WO2008154960A1

WO2008154960A1 - Battery charging method with constant current and constant voltage

Info

Publication number: WO2008154960A1
Application number: PCT/EP2007/056206
Authority: WO
Inventors: Stephan Leuthner; Philipp Kohlrausch; Jasim Ahmed; Mario Roessler; John Christensen
Original assignee: Robert Bosch Gmbh
Priority date: 2007-06-21
Filing date: 2007-06-21
Publication date: 2008-12-24

Abstract

The invention relates to a method for charging a battery by delivering electrical energy to the battery in a charging step, the charging step comprising a constant voltage step, during which a constant charging voltage is applied to the battery, and a current charging step, during which a specific charging current is applied to the battery. One and the same charg- ing step comprises the constant voltage step and the current charging step: The constant voltage step is carried out before the current charging step and the charging step is com- pleted upon completion of the current charging step.

Description

Title

Battery Charging Method with Constant Current and Constant Voltage

Background of the invention

The technical field of the invention is the operation of rechargeable batteries, in particular a method for charging a battery to its full capacity in the shortest possible time without damaging the battery.

Batteries are typically charged at a constant current until the voltage at the battery terminals reaches a specified cut-off voltage. After having reached the specified cut-off voltage, the battery is charged with a constant voltage, for example the voltage which has been reached in the constant current charging process. The change from constant current to constant voltage after having reached a specified cut-off voltage is a measure known in the art to avoid overcharge situations.

However, finishing the charging process with a constant voltage that eliminates the risk of overcharging leads to a tapering current and it takes an infinite amount of time (due to an asymptotic progress) for the open circuit potential to reach a state of charge of 100 %. Therefore, terminating the charging process with a constant voltage leads to a long duration of the complete charging operation.

Another approach to avoid damages due to overcharging situations is to use a pulse charg- ing mechanism. According to this approach, the average charging current is reduced according to the duty cycle of the current pulses. However, this approach is based on an asymptotic approximation to the maximum voltage in order to avoid overcharging situations, too.

In US 5,828,202, US 5,905,364 and "Charging, Monitoring and Control" by van Schalk- vijk, Advances in Lithium-ion Batteries, Eds. Kluwer Academic, New York, 2002, charging mechanisms for rechargeable batteries are disclosed which all terminate with a constant voltage charging step. In "The effects of pulse charging on cycling characteristics of commercial lithium- ion batteries", by Li, J. et al, Journal of Power Sources, 102 (2001) 302 and "Rapid Charging of Lithium-ion Batteries Using Pulsed Currents: A Theoretical Analysis" by Purushotaman, B. K. et al, Journal of the Electrochemical Society, 153, (2006) 533, pulse-charging mechanisms are described in which the duty cycle of the applied current is reduced when approaching the end of the charging process.

It is therefore an object of the invention to reduce the overall charging time for rechargeable batteries, in particular to provide a method with an accelerated charging step at the end of the charging operation.

Summary of the invention

According to the invention, a battery is charged in a charging step, the charging step comprising: a voltage charging step, in which a specific charging voltage is applied to the battery, and a current charging step, in which a specific current is applied to the battery. In contrast to the state of the art, the last step, i.e. the step carried out upon termination of the charging operation, is a current charging step. According to the invention, the constant voltage charging step is carried out before the current charging step. In order to avoid damages (by overcharge situations), the current charging step is terminated or completed until a specified total amount of charge has been transferred to the battery. This order of charging steps ensures that the open circuit potential of the battery or the state of charge of the battery reaches the desired value within a reduced period of time. Thus, the invention provides a fast charging method, in particular at the time period preceding the termination of the charging step.

In a preferred embodiment, the specific charging current of the current charging step is a constant current, a current linearly decreasing as a function of time, i.e. a ramp, or a current with a predefined progression. Alternatively, a model as defined in this description could be used to periodically determine the appropriate charging current, charging voltage, and/or the change between current charging step and voltage charging step or vice versa. The constant voltage step can also by a voltage charging step with a charging voltage having a predefined progression.

In the following, the invention is described with constant voltage / constant current steps for the sake of simplicity. Of course, the constant voltage step could be any voltage charging step as defined above or as defined in the claims. Further, the constant current step could be any current charging step as defined above or as defined in the claims. In this description and in the claims, the term "charging step" alone, without the prefixes "voltage" or "current", denotes the combination of at least one current charging step and at least one voltage charging step.

