,966 B2 discloses a system and a method for the management energy relations (charge balancing and/or charging) of serially coupled electrical energy- storing units, which system comprises energy-control units connected between the poles of the individual energy-storing units, which energy-control units are provided with a control circuit and an energy-transfer circuit, as well as a central control being in communicating connection with the control circuits of the energy-control units, gathering information about the individual energy-storing units and sending commands to the control circuits based on the received information.

It is the advantage of the solution according to this latter document that it allows for the alteration and balancing of the charge level of the individual cells in a selective manner. It is, however, a disadvantage of the solution that the additional energy useable for charge balancing is drawn in the form of direct current from the charging voltage connected to the whole battery, which can be conducted to the given cell or away from it through adjacent cells and/or energy-control units assigned thereto only in a manner as described in the document. Energy transfer among adjacent units results in energy loss after every step, and requires extremely complex control algorithms. It is, therefore, clearly evident that energy feeding and releasing using direct current brings relatively unfavorable results.

There is also such a solution presented in the above listed patent publications, wherein secondary coils reeled in a common transformer are assigned to the individual cells, and selective charge balancing or charging may take place across these secondary coils. In the case of this solution, the control signal is to be introduced into each inductance, presenting a further disadvantage of additional cable demand in addition to the disadvantageously high space demand and cost of the transformer itself.

DESCRIPTION OF THE INVENTION

It is an object of the invention to provide a charge balancing and/or charging system and method, which decrease or eliminate deficiencies and difficulties of the solutions according to the prior art. It is an object of the invention to provide a system and method enabling the selective charge control of the individual energy- storing units, such as battery cells, at the least possible hardware and wiring demand. It is a further object of the invention to provide a control for the charge balancing and/or charging with the least possible loss, as well as utilizing the advantages of a central control controlling the operation by means of communication technologies realized in the simplest possible way. It is a yet another object of the invention to provide a system and a method, which allow for the energy released from the individual energy-storing units in the course of charge balancing to be transferred in a selective way to any other arbitrary energy-storing unit in the simplest possible way.

The objects of the present invention are achieved by means of the system according to claim 1 and by means of the method according to claim 13. Preferred embodiments are defined in the dependent claims. Exemplary preferred embodiments of the invention are hereunder described in reference to drawings, where

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is a schematic construction of a system according to the invention,

Fig. 2 is a scheme of an energy-storing unit and an energy-control unit of a system according to the invention,

Fig. 3 is a scheme of a preferred energy-transfer circuit,

Fig. 4 is a scheme of an another preferred energy-transfer circuit,

Fig. 5 is a scheme of a further preferred energy-transfer circuit, Fig. 6 is a scheme of a yet another preferred energy-transfer circuit, Fig. 7 is a schematic construction of a further embodiment,

Fig. 8 is a scheme of an energy-storing unit and an energy-control unit of the embodiment in Fig. 7,

Fig. 9 is a scheme of a preferred energy-transfer circuit of the embodiment in Fig. 7, Fig. 10 is a scheme of an another preferred energy-transfer circuit of the embodiment in Fig. 7,

Fig. 11 is a scheme of a further preferred energy-transfer circuit of the embodiment in Fig. 7, and

Fig. 12 is a scheme of a yet another preferred energy-transfer circuit of the embodiment in Fig. 7.

MODES OF CARRYING OUT THE INVENTION

According to the exemplary scheme illustrated in Fig. 1 , the system is used for charge balancing and/or charging of serially coupled electrical energy-storing units 10, preferably battery cells (in the given case units consisting of parallel coupled cells). If the system is used for charge balancing, the main charging voltage is connected to the two poles of the series made up of energy-storing units 10. At the same time, the system according to the invention may also be applied for providing the main charge; in which case no further charging voltage need to be connected to the poles of the series.

