WO2016173968A2

WO2016173968A2 - Method for electrically charging a battery using an intermittent energy source and corresponding charge control device

Info

Publication number: WO2016173968A2
Application number: PCT/EP2016/059135
Authority: WO
Inventors: Georges KANAAN; Fathia KAROUI HENCHIRI; Laurent Vinit
Original assignee: Commissariat A L'energie Atomique Et Aux Energies Alternatives
Priority date: 2015-04-27
Filing date: 2016-04-25
Publication date: 2016-11-03
Also published as: FR3035550B1; WO2016173968A3; FR3035550A1

Abstract

The invention relates to a method for electrically charging a battery using an intermittent energy source, according to a time-variable charge profile. The following steps are performed iteratively at successive instants: a) determination of a limit charge current on the basis of a variable characteristic of a current charge level of the battery; b) acquisition of a current delivered by the source; c) a comparative test between the acquired current and the determined limit charge current; d) generation of a charge current having an amplitude that is either equal to the determined limit current if the acquired current is greater than or equal to the limit current, or that is equal to the current supplied by the source if the acquired current is less than the limit current.

Description

TITLE: Method of charging a battery with an intermittent energy source and corresponding charge control device

The present invention relates to a method for charging a battery and a device for controlling the electrical charge of a battery, for example a lithium-ion battery, lead, Ni-Cd or Ni-MH.

As a standard, a battery such as a lithium-ion battery is charged by the implementation of two successive charging phases:

a first charging phase of charging at a constant charging current until the battery voltage reaches an upper voltage limit, for example of the order of 4.2V; a second charging phase of charging at constant voltage until the charging current falls to about 3% of its nominal value.

Such a charging process is relatively slow. It typically requires several hours of charging. To reduce the charging time, however, it is not recommended to use a charging current that is too high, exceeding its nominal value, or a charging voltage higher than the upper limit of the battery voltage. Indeed, it would have the effect of degrading the battery, due to the accumulation of lithium metal at the electrode-electrolyte interface, which increases the internal resistance and reduces the life of the battery. To reduce the charging time of a lithium-ion battery while preserving the battery, it is known to apply charge current pulses separated by relaxation periods, as described in the document BK Purushothaman and U. Landau, "Rapid Charging of Lithium-Ion Batteries Using Pulsed Currents A Theoretical Analysis," J. Electrochem. Soc., Vol. 153, no. 3, pp. A533-A542, Mar. 2006. The amplitude and width of the pulses as well as the duration of the relaxation periods can be modified in order to influence the diffusion of Li ⁺ ions through the graphite / SE1 interface (of the English "Solid Electrolyte Interface ") and, therefore, vary the charging speed and battery life.

To charge a lithium-ion battery, it is possible to use a temporally intermittent source of energy, based for example on solar or wind energy, providing energy that is not constant and regular over time. In this case, the charging current available at the output of the energy source varies temporally: it may be insufficient or too high compared to a prescribed charge current prescribed according to the charging method.

A first solution consists in charging the battery only with the current delivered by the intermittent energy source by limiting the charging current to a threshold, for example 2A, when it exceeds this threshold. Similarly, the charging voltage can be limited to a voltage threshold.

A second solution is to charge the battery with the current delivered by the intermittent energy source, only when this current is between two predefined values, for example between 1.8 A and 2.2 A. We can proceed in a similar way for the charging voltage.

Both solutions are suitable for charging methods using a constant charging current. On the other hand, they prove to be less efficient when the charging is carried out by a current (or a voltage) which is not constant, for example in a pulsed manner and / or according to a temporally variable charge profile.

The present invention improves the situation.

For this purpose, the invention relates to a method for charging a battery electrically by an intermittent energy source according to a charge profile that varies over time, comprising a first configuration step, during which a charge curve is parameterized. reference that defines a load profile theoretical variable as a function of time, a second configuration step, in which a relationship between the reference load curve and a characteristic variable of the charge level of the battery is determined, and steps a), b), c) and d) following, which are performed iteratively, at successive times, until a predefined load, including a full charge, of the battery is obtained:

a) determining a load current limit using said reference curve and from the characteristic variable of the current battery charge level;

(b) acquisition of a current delivered by the source;

c) comparative test between the acquired current and the determined load limit current;

d) generating a charging current whose amplitude is equal to the determined limit current, if the acquired current is greater than or equal to the limit current, equal to the current {i _PV t _N )) supplied by the source (2 ), if the acquired current is lower than the limit current.