Preferably, the battery voltage (i.e. the voltage present at the battery terminals) used within the constant voltage step is monitored or tracked, and the current supplied to the battery in the constant current step is determined, known or measured. In this way, the SOC (state of charge) is continuously monitored and is preferably used as criterion for terminating the last charging step, i.e. the constant current step. In combination with the duration of the constant current step, i.e. the time between the beginning of the last constant current step and the end of the last constant current step before terminating the charging step, any overcharging situations can be avoided. Further, the voltage and the current parameters are chosen as to avoid detrimental and/or undesired side reactions which are pertaining to the over- current situation and/or are pertaining to battery operations with impaired efficiency. Examples for an impaired efficiency are the reduction of battery capacity or other side effects like deposition of Lithium at the negative electrode of a Lithium-ion battery. Together with the duration of the time period of the constant current step, the constant current can be selected according to known relationships realized in a table or in approximation equations. In the constant current step, the voltage at the battery terminal, in particular the behavior of the voltage over time is tracked and examined in order to detect the begin of undesired side reactions or other situations which would impair the capacity characteristics of the battery.

Further, the temperature of the battery as well as the progress, e.g. the change rate, of the temperature is monitored to detect unwanted mechanisms in the battery. In general, for this detection, internal parameters are estimated from external parameters using models, approximations or other interrelationships, preferably based on reactions taking place in the battery.

Additionally or alternatively, the internal resistance of the battery is monitored by determining and/or tracking the current as well as the voltage at the battery and by monitoring the ratio of the voltage to the current. Of course, if the constant voltage or the constant current is used, this constant value does not have to be measured or determined but is already known. Preferably, the charge transferred to the battery is tracked by constantly integrating or periodically summing a current value over time which is known or which is constantly or periodically measured. In this way, overcharge situations can be determined by amount of charge transferred to the battery in relation to the capacity of the battery. Thus, the state of charge (SOC), i.e. the relationship between available amount of charge to overall capacity, is monitored. According to one embodiment, the constant current step is terminated after the state of charge (SOC) of the battery has reached a certain limit, i.e. 90 %, 92 %, 95 % or 98 %. The overall capacity of the battery can be estimated or determined by monitoring respective operational parameters of the battery during a discharge or charge process. For estimating the overall capacity of the battery, the same parameters can be used which are also used for the detection of unwanted side effects or overcharge situation as described above. In the same way, the SOC can be monitored by tracking the charge transfer to the battery as well as by monitoring the operational parameters which are also used for determining the occurrence of undesired side effects or overload situations.

Preferably the charging parameters, e.g. the duration of one or more constant voltage steps, of one or more constant current steps, the magnitude of the constant current and/or the magnitude of the constant voltage, are optimized in view of physical constraints of the battery. The physical constraints give the maximum allowable charging parameter that leads to an operating point close to an operating point related to undesired side reactions. The maximum allowable charging parameter can be related to an acceptable degree of undesired side reactions, to a negligible degree of undesired side reactions or to an operating point spaced apart from parameters related to undesired side reactions and located within a safety margin.

Thus, a preferred embodiment of the method does not only monitor external operational parameters, e.g. the charging current flowing into the battery, the voltage at the terminals of the battery and the temperature measured at a surface of the battery, and adequately sets the charging voltage / charging current. Rather, the selection of the operating point (defined by charging current, terminal voltage, battery temperature...) is focussed on internal parameters and/or reactions taking place in the battery. A relationship is used to map the external parameters onto internal parameters, the relationship being based on a model, on empirical data, approximations, or on numerical extrapolation. The same model, empirical data or numerical approximation / extrapolation can be used to determine, if the internal parameters are within allowable intervals or if the internal parameters are close to or in areas, which correspond to undesired side effects, e.g. lithium deposition, solvent, salt or impurity reduction or solvent, salt or impurity oxidation. These side effects give the internal physical constraints for the internal parameters.