The system according to the invention comprises energy-control units 12 connected between the poles of the individual energy-storing units 10 as well as a central controller 16 being in communicating connection with the energy-control units 12 - with their control circuits to be described later - and gathering information about the individual energy-storing units 10, and sending commands to the control circuits based on the received information. The central controller 16 preferably collects information about the charging status of the individual energy-storing units 10, battery cells, and sends commands to the control circuits of the energy-control units 12 based on the received information to charge the individual battery cells or to balance their relative charge status. The control circuits of the energy-control units 12 are preferably in chain-like, voltage isolated, two-way communicating connection with the central controller 16. Communication can be made through a communication line 14 by means of any suitable protocol in either an analogous or digital form. Voltage-isolation is preferably established by means of so-called uplink or downlink coupling capacitors 18 connected between the individual energy-control units 12, namely between their respective control circuits. The coupling capacitors 18 are high-voltage capacitors, therefore, in the case of an accidental line break between the cells, the circuits are protected from blowing out. In the case of a line break or contact failure, the whole voltage of the battery pack would appear at the two points of the broken contact. As the circuits of the energy-control units 12 operate at low-voltage, typically at 2.5 to 4.2 V (Li-ion), the high-voltage up to 600 to 1000 V, depending on the number of cells, appearing on them would cause instantaneous damage. Due to the serial coupling capacitors 18, which capacitors have a small value, e.g. some nF, no current of such magnitude may appear that would damage the electronics.

The energy-control units 12 preferably gather information not only regarding the charge status of each individual energy-storing unit 10, but also other parameters, by way of example cell temperature - informative with respect to the status of individual cells - is even measured by means of thermometers 20.

The central controller 16 may provide additional control functions by means of control signals 30, moreover via a communication channel 32 it can be connected to e.g. a not illustrated display system.

According to the invention, a high-frequency (preferably a frequency exceeding 1 kHz, or even up to 25 - 100 - 1000 kHz), capacitively (Figs. 1 to 12), preferably resonantly (Figs. 7 to 12) coupled central alternating current energy source is used for the purposes of charge balancing and/or charging, by means of which active balancing and low-loss selective charge can be realized in an extraordinarily advantageous manner. According to the invention several alternatives can be imagined: - Energy may originate from one central source only, therefore, the individual cell controllers are merely able to derive energy from the central source, if necessary.

- Energy can be sent in two directions, therefore, discharging of the higher-energy cell and charging of the lower-energy cell can be realized simultaneously.

Furthermore, balancing and charging according to the invention constitute an improvement in that the efficiency of the system is much greater than that of any of the above-described solutions. The capacitive energy feeder - being a resonant energy feeder in a preferred embodiment - may also operate at a voltage exceeding cell voltage, therefore, the losses coming from the remnant voltages of the semiconductors may be reduced significantly beside that this solution bears only at most minimal cost increase compared to passive balancing. In practice, high-voltage capacitive coupling may mean e.g. that beside a cell voltage of typically a few V, the alternating current energy source may have a voltage of up to 500 - 1000 V; the energy-transfer capacitive elements realizing the capacitive coupling are to be selected with a corresponding voltage. The energy- transfer capacitor 24 together with energy-reconductor capacitor 28 shown in the figure and to be described later, are a part of the energy-control unit 12 and are specially highlighted in the drawing for the purposes of illustration only.

In the case of a high-voltage energy source, feeding into the low-voltage energy- storing units 10 can only be realized by dividing down and converting the voltage. This, according to the invention, is realized so that the energy-transfer capacitors 24 are switched by means of a controlled switching means, preferably via pulse- width modulation.

According to the invention, therefore, the energy-control units 12 are preferably provided with a control circuit 40 comprising a microcontroller and an energy- transfer circuit 50, in a way as shown in Fig. 2. The energy-transfer circuits 50 are connected to a feed conductor 22 coupled to the alternating current energy source and comprise: - an energy-transfer capacitor 24 connected to the feed conductor 22 enabling capacitive energy flow between the alternating current energy source and the given energy-storing unit 10, as well as

- a switching means operated by means of the control circuit 40 and suitable for realizing energy transfer in a selective manner.