According to the invention, for each successive instants, the amplitude of the charging current is determined by the following compromise:

if the current delivered by the source is greater than or equal to a limit current, the amplitude of the charging current is equal to this limit current; if the current delivered by the source is lower than the limit current, the amplitude of the charging current is equal to the current delivered by the source.

In addition, the limit current is updated according to a variable characteristic of the current charge level of the battery.

As a result, the battery is charged according to a real charge profile that is as close as possible to a predefined charge profile. Thus, one can preserve the life of the battery and / or improve the speed of the load. As a standard, a load, for example a pulse load or a continuous load, follows a load profile which is defined by a reference load curve. This is used to determine the updated load limit current for each pulse from the characteristic variable of the current battery charge level.

The characteristic variable of the current charge level of the battery can be a state of charge or SOC (of the English State of Charge) of the battery, the SOC makes it possible to represent the level of charge of the battery with a lot of precision regardless of the temperature, current and / or aging of the battery.

Advantageously, during step a), the load limit current is determined by determining the point of the corresponding reference load curve by said relation to the characteristic variable of the current charge level of the battery.

In a particular embodiment, during the second configuration step, the battery is charged by applying a charge current whose amplitude is defined by the reference charge curve, and the evolution of said charge is determined in parallel. characteristic variable of the current charge level of the battery. Advantageously, the reference load curve defines a load current whose amplitude is decreasing over time.

The state of charge of the battery can be determined by a method of the group comprising a coulometric method of temporal integration of the charging current and a method of temporal derivation of the voltage of the battery. In a particular embodiment, the battery is charged by pulses of electric current and, for each current pulse (Ibatt (N)), the steps a) to d) are performed. In this case, advantageously, the reference load curve is a current pulse envelope curve.

In another particular embodiment, the battery is charged by a direct current (non-pulsed).

Advantageously, the method is applied to one of the batteries of the group comprising a lithium-ion battery, a lead battery, a Ni-Cd battery and a Ni-MH battery. The invention also relates to a device for controlling an electric charge of a battery by an intermittent energy source according to a time-varying charge profile, comprising a module for parameterizing a current reference charge curve. which defines a theoretical variable load profile as a function of time, a module for determining a relation between the points of the reference load curve and a characteristic variable of a battery charge level, a module of determining a load current limit by means of said reference curve and from the characteristic variable of the current charge level of the battery, a module for acquiring a current delivered by the source, a module of test for comparing the acquired current and the determined load limit current, and a generation module, from the current delivered by the source, of an electric charge current whose amplit ude is equal to the determined limit current, if the acquired current is greater than or equal to the limit current, equal to the current supplied by the source, if the acquired current is lower than the limit current. The load control device advantageously comprises all or part of the following additional features:

the load current limit determining module is arranged to determine the point of the corresponding reference load curve by said relation to the characteristic variable of the current charge level of the battery;

the generation module is arranged to generate charging electric current pulses. The invention will be better understood with the aid of the following description of a particular embodiment of the method of charging a lithium-ion battery with an intermittent energy source according to the invention, with reference to the accompanying drawings on FIG. which :

FIG. 1 represents a flowchart of the steps of the charging method, according to a particular embodiment of the invention;

FIG. 2 represents a reference envelope curve of charge current pulses assuming that the power generation is sufficient for the duration of the charge;

FIG. 3 represents the time evolution of the state of charge of the battery, corresponding to a charge according to the envelope curve of FIG. 2;

FIG. 4 represents an example of the time evolution of the current supplied by the intermittent energy source for several hours and a real envelope curve of charge current pulses obtained according to the invention;

FIG. 5 represents the temporal evolution of the state of charge by application of charge current pulses according to the real envelope curve of FIG. 4;

FIG. 6 represents a functional block diagram of a system comprising an intermittent energy source, a charge control device and a battery, according to a particular embodiment of the invention. The charging method of the invention makes it possible to charge a lithium-ion type battery 1, for example a Samsung-22F lithium-ion battery, using an intermittent energy source 2, according to a profile. temporally variable charge.

By intermittent energy source, we mean a source which, during production, produces energy that is not constant and regular over time. For example, it may be a photovoltaic device for transforming solar energy into electrical energy or a wind device for transforming wind energy into electrical energy. The electrical energy produced is intermittent. In the particular embodiment described here, the intermittent energy source 2 comprises a photovoltaic device.