Examples of the internal parameters are the electrode-electrolyte potential drop in the negative electrode, which has to be maintained above certain limits to prevent or minimize side reactions that can occur at the negative / positive electrode. In order to avoid lithium deposition, the potential drop must be above 0 V vs. the Lithium metal potential. To avoid sol- vent, salt, or impurity reduction at the anode, the potential drop must be maintained above a certain limit defined by the open-circuit potential (OCP) of the side reaction. In this example, the OCP is one of the internal parameters essential for deriving an internal status of a battery model. This limit is preferably necessarily below the OCP of the respective side reac- tion, but it should be chosen to minimize the extent of reaction such that the resulting unde- sired reactions can be neglected or tolerated. The extent of reaction can be determined by an appropriate kinetic relationship, e.g., the Butler-Volmer or Tafel equations known to a person skilled in the art. Further, these relationships can be part of a model and/or can be implemented as empirical data reflected by numerical approximations, tables, or extrapola- tions. Further, heuristic algorithms, neural networks and fuzzy logic can be used for extrapolation, approximation or model mapping.

A further example of the internal parameters is the electrode-electrolyte potential drop in the positive electrode: This must be maintained below certain limits to prevent or minimize oxidation side reaction at the positive electrode. To avoid solvent, salt, or impurity oxidation, the potential drop must be maintained below a certain limit, similar to the strategy regarding the electrode-electrolyte potential drop in the negative electrode.

Another example of the internal parameters is the internal temperature and rate of heat gen- eration, which must be maintained below certain safety thresholds. The thresholds are dependent on the chemistry of the battery and could be defined based on empirical and/or model-based evidence for safety hazards and/or premature degradation of the cell. An energy balance on the cell relates measurable quantities, i.e. external parameters (e.g. external temperature, voltage, and current) to the internal temperature and rate of heat generation. The energy balance can be a part of the model, which can be implemented as one equation of a system of equations, which reflects the behaviour of the battery, or can be implemented as a table, approximation functions or recursive approximation algorithms, which can be at least partly based on empirical data.

In order to determine whether the constraints are satisfied, i.e. if the internal parameters match to allowable values or limits, preferably an electrochemical model of the battery is used reflecting, emulating or simulating the most important or all known behaviours of the battery. The electrochemical model that includes material balances, charge balances, energy balances, and/or constitutive equations for mass transport, charge transport, and kinetics is used to calculate internal battery states such as the temperature, state of charge (SOC), and electrode and electrolyte potentials based on the temperature of the environment and the current that is passed through the cell. These examples of internal states cannot be meas- ured directly for a cell, but are essential in order to determine the status and the resulting maximum allowable charging current / charging voltage of the battery.

As an additional or alternative measure for determining whether the constraints are satisfied, external parameters or states can be used. Preferably, these states can be directly measured or can be determined indirectly. E.g. the overall amount of charge transferred to the battery can be determined indirectly, by integrating the charging current. Further examples of these external parameters or status are the current passed through the battery, the voltage of the battery, and the temperature external to the battery, e.g. the temperature of an outer surface of the battery. These states are related to the internal states via model equations, approximations, or empirical data / extrapolation.

According to another embodiment of the method, the method comprises a charging step with a plurality of constant current steps and constant voltage steps which are carried out alternatingly and repeatedly within the same charging step. A constant current step is terminated and a constant voltage step is started if one of the monitored operational parameters shows a specific behavior which can be exceeding or undershooting a certain limit, e.g., if the temperature or the voltage in the constant current step reaches a certain limit or if the rate of change of these or of other operational parameters exceeds a certain threshold, e.g. if the voltage at the battery terminals suddenly shows a steep edge. According to the model, the occurrence of such an event is explicitly or implicitly linked with a side reaction implemented in the model. Preferably, the constant voltage step comprises to charge the battery with a constant voltage value which corresponds to the voltage at the battery terminals at the end of the constant current step or corresponds to this value minus a certain margin. In the same way, the constant voltage step is terminated and a constant current step is started if, in the constant voltage step, a certain operational parameter, for example the charging current, the rate of change of the charging current, the temperature and/or the amount of charge transferred into the battery during this step has reached a certain upper or lower limit which corresponds to a certain behavior relating to an undesired side effect or to a charging current lying below a lower limit. In particular, also behaviors can be detected and used as trigger for changing the constant current step to the constant voltage step and vice versa which occur before the undesired effect takes place or the overcharge situation begins. In this way, critical situations can be detected beforehand and measures can be taken (i.e. changing constant current to constant voltage and vice versa) in order to avoid a very low charging current or a very low increase of SOC (leading to a unacceptable long duration of the charging process). On the other hand, undesired side reactions or overcharge situations (caused by very high charging currents) can be detected beforehand. Alternatively or in combination thereto, the charging steps can be exchanged (i.e., a constant current step is terminated and a constant voltage step is started or vice versa) in order to avoid the occurrence of undesired side reactions, overcharge situation or too low charging currents. Further, the charging steps can be exchanged (i.e. a constant current is terminated and a constant voltage step is started or vice versa) if a certain amount of time has lapsed since the start of the respective constant voltage or constant current step.