The high-frequency energy originating from the central source according to the present invention is preferably of rectangular pulse form, as in this case the transferable energy packages are easy to calculate and realize via pulse-width modulation. The alternating current energy source is not shown in Fig. 1 , and is a part of the central controller 16 in the preferred embodiment described herein. Of course, the alternating current energy source can be separately realized from the central controller 16. The potential of the alternating current energy source may exceed the cell voltage, even can be a multiple of the cell voltage, in this way the loss of the rectifiers in the energy-control units 12 can be lowered even to 1 %. A pulse-width modulation controlled voltage-converter is arranged In the energy- control units 12, switching on only in the case when equalizing energy is required by the given cell. The voltage is transformed down to the cell potential with very good efficiency, thereby providing the necessary balancing current. By means of the pulse-width modulation a filling factor of 0 to 99 % can be set, thereby the voltage of the alternating current energy source can be divided down to even extraordinarily small values. This allows for the use of a feed voltage of, for example, even 500 to 1000 V. The energy transfer can be two-directional, when high-frequency feedback to the central equalizing high-frequency network is possible via additional circuit elements. The maximum energy feeding factor of the central high-frequency energy supply network is proportional to the frequency and to the value of the energy-transfer capacitor 24, while it depends quadratically on the amplitude.

An energy-reconductor capacitor 28 seen in Fig. 1 is required when the full high- frequency current is not intended to be passed across the cells. The energy- reconductor capacitors 28 are connected to a lead-off conductor 26, which latter is connected e.g. to a means constituting a part of the central controller 16 and being suitable to lead off the alternating current energy.

In Fig. 2 a scheme of a more simple embodiment of the invention can be seen, wherein the energy-transfer circuit 50 comprises, in addition to the energy-transfer capacitor 24, only rectifiers and a switch 52 being operated by means of the control circuit 40 in a pulse-width modulation manner. The switch 52 is preferably realized with a FET. In this embodiment, the full charging current flows through the diodes, therefore the efficiency is not the best. The energy-transfer circuit 50 comprises a puffer inductance 54 placed between the energy-transfer capacitor 24 and the positive pole of the energy-storing unit 10. This latter circuit element has a filtering and energy-storing function.

The embodiment comprises a fuse 42 for the case of any internal cell line break or contact break, as well as a suppressor diode 44 for dissipating any accidental over- voltage imposed on the cell. The voltage of the energy-storing unit 10 and the signal of the thermometer 20 are introduced into the control circuit 40, which are then each converted into digital format by means of A/D converters A/D_v and A/D_T, respectively.

Fig. 3 shows another embodiment of the energy-transfer circuit 50 realizing a highly efficient one-way feed. The energy-transfer circuit 50 comprises:

- an internal feed capacitor 56, chargeable via the energy-transfer capacitor 24, one pole thereof being connected to the negative pole of the energy-storing unit 10, and establishing an internal feedpoint 57 with its other pole, as well as

- a first and a second controlled feed switch 58, 60 as a switching means connected in series between the internal feed point 57 and the negative pole of the energy-storing unit 10.

Here, one pole of the puffer inductance 54 is interconnected between the first and the second controlled feed switches 58, 60, the other pole being connected to the positive pole of the energy-storing unit 10.

The charge of the internal feed capacitor 56 is realized through a supplying half diode bridge connected between its poles, wherein one pole of the energy-transfer capacitor 24 is connected to the feed conductor 22 whilst the other pole is connected between the diodes of the supplying half diode bridge.

The feed switches 58, 60 have three states:

1. Both feed switches 58, 60 being in decoupled position, no energy is drawn across the energy-transfer capacitor 24.

2. Feed switch 58 being in closed position and switch 60 being in decoupled position, puffer inductance 54 is being charged.

3. Feed switch 58 being in decoupled position and feed switch 60 in closed position, the energy stored in the puffer inductance 54 is fed into the energy-storing unit 10.

The voltage of internal feed-point 57 exceeds the cell voltage. Voltage division between the voltages of internal feed-point 57 and energy-storing unit 10 is determined by the filling factor of the switching of feed switches 58, 60. The voltage difference evolving in this way defines the direction and value of the current. In the illustrated embodiment, the filling factor should be determined so that the current flows from the direction of the internal feed point 57 towards the energy-storing unit 10. In this embodiment, therefore, a switching means is employed having switching elements, enabling in a controlled manner the internal feed capacitor 56 to be charged or the energy stored within to be fed into the given energy-storing unit 10.

The filtering capacitor 62 shown in the figure gives protection against high- frequency noise, as well as it can be suitable for feeding a microprocessor of the control circuit 40. The energy-reconductor capacitor 28 in the energy-transfer circuit 50 is connected to the negative pole of the energy-storing unit 10, and is suitable, in the case of high impedance cells, for the alternating current to leave the battery through it.