In the particular embodiment described here, the charge of the battery 1 is impulse. In other words, the battery 1 is charged by a succession of pulses of charging electric current, denoted I _ba tt ( ^N ) where N represents the order number (or index) of a pulse, N being here equal to 0, 1, 2, ....

The photovoltaic device 2 produces at the output an electric current i _PV t) which varies over time as a function, in particular, of the solar irradiation, the characteristics specific to the device 2, and possibly wind. This current i _PV t) is intended to be used to charge the battery 1 via a charge control module 3.

The charge control module 3 is intended to receive as input the electric current i _PV t) delivered by the photovoltaic device 2 and to output current pulses l _ba tt ( ^N ) of charge of the battery 1. It is adapted to implement an algorithm for determining the amplitudes of the charging current pulses I _ba tt W, as will be described later. The Charge current pulses can be separated from each other by pauses (at zero load current) or possibly by discharge phases. With reference to FIG. 1, during a first configuration step E0, a reference load curve, denoted by l _C h_ref, or envelope curve, is parameterized. In the embodiment described here, the reference load curve constitutes a charge current pulse envelope curve in the charge control module 3. This reference load curve represents the evolution of the current amplitude. charge (here the evolution of the amplitudes of the charging current pulses) as a function of time (or here, alternatively, as a function of the pulse order number N) in the theoretical hypothesis where the solar irradiation received by the photovoltaic module 2 would be sufficient to allow charging the battery 1 according to the limit of the reference curve. In other words, if the solar irradiation is sufficient, the amplitude of the charging current is defined by points of this reference curve _C the h_ref- In this case, the respective amplitudes of successive charging current pulses, ^ _N where t _N represents the moment of application of the index pulse N, are defined by points of the reference curve l _C h_ref-

The reference charge curve l _C h_ref is stored in a memory of the control module 3. It comprises a set of points, each of these points being defined by a current amplitude expressed in amperes and by a time t (or here a order number N). It can be defined by an equation and / or by data relating to the points of the curve.

The reference load curve l _C h_ref, or envelope curve, defines a theoretical or virtual load profile as a function of time. It represents a charge according to a temporally variable profile, that is to say to charge current (or similarly to charge voltage) not constant in time. Thus, according to this theoretical load profile, the charging current varies over time. For example, the charging current decreases with time. In the In the case of a pulse load, it follows that the charge current pulses have amplitudes that vary over time. For example, successive pulses have decreasing amplitudes. The variation of the charging current may for example be of the exponential or linear type. In the case of an exponential variation, the envelope curve is defined by an equation containing an exponential factor as a function of the time or the sequence number of the pulse, for example:

^bxN + c axis

or

- N is the sequence number of a charge pulse;

- / (N) is the speed (or amplitude) of the ^Nth load pulse;

a is a negative real of absolute value strictly less than 1;

b is a positive real of absolute value strictly less than 1;

- it is a real positive value close to a maximum current.

In the case of a linear variation, the reference load curve may comprise a line segment with decreasing slope as a function of time (or here of the pulse order number) or preferably several line segments having slopes. successive more and more important over the load. In particular, the reference load curves may be similar to the envelope curves described in French Patent Application No. 1462984.

As will be explained hereinafter, the reference load curve I _C h_ref makes it possible to determine and update, or update, a load limit current. The load current limit can be updated at successive times. In the embodiment described here, the load limit current is updated for each current pulse to be applied. An example of envelope curve l _C h_ref is illustrated in FIG. 2 as a function of time t. This curve is defined by the following relation (as a function of the pulse sequence number): -0.0183 x _e ^{0 002xw} + 1.018. In Figure 3, has represented the temporal evolution of the corresponding state of charge (SOC), that is to say during an impulse load according to the load profile defined by the envelope curve the h_ _C _ref of Figure 2. in a second configuration step E1, a relationship between the reference load curve the _C h_ref (or envelope curve) and a characteristic variable of the battery charge level is determined 1. In the embodiment described here, the characteristic variable of the charge level of the battery 1 is the state of charge (SOC) of the battery 1. However, any other variable characterizing the level of charge of the battery 1 could be used.