According to an embodiment of the invention, a charging current and/or a charging voltage of a battery is determined by applying a model of the battery, the model reflecting the behavior of the battery. The model can be based on a set of equations providing all or most physical or chemical reactions in the battery. A first function of the model is to map external parameters onto internal parameters and vice versa such that external measurements can be interpreted in terms of internal processes and internal values (derived or estimated) can be measured as external parameters. A second function of the model is to implement side reactions, i.e. unwanted effects occurring e.g. in overcharge situations. These side reactions can be determined by comparing current internal parameters with limits related to side reactions. Due to the correspondence between internal and external parameters (1^st function), measured external parameters (in particular the developing of the parameters over time) provide information related to side reactions, e.g. the degree of occurrence of side reactions. Using these relations, the charging current can be exactly matched with the internal processes in the battery and the internal processes can be monitored and controlled directly. In contrast to a charging control based on external parameters only, this allows one to exactly define internal process parameters and to control the occurrence (or the inhibition) of side reactions in detail. In addition to a charging control based on the mapping of external parameters on internal parameters and on an model, a charging control deriving charging parame- ters from external parameters can be used in combination with the model or as redundant control entity.

The model can be implemented as an online simulation tracking internal parameters, external parameters and side reaction implemented by a set of variables defining a set of reactions in the battery. Further, the model can be implemented as numerical or arithmetical approximation reflecting the behavior of the battery, i.e. the internal processes of the battery. Still further, the model can be implemented by a table the entries of which reflect the behavior of the battery and provide relationships between external parameters and the charging current and/or the charging voltage, the relationships reflecting the side reactions and the processes in the battery. The implementation of the model can be based on empirical data and/or on physical laws. Additionally, interpolation and extrapolation algorithms can be used. In particular, extrapolation can be combined with a table of entries. In an embodiment, in which only a predefined table (in combination with an extrapolation algorithm) is used, the behav- ior is not simulated or tracked online, i.e. during charging. Rather, the behavior has been evaluated beforehand and is stored as entries in the table or as approximation equations or as parameters defining the approximation equations. In this case, the control is carried out by up-to-date internal and/or external parameters and by relationships and rules which have been completely predefined by a previous simulation and/or by empirical data related to the battery's behavior.

Based on the model, the internal/external parameters and the resulting error with regard to the limits, the voltage source or the current source providing the charging current / charging voltage of the battery are preferably controlled according to the control mechanisms such that the appropriate internal states, e.g. the desired current as estimated as mentioned above is passed to the cell at every step in the charging process. The updating of the model, the limits, the internal parameters and the external parameters can be performed by continuous, repeated or periodic calculation or measurement. In general, the reference value resulting from the application of the model on external and/or internal parameters with regard to the limits can be used as controlling parameter for the delivery of electrical energy. The delivery of electrical energy according to the control can be continuous, e.g. a current source delivering a current to the battery, which is constantly or periodically updated by the reference value. Alternatively, the delivery of the electrical energy according to the control can be in discrete steps, e.g. constant current and constant voltage steps, which are applied alternat- ingly and which are defined by discrete, single values.

In order to estimate the values of the internal parameters from the external parameters, appropriate relationships or mechanisms are used. Such mechanisms comprise a control strat- egy, which is implemented to use the measured external values of the voltage, current, and temperature to correct both, the measurable and immeasurable states of the model. Several approaches may be employed, including any approximate linear or recursive mechanism for error reduction, e.g. an algorithm for minimizing the mean square error, preferably a KaI- man filter, or an extended Kalman filter (for nonlinear systems) or a moving boundary esti- mator. Ideally, the electrochemical model perfectly replicates the physical processes of the battery, in which case no such filter is required. In an implementation, a suboptimal model could be used and error reduction algorithms could be used. There is a trade-off between the quality of the model and the requirements of the control strategy, such that both can be adapted to the requirements of the application.

In order to determine the difference between reference and instantaneous value regarding internal parameters (= instantaneous value) and the physical constraints, i.e. limits (= reference), estimates of the appropriate internal states (e.g., electrode-electrolyte potential drop at a particular location in the cell) are compared to the constraints, e.g. physical constraints defined above. The current passed through the cell is corrected or set in order to prevent the states of interest from moving beyond the constraints or limits. A single or multiple constraints may be employed for any given application. The particular errors between limit and instantaneous value of the particular internal parameters can be weighted according to relevance and averaged. The control mechanisms can be open loop control mechanisms or closed loop control mechanisms.