In Fig. 4 an embodiment of the energy-transfer circuit 50 is shown, which realizes a two-directional energy feed with high efficiency. This embodiment differs from the one shown in Fig. 3 in that the switching means further comprises releaser switches 64, 66 by-passing the diodes of the supplying half diode bridge. These provide a high-frequency energy releasing from the internal feed point 57 across the energy- transfer capacitor 24. In a release mode, the filling factors of feed switches 58, 60 are to be set so that voltage should reach the desired AC amplitude at internal feed point 57. As in such cases energy does not flow from the feed conductor 22, the voltage of the internal feed point 57 is determined only by the voltage of the energy- storing unit 10 and the filling factor. In the case of a cell voltage of 4 V, by way of example at a filling of ¼, there will be a voltage of 16 V at the internal feed point 57. If in this situation, releaser switches 64, 66 are switched at high-frequency in antiphase, then the rectangular signal of a set voltage will appear at the energy-transfer capacitor 24. Thus, a switching means is controlled, which also comprise switching elements releasing the energy stored in the internal feed capacitor 56 to the alternating current energy source. The energy sent in this way can be re-converted by another similar energy-control unit 12 in the system into the appropriate form and can be filled into the respective energy-storing unit 10.

The energy-transfer circuit 50 of a particularly preferred embodiment of the invention is shown in Fig. 5. This embodiment realizes a one-directional, so-called full bridge feed with high efficiency. Here, the energy-reconductor capacitor 28 in the energy-transfer circuit 50 is interconnected between the diodes of a lead-off half diode bridge connected between the poles of the internal feed capacitor 56. It is a great advantage of this embodiment, that the recurring high-frequency current does not flow through the impedance of the battery cells, but through the energy- reconductor capacitor 28. In this circuit arrangement, the reconducted AC signal is to be realized as a signal in a full anti-phase to the input AC signal having identical amplitude. In this case a double energy feeding is possible due to the anti-phase mode, while on the other hand AC will return through the energy-reconductor capacitor 28 only.

In Fig. 6 the energy-transfer circuit 50 of another particularly preferred embodiment is shown, enabling a two-directional, full bridge feed with high efficiency. In this solution all of the elements are doubled, therefore, the switching means comprises reconductor switches 68, 70 by-passing the diodes of the lead-off half diode bridge in a controlled manner. The control of the releaser switches 64, 66 and the reconductor switches 68, 70 are in full anti-phase. When one circuit has the upper switch ON, the other circuit has the lower switch ON. This embodiment enables even double energy releasing, whilst alternating current does not flow through the cells.

The central controller 16 comprises electronics similar to the above, although dimensioned to higher powers. The central controller 16 is able to supply energy to more energy-storing units 10 and cells simultaneously, as the energy-transfer capacitors 24 of the individual energy-control units 12 can be connected in parallel with the feed conductor 22. A control signal can be sent by the central controller 16 to a consumer or to a main charger to stop consuming or charging, once a previously set threshold is reached by the battery pack. In case of smaller consumers, the central controller 16 itself can cut the connection of the battery pack. This generally means introduction of additional switches, which decrease the efficiency of the system. It seems to be a better solution, if an intelligent consumer and a main charger is instructed by means of a control signal. The central controller 16 can also measure the input and output current of the pack, as well as store the measurement data for the purpose of later analysis. In the course of the method according to the invention, therefore, information is gathered via the control circuit 40 about the charge status of the energy-storing units 10, the information being sent to the central controller 16, based on which information the charge level of the individual energy-storing units 10 is modified in a selective manner by means of the central controller 16. Charge balancing and/or charging is at least in part realized from an alternating current energy source in such a way that an energy-transfer capacitor 24 enabling capacitive energy flow is provided between the individual energy-storing units 10 and the alternating current energy source . in a selectively controlled way via the central controller 16. The connection of the energy-transfer capacitor 24 is preferably realized by means of a switching means controlled via pulse-width modulation. Based on the commands of the central controller 16, the individual control circuits 40 provide the filling factor to be realized by means of the switching means and required for the desired operation of the given energy-control unit 12. The embodiments in Figs. 7 to 12 differ from those in Figs 1 to 6 in that

- the energy-transfer capacitive element is an energy-transfer oscillator 24' enabling resonant energy flow between the alternating current energy source and the energy-storing unit 10,

- the energy-reconductor capacitive element is an energy-reconductor oscillator 28', and

- the switching means are controlled by frequency modulation.