During step E1, a preliminary charge of the battery 1 is carried out by means of a charging current according to the charging time profile defined by the reference curve I _C h ref of FIG. 2. battery 1 with a charging current whose amplitude is defined by the reference curve the _C h_ref- in the embodiment described herein, is charged by means of charging current pulses. The amplitudes of the successive current pulses correspond to successive points of the envelope curve. To implement the configuration step E1, the empty battery 1 (SOC = 0%) is connected to a power source capable of supplying a sufficient electric current, via the charge control module 3. During the preliminary load, the module 3 generates the charging current (here current pulses) from the current supplied by the source, according to the charging time profile defined by the reference curve l _C h_ref-

During this preliminary charge, the control module 3 determines in parallel the evolution of the characteristic variable of the charge level of the battery 1, here the SOC or state of charge of the battery 1. The state of charge or SOC of the battery 1 is evaluated, in known manner, for example by a coulometric method based on the temporal integration of the charging current applied to the battery 1 or, in the case of an impulse load, by a method based on the calculation of the derivative of the voltage across the battery as described in document FR2908243.

From the reference load curve l _C h_ref as a function of time and of the temporal evolution of the state of charge or SOC of the battery 1 during the preliminary charge, the charge control module 3 determines a relation between the curve reference load l _C h_ref and state of charge SOC in the form of a function / such that:

h ref ⁼ fi ^ OC) The / function could alternatively be determined by calculation or by simulation.

We will now describe a charging process according to a particular embodiment of the invention. In the particular example described here, the load is impulse. This charging process comprises a succession of steps E3 to E10 or E1 1 intended to be performed iteratively, or in a loop, in order to determine the amplitude of the charging current at successive times. We denote N the index of each loop or iteration of execution with N = 0, 1, 2, Here, N also represents the order number of a load pulse.

Initially, during a step E2, the battery 1 is empty (in other words, its SOC is zero) and it is posited that:

- N = 0;

- Iiim.o = lch_ref (to), where t ₀ represents the start time of load and I Mm_o corresponds to an initial limit current equal to the value of the reference envelope l _C h_ _re f at the initial moment t ₀ as shown in Figure 2.

The following steps E3 to E10 are executed in a loop at successive instants, here every x seconds. The value of x is for example less than or equal to 30 seconds. Step E3 is a step of acquiring the current produced by the intermittent energy source 2. For example, the control module 3 measures the current i _PV {t _N ) at the output of the photovoltaic device 2. The instant I N is such that:

t _N = to + Nx

During a test step E4, the acquired current i _PV {t _N ) is compared with a limit current ln _m , N valid for the loop of index N being executed. When executing the first loop (N = 0), the current limit ln _m , o is equal to l _C h_ref (to) - If the current produces i _PV t _N ), where N = to + Nx, is greater than equal to the limit current l | _{im, N} valid for the index loop N being executed (branch "Yes" in FIG. 1), then the amplitude of the charging current (in this case the amplitude of the pulse of current of index N or of order number N to be applied), denoted I _b a _tt (N), at the limit current value l | _{im, N} , during a step E5. Thus, at time t _N , the amplitude of the charging current (in this case the amplitude of the current pulse of order N) is limited to the limiting current I | _{im, N.}

If the current produced i _PV {t _N ) is strictly lower than the limit current l | _{im, N} valid for the index loop N being executed (branch "No" in FIG. 1), then the amplitude of the load current to be applied (here to the current pulse of index N or of order number N) the value of the current i _PV t _N ), produced by the photovoltaic module 2, during a step E6.

During a step E7, the control module 3 generates a charge current (in this case a charge current pulse) having the amplitude determined in step E5 or E6, from the current delivered by the device photovoltaic, and applies this current to the battery 1.

The method then proceeds to an ACTU operation for updating or updating the limit current l ii _m , N + 1 valid for the next loop of index N + 1. The current limit update comprises several steps E8 to E10 described below. The updating operation ACTU first comprises a step E8 for determining the current state of charge of the battery 1, denoted SOC _N , following the application of the charging current I _b att (N). The state of charge SOCN can be determined by different methods of calculation, for example:

a coulometric method based on the calculation of the capacity of the battery at the instant t current by integration of the applied current in time;

a calculation method based on the evaluation of the time derivative of the voltage at the terminals of the battery, as described in the patent FR2908243.