According to the control mechanisms, the model, the internal/external parameters and the resulting error with regard to the limits, the voltage source or the current source providing the charging current / charging voltage of the battery is preferably regulated such that the appropriate internal states, e.g. desired current estimated as mentioned above, is passed to the cell at every step in the charging process. The updating of the model, the limits, the internal parameters and the external parameters can be performed by continuous, repeated or periodic calculation or measurement.

In one embodiment of the invention, the state of charge, SOC, is determined or measured and provides the criterion for terminating the charging step. According to a preferred control structure or model, the SOC of the battery is linearly related to a subset of the internal states (i.e., the concentration of lithium in the solid phase of the electrodes) and can be calculated easily using the approach described above with regard to the determination whether the constraints are satisfied. Once the SOC exceeds the desired threshold (e.g. defined as 100% or 95 % of the battery's capacity), the charging step is terminated.

According to another embodiment which can be used with the embodiments described above, the method comprises a charging step with a plurality of constant current steps and constant voltage steps which are carried out alternatingly and repeatedly within the same charging step. A constant current step is terminated and a constant voltage step is started if one of the monitored operational parameters shows a specific behavior which can be ex- ceeding or undershooting a certain limit, e.g., if the temperature or the voltage in the constant current step reaches a certain limit or if the rate of change of these or of other operational parameters exceeds a certain threshold, e.g. if the voltage at the battery terminals suddenly shows a steep edge. Preferably, the constant voltage step comprises to charge the battery with a constant voltage value which corresponds to the voltage at the battery termi- nals at the end of the constant current step or corresponds to this value minus a certain margin. In the same way, the constant voltage step is terminated and a constant current step is started if, in the constant voltage step, a certain operational parameter, for example the charging current, the rate of change of the charging current, the temperature and/or the amount of charge transferred into the battery during this step has reached a certain limit which corresponds to a certain behavior relating to an undesired side effect or an overcharge situation. In particular, also behaviors can be detected and used as trigger for changing the constant current step to the constant voltage step and vice versa which occur before the undesired effect takes place or the overcharge situation begins. In this way, critical situations can be detected in beforehand and measures can be taken (i.e. changing constant current to constant voltage and vice versa) in order to avoid the occurrence of the undesired effects or the overcharge situation. Alternatively or in combination thereto, the charging steps can be exchanged (i.e., a constant current step is terminated and a constant voltage step is started or vice versa) in order to avoid the occurrence of undesired effects or the overcharge situation. Alternatively or in combination thereto, the charging steps can be exchanged (i.e. a constant current is terminated and a constant voltage step is started or vice versa) if a certain amount of time has lapsed since the start of the respective constant voltage or constant current step.

Apart from the protection against undesired side effects or overcharge situations, the present invention allows the maximum allowable voltage or the maximum allowable current for a minimum charge duration. In other words, the present invention allows to follow the maximum allowable voltage or current and therefore to follow the border to operational parameters corresponding to undesired effects or overcharge situations as close as possible. The current and voltage values can be determined a priori, either experimentally or via simulation, or adaptively using an on-board battery management system. In general, the current used during the constant current step and the voltage used during the constant voltage step are predetermined and can be given in a table as entries corresponding to one or more respective operational parameters at which the respective current or voltage should be applied. The relationship between the operational parameters (voltage, current, SOC, charge, temperature or the respective changing rates) is pre-stored in a memory and gives the optimum current or the optimum voltage to each operating point defined by the operational parameter or operational parameters, i.e. the maximum allowable current or maxi- mum allowable voltage. Instead of storing this relationship in the memory as a table, predefined numerical approximations can be used. Further, criterions of internal and/or external parameters can be used, as well as limits and constraints as described above. The relationships between the internal, external parameters, limits and/or constraints can be based on the above mentioned models, which can be implemented as tables, approximations, set of equa- tions, together or without the control mechanisms described above.