These embodiments provide all the advantages mentioned in connection with the earlier disclosed embodiments.

The method and system according to the invention have exemplary advantages such as follows:

- Energy feeding with high efficiency is realized tending in the direction of the cells by means of capacitive/resonant coupling.

- High-voltage direct current protection is realized by means of the capacitors by the feeding and the communication.

- Safety fuse and suppression overvoltage protection can be included for the case of any possible contact failure, where the electronics would experience high- voltage, high-current signal.

- A process of selective energy transfer is provided from the overcharged cells towards low-voltage cells.

By means of the active, intelligent system according to the invention, an extraordinarily efficient and favorable priced battery management system (BMS) can be realized. The system is able of receiving/using energy from systems other than the charging unit, by way of example in the case of vehicles from regenerative breaking, solar cells or shock-absorber energy recovery systems.

The system according to the invention may communicate or cooperate with intelligent consumers (e.g. motor controls, DC-DC and DC-AC converters) via standard CAN buses or in any other, more advanced way. The system can integrate SOC (state of charge) or SOH (state of health) diagnostic systems, as well. The energy-control unit 12 according to the invention consumes approx. 5 mA in its active mode and approx. 10 μΑ in its passive mode, which is significantly lower than the self-discharging current of an average battery cell. Of course, the present invention is not limited to the preferred embodiments detailed hereabove, but additional modifications and variations are possible within the scope as defined by the claims. The energy-transfer capacitors 24 or energy- transfer oscillators 24' can be connected to the alternating current energy source not only by means of a common feed conductor 22, but may also be individually connected via separate feed conductors 22, or in any other suitable manner. The system according to the invention may promote charge balancing not only by way of selective additional charge but also by selective discharges; therefore it may be combined with any known passive balancing technologies. The internal feed point 57 may be configured not only by means of the feed capacitor 56 described herein, but also by way of any other suitable connection arrangement.

Claims

1. A system for charge balancing and/or charging of electrical energy-storing units (10) connected in series, said system comprising

- energy-control units (12) interconnected between the poles of the energy-storing units (10), the energy-control units (12) having a control circuit (40) and an energy-transfer circuit (50), as well as

- a central controller (16), being in communicating connection with the control circuits (40) of the energy-control units (12) and gathering information about the individual energy-storing units (10) and sending commands to the control circuits (40) based on the received information,

c h a r a c t e r i z e d in that

the energy-transfer circuits (50) are connected to a feed conductor (22) being coupled to an alternating current energy source, and comprise

- an energy-transfer capacitive element (24, 24') being connected to the feed conductor (22) and enabling energy flow between the alternating current energy source and the energy-storing unit (10), as well as

- a switching means suitable for selective energy transfer, being operated by the control circuit (40).

2. The system according to claim 1 , characterized in that the energy-transfer circuit (50) comprises a puffer inductance (54) interconnected between the energy-transfer capacitive element (24, 24') and the positive pole of the energy-storing unit (10).

3. The system according to claim 2, characterized in that the energy-transfer circuit (50) comprises

- an internal feed capacitor (56) chargeable by the energy-transfer capacitive element (24, 24'), one pole thereof being connected to the negative pole of the energy-storing unit (10), and the other pole thereof establishing an internal feed point (57), as well as

- a first and a second controlled feed switch (58, 60) being connected in series between the internal feed point (57) and the negative pole of the energy-storing unit (10), as a switching means, wherein one of the poles of the puffer inductance (54) is connected between the first and the second controlled feed switch (58, 60), while the other pole is attached to the positive pole of the energy-storing unit (10).

4. The system according to claim 3, characterized in that the charge of the internal feed capacitor (56) is realized through a supplying half diode bridge connected to the poles thereof, wherein the energy-transfer capacitive element (24, 24') being connected to the feed conductor (22) with its one pole and being connected between the diodes of the supplying half diode bridge with its other pole.

5. The system according to claim 4, characterized in that the energy-transfer circuit (50) also comprises an energy-reconductor capacitive element (28, 28'), and the system comprises a means suitable for leading off alternating current energy, to which means the energy-reconductor capacitive element (28, 28') are connected via a lead-off conductor (26).