The step E8 for determining the state of charge SOC _N current of the battery 1 is followed by a test step E9 intended to check whether the battery 1 is not completely charged (in other words if SOCN <1 00% ).

In the case of a positive test E9 (branch "Yes" in FIG. 1), in other words if the battery 1 is not completely charged (SOCN <100%), the process proceeds to a step E1 0 for determining the limiting current lii _m , N + i valid for the next loop of index N + 1. The limiting current l ii _m , N + i is determined by means of the function / predetermined in step E1, as a function of the current charge state SOC _N determined in step E8, by the relation next :

Taking into account the relation between the reference load curve and the state of charge, such that l _ref = f (SOC), during step E1 0, the point of the reference curve I is finally determined. _{ch ref} which corresponds by the relation / to the state of current load SOC _N of the battery 1. This determined point of the reference envelope curve constitutes the load limit current lu _{m> N + 1} valid for determining the next amplitude of the charging current (here the amplitude of the load pulse of index N + 1). In case of negative E9 test (branch "No" in Figure 1), ie if the battery is fully charged (SOC _N = 100%), the process goes to an end step E1 1. Following step E10, the loop index is incremented by 1 to go to

N + 1. The method then returns to the step E3 of acquisition of the current produced by the photovoltaic module, i _PV (t _{N + 1} ), for the moment following N + I. The steps E4 to E10 or E1 1 are then repeated for the index N + 1. The process ends here when the battery is fully charged, its state of charge having reached 100%. One could however consider stopping the load to a predefined maximum load state and less than 100%.

FIG. 4 shows, as a function of time t:

The time evolution of the current i _PV t) produced at the output of the photovoltaic module 2;

- The curve l _C h_reei real evolution of the amplitude of the charging current, corresponding here to the actual envelope curve of the successive charging current pulses, as a function of time;

different theoretical curves of time evolution of the charging current (in this case envelopes of charge pulses), l _C h_thi, l _C h_th2 and l _C h_th3, corresponding to different states of charge of the battery 1, having portions with the real evolution curve lch_reel-

FIG. 5 shows the temporal evolution of the state of charge of the battery 1 during an impulse load according to the real envelope 1 _C h_reei represented in FIG. 4. We will now describe, with reference to FIG. 6, the load control device 3 according to a particular embodiment of the invention. The charge control device 3 comprises:

an input interface 30 for connection to a power source,

a first configuration module 31,

a second configuration module 32,

a module 33 for acquiring the current i _PV t) delivered by the source,

a test module 34,

a module 35 for adjusting the charging current I _b att (N),

a module 36 for updating or updating a current limit l | _{im, N} ,

a module 37 for generating the charging current,

an output interface 38 for connection to a battery to be charged,

- and a user interface 39.

The charge control device 3 is intended to be interposed between a power source, for example the photovoltaic module 2, and a battery to be charged, for example the battery 1.

The first configuration module 31 allows a user to set a reference load curve l _C h_ref as a function of time (or, alternatively, in the particular case of an impulse load, depending on a number of pulse) by means of the user interface 39. The configuration module 30 is arranged to implement the step E0. It integrates a data storage memory relating to the reference load curve. The second control module 32 is adapted to control the execution of step E1 in order to determine the function / defining the relationship between the reference load curve the _C h_ _ref and the SOC of the battery state of charge 1. The acquisition module 33 is adapted to measure the current i _PV t) at the output of the photovoltaic module 2 at successive instants t _N. It is intended to implement step E3. The test module 34 is adapted to implement the test step E4. Thus, the test module 34 is able to compare the acquired current i _PV {t _N ) with a limit current I _{Um> N} valid for the loop N being executed.

The adjustment module 35 is intended to adjust the amplitude of the charging current as a function of the result of the test performed by the module 34. It is thus adapted to implement the steps E5 and E6. The update or update module 36 is adapted to update the value of the limit current. It is intended to implement steps E8, E9 and E10. It incorporates a block 360 for determining the state of charge of the battery and a block 361 for calculating the current limit ½ _m , w + i © ⁿ depending on the state of charge SOC _N , using the function / determined by module 32.

The charge current generation module 37 is intended to generate a charging current (for example current pulses or a direct current), from the current delivered by the energy source to which the device 3 is connected. the amplitude of the generated charging current is equal to the determined limit current ln _m , N, if the acquired current is greater than or equal to this limit current, equal to the current i _PV {t _N ) supplied by the source, if the current i _PV {t _N ) delivered by the source is lower than the limit current ln _m , N.