In order to monitor or determine the absolute amount of charge transferred to the rechargeable battery or the corresponding relative measured SOC (state of charge, i.e. the percent- age of charge available from the battery in relation to the overall amount of storable charge), a so-called Coulomb counter is preferably used that periodically or constantly tracks the current flowing to the battery or the current flowing from the battery. The current values are integrated in order to determine the amount of charge and, therewith, determin- ing the SOC. Alternatively, the SOC can be estimated from the battery voltage, the battery current and an estimated value of the battery's internal resistance. Further, the battery's internal resistance can be measured by using the battery voltage and the battery current.

Further, if the constant current and the constant voltage step are applied alternatingly, the sequence, in particular the duration of each step can be adapted to the battery degradation due to aging. Thus, the limits triggering the change from a constant current to a constant voltage step or vice versa can be lowered or increased in order to address the change of characteristics due to aging. Further, the temperature of the battery can be addressed when determining the limits which define the changing points from constant current to constant voltage or vice versa.

In one embodiment of the invention, the open circuit potential (OCP) of the battery terminal is measured for determining the current status of the battery, in particular in order to determine the SOC or the internal resistance of the battery. The OCP can be measured directly by measuring the terminal voltage at a current I = O A. Alternatively, the OCP can be calculated from measuring at least two voltages at two different current values. The measured open circuit potential can be used in order to determine any of the above-mentioned limits, the charging current and/or the charging voltage. Further, the SOC can be derived from the OCP, for example using a table with predefined entries or a respective predefined numerical approximation, which have established by a model simulation and/or by empirical studies. The derived SOC can be used to determine whether the charging process should be stopped if a maximum allowable SOC is reached.

In the above, the term battery is used for any kind of accumulator, galvanic cell or capacitor which is based on the chemical storage of energy and is based on electrodes / electrolyte. Of course, the models used for mapping the external on the internal parameters and for providing the control mechanisms as well as the limits have to be adapted to the reactions occurring in the particular battery type. Examples for batteries comprise rechargeable Li-Ion-, Lithium-Polymer-, NiMH-, NiCd-, wet or gel Pb-accumulators as single cells or as cell packages. Brief description of the drawings

Exemplary embodiments of the invention are depicted in the figures and are described in detail in the following description.

Figure 1 shows the development of the state of charge (SOC) vs. time. The dashed curve shows the course or progression of the SOC for a constant current charging step followed by a constant voltage charging step as known from the prior art. This course (denoted as CCCV) shows a linear progression-until point P is reached. Before point P, a constant cur- rent charging step is carried out, while after point P, a constant voltage charging step is carried out. Of course, P can be located more to the left or more to the right, provided that P is element of the dashed line an the progression before reaching P is linear. For example, P can be located at a point with a SOC of 0,9 (late activation of the last current charging step) or can be located at a point with a SOC of 0,3 (early activation of the last current charging step). The slow approximation of the SOC to a certain value after P has been reached is caused by the decrease of charging current, due to a constant voltage applied to a battery. In other words, the voltage difference between voltage source and open circuit voltage of the battery, which defines the charging current, decreases exponentially according to I ~ e "^τ. Therefore, the charging rate, i.e. the rise or increase of the SOC over time, decreases.

According to the state of the art, overload situations are prevented by charging the battery with a constant voltage, which corresponds to open circuit voltage of a battery with SOC = 100 %. In this way, if the SOC is 100 %, the voltage difference is between voltage source and battery is 0 V leading to a charging current of 0 A. In particular at the end of the charging process, the charging method according to the state of the art has a very low increase of SOC.

In Figure 1 , the continuous line shows the course of the SOC over time, if the charging method according to the invention is employed. It can be seen that the SOC increases nearly linearly over time. Thus, in particular at the end of the charging process, the charging method according to the invention is by far less time consuming in comparison to prior art charging methods. As mentioned above, the charging according to the invention is terminated if a certain SOC has been reached, for example a SOC of 0,98 ( = 98%). Until this point, a constant charging current (CC) is applied. The exact point of termination of the charging process using the constant current source is determined by monitoring the SOC using an integration of the charging current, by applying external parameters (for example the charging current, the terminal voltage of the battery, the inner voltage of the battery and the [estimated] open circuit voltage) to a model or to a set of model equations, approximation equations, estimations of inner parameters, a comparison of inner parameters with respective limits defined by undesired side reactions, or by applying external parameters to correspondence tables which give an estimation for the current SOC. Further, the point of termination of the charging process can be derived using a periodically updated simulation based on a battery model, by deriving internal status information from external parameters or by the interpretation of external parameters only.

In particular, in the case that the determination of the SOC is defined by interpretation of external parameters only, e.g. by integrating the current, a safety margin is used, for example 2 % as shown in Figure 1 , and aging processes are taken into account when determining the end of the charging process. Preferably, the internal resistance of the battery is monitored periodically or continuously in order to determine the approach of the SOC to the final state, for example, 98 %, 95 % or 100 %. The determination of the SOC can involve several different external parameters (battery voltage, charging current, battery temperature, internal resistance of the battery, etc.) and/or can involve a plurality of measurement points of the same parameter taken in the course of the same charging step.

As mentioned above, parameters like open circuit voltage, battery temperature and amount of charge determined by integrating the charging current over time can be combined by appropriate approximation equations. Further, a plurality of measurement points of the same parameter can be used for, for example, a recursive algorithm for weighted minimum square mean error reduction or can be averaged. Preferably, an algorithm is used which updates both, internal parameters, for example the SOC, as well as the model of the battery and the respective changes in the battery characteristics due to aging and/or due to high temperature.

Claims

1. Method for charging a battery by delivering electrical energy to the battery in a charging step, the charging step comprising: a voltage charging step, during which a defined voltage is applied to the battery, and a current charging step, during which a defined charging current is applied to the battery, the same charging step comprising the voltage charging step and the current charging step, wherein the voltage charging step is carried out before the current charging step and wherein the charging step is completed upon completion of the current charging step.

2. Method of claim 1, further comprising: a) constantly or repeatedly determining internal parameters including an absolute amount or relative percentage of charge present in the battery, an open circuit volt- age at the terminals of the battery, and/or an electrode/electrolyte voltage drop at an anode or a cathode of the battery, wherein b) determining the internal parameters includes monitoring external parameters, the external parameters comprising a terminal voltage, a terminal current, a temperature of the battery and/or an amount of charge being transferred to the battery; the step of determining the internal parameters further including a step of applying a model of the battery to the external parameters and/or performing diagnostic battery tests, at least one of the internal parameters having limits being linked with side reactions, the side reactions comprising lithium deposition, oxidation or reduction of solvent, salt or impurity or premature degradation by excessive battery temperature; c) comparing at least one of the internal parameters to at least one of the respective limits; and d) determining a reference value, the reference value comprising a maximum charging voltage, a maximum duration of the current charging step, a maximum duration of the charging step an instantaneous charging current and/or a maximum charging current being selected according to the internal parameters, the respective side reactions and/or a safety margin, the determination of the reference value comprising using the model and/or an arithmetic relationship between the reference value and at least one of the internal parameters, wherein steps b) - d) are carried out at least once or periodically within each charging step.

3. Method of claim 1, wherein the defined charging current of the current charging step is a predefined charging current comprising a constant current, a current linearly decreas- ing as a function of time or a current with a predefined progression, and wherein the defined charging voltage of the voltage charging step is a predefined charging voltage comprising a constant voltage, or a voltage with a predefined progression.

4. Method of claim 1, wherein the charging step comprises a plurality of current charging steps and voltage charging steps, which are carried out repeatedly and alternatingly within the same charging step.

5. Method of claim 1, wherein the charging step is completed upon the occurrence of an external event, the event comprising: expiration of a predefined time period starting with the charging step, with the ongoing voltage charging step or with the ongoing current charging step; reaching a predefined maximum voltage or a predefined maximum voltage change rate occurring at the battery's terminals; reaching a battery temperature limit or a limit of a battery temperature change rate; reaching a predefined relative state of charge or a predefined absolute amount of stored charge; detecting an internal resistance of the battery lying outside a predefined internal resistance interval; and/or estimating an internal parameter value comprising a rate of internal heat generation, an electrode/electrolyte potential drop at the anode or an electrode/electrolyte potential drop at the cathode, the internal parameter value being linked with a degradation mechanism according to a battery model or a diagnostic technique.

6. Method of claim 1, further comprising: continuously, repeatedly and/or periodically measuring the voltage present at terminals of the battery, the current flowing to the terminals of the battery, measuring the time having passed since the begin of the charg- ing step and/or the begin of the ongoing current charging step or ongoing voltage charging step and/or determining a relative and/or absolute state of charge by continuously, repeatedly and/or periodically measuring the voltage and the current at the terminals of the battery and continuously, repeatedly and/or periodically calculating the internal resistance of the battery based on the measured current and the measured volt- age.

7. Method of claim 1, further comprising continuously, repeatedly and/or periodically measuring the current flowing to