6. The system according to claim 5, characterized in that the energy-reconductor capacitive element (28, 28') in the energy-transfer circuit (50) is connected to the negative pole of the energy-storing unit (10).

7. The system according to claim 5, characterized in that the energy-reconductor capacitive element (28, 28') in the energy-transfer circuit (50) is interconnected between the diodes of a lead-off half diode bridge interconnected between the poles of the internal feed capacitor (56).

8. The system according to claim 7, characterized in that the switching means comprises releaser switches (64, 66) by-passing the diodes of the supplying half diode bridge in a controlled manner, and/or reconductor switches (68, 70) bypassing the diodes of the lead-off half diode bridge in a controlled manner.

9. The system according to any of claims 1 to 8, characterized in that the control circuits (40) of the energy-control units (12) are in a chain-like, voltage isolated, two-directional communicating connection with the central controller (16).

10. The system according to claim 9, characterized in that the voltage isolation is realized by means of coupling capacitors (18) connected between the individual control circuits (40).

11. The system according to claim 7, characterized in that the central alternating current energy source supplies voltage exceeding a frequency of 1 kHz, preferably in rectangular signal form, and the switching means are controlled by pulse-width or frequency modulation.

12. The system according to any of claims 1 to 11 , characterized in that the energy- transfer capacitive element is an energy-transfer capacitor (24) enabling capacitive energy flow between the alternating current energy source and the energy-storing unit (10), and the energy-reconductor capacitive element is an energy-reconductor capacitor (28).

13. The system according to any of claims 1 to 11 , characterized in that the energy- transfer capacitive element is an energy-transfer oscillator (24') enabling resonant energy flow between the alternating current energy source and the energy-storing unit (10), and the energy-reconductor capacitive element is an energy-reconductor oscillator (28').

14. The system according to any of claims 1 to 13, characterized in that the energy- storing units (10) are battery cells, and the central controller (16) is configured for gathering information about the charge status of the individual battery cells, and sending commands based on the received information to the control circuits (40) to charge the battery cells or to balance their charge levels relative of each other.

15. A method for charge balancing and/or charging electrical energy-storing units (10) connected in series, comprising the steps of gathering information about the charge status of the energy-storing units (10), sending the information to the central controller (16), and based on this information modifying the charge levels of the energy-storing units (10) in a selective manner by means of the central controller (16) via energy-control units (12) assigned to the energy-storing units (10), c h a r a c t e r i z e d in that charge balancing and/or charging is realized at least in part from an alternating current energy source, by selectively connecting an energy-transfer capacitive element (24, 24') enabling energy flow between each of the energy-storing units (10) and the alternating current energy source.

16. The method according to claim 15, characterized in that the energy-transfer capacitive element (24,24') is connected by means of a switching means controlled via pulse-width or frequency modulation.

17. The method according to claim 16, characterized by determining the charge balancing and/or charging demand of the energy-storing units (10) by means of the central controller (16), sending a charge balancing and/or charging command corresponding to the given charge balancing and/or charging demand to the energy-control units (12) assigned to the energy-storing units (10), and connecting the energy-transfer capacitive element (24, 24') within the energy-control units (12) by the switching means via pulse-width or frequency modulation of a charge factor corresponding to the command.

18. The method according to claim 17, characterized by applying

- an internal feed capacitor (56) chargeable by the energy-transfer capacitive element (24, 24') and establishing an internal feed point (57) between the individual energy-storing units (10) and the alternating current energy source, as well as

- a switching means having switching elements enabling in a controlled manner the charge of the internal feed capacitor (56) or the charge of energy stored therein into the given energy-storing unit (10).

19. The method according to claim 18, characterized by controlling a switching means comprising switching elements enabling releasing of energy from the internal feed capacitor (56) towards the alternating current energy source.

20. The method according to any of claims 15 to 19, characterized by establishing communication between the central controller (16) and energy-control units (12) comprising the energy-transfer capacitive element (24, 24') and the switching means and being interconnected between the poles of the energy-storing units (10).

21. The method according to any of claims 15 to 20, characterized in that by means of the alternating current energy source a voltage of a frequency exceeding 1 KHz is provided preferably in rectangular signal form.