The generated charging current is transmitted to the battery to be charged through the output interface 38.

The user interface 39 conventionally comprises a keyboard, a screen, a graphical interface for entering and retrieving data. Of course, a central control unit, not shown, is intended to control the operation of the elements 30 to 39 of the load control device 3. It is intended in particular to control the execution of iteratively, at successive times, steps E3 to E10 by the modules 33, 34, 35 and 36, until a predefined load, for example a complete load. The invention could be applied to a battery other than a lithium-ion battery, in particular to lead, Ni-Cd or Ni-MH batteries.

In the particular embodiment which has just been described, the load is impulse. The invention also applies to a continuous load (that is to say non-impulse) according to a charge profile that varies over time.

Claims

Method for electrically charging a battery (1) using an intermittent energy source

(2) according to a load profile varying over time, comprising a first configuration step (E0), during which a reference load curve is parameterized which defines a theoretical variable load profile as a function of time, a second step configuration, during which a relationship is determined between the reference charge curve (l _C h_ref) and a characteristic variable of the battery charge level, and the following steps a), b), c) and d), which are executed iteratively, at successive times, until a predefined charge, in particular a complete charge, of the battery (E1 1) is obtained: a) determination (E2, E10) of a charging limit current at the using said reference curve and from the characteristic variable of the current charge level of the battery (E8);

b) acquisition

(E3) a current {i _PV t _N )) delivered by the source (2);

c) comparative test (E4) between the acquired current {i _PV t _N )) and the determined load limit current;

d) generation (E7) of a load current whose amplitude is either equal to the determined limit current, if the current acquired is greater than or equal to the limit current, or equal to the current {i _PV t _N )) supplied by the source (2), if the acquired current is less than the limit current.

Method according to the preceding claim, characterized in that the characteristic variable of the battery charge level is a state of charge (SOC _N ) of the battery (1).

Method according to one of the preceding claims, characterized in that, during step a), the load limit current is determined, by determining the point of the reference load curve corresponding by said relationship to the characteristic variable of the current battery charge level.

4. Method according to one of the preceding claims, characterized in that, during the second configuration step (E1), the battery (1) is charged by applying a charging current whose amplitude is defined by the reference charge curve (l _C h_ref) and the revolution of said characteristic variable of the current charge level of the battery (1) is determined in parallel.

5. Method according to one of the preceding claims, characterized in that the reference load curve defines a load current whose amplitude decreases over time.

6. Method according to one of claims 2 to 5, characterized in that the state of charge of the battery is determined by a group method comprising a coulometric method of temporal integration of the charging current and a derivation method time of the battery voltage.

7. Method according to one of the preceding claims, characterized in that the battery (1) is charged by means of pulses of electric current and, for each current pulse (I _b att(N)), the steps are carried out a) to d).

8. Method according to the preceding claim, characterized in that the reference load curve is an envelope curve of current pulses.

9. Method according to one of the preceding claims, characterized in that it is applied to one of the batteries from the group comprising a lithium-ion battery, a lead-acid battery, a Ni-Cd battery and a Ni-MH battery .

10. Device for controlling an electrical charge of a battery (1) by an intermittent energy source (2) according to a charge profile varying over time, comprising a module for setting a reference charge curve of the charging current which defines a theoretical variable charging profile as a function of time, a module for determining a relationship between the points of the reference charging curve (l _C h_ref) and a variable characteristic of a battery charge level, a module (36) for determining a charging limit current using said curve reference and from the characteristic variable of the current charge level of the battery, a module (33) for acquiring a current {i _PV t _N )) delivered by the source (2), a test module ( 34) intended to compare the acquired current {i _PV t _N )) and the determined charging limit current, and a module (37) for generating, from the current delivered by the source, an electric charging current of which l the amplitude is either equal to the determined limit current, if the acquired current is greater than or equal to the limit current, or equal to the current {i _PV t _N )) supplied by the source (2), if the acquired current is less than the limit current .

Device according to the preceding claim, characterized in that the module (36) for determining a charging limit current is arranged to determine the point of the reference charging curve corresponding by said relationship to the characteristic variable of the current charging level drums.

Device according to one of claims 10 and 1 1, characterized in that the generation module is arranged to generate pulses of electric charging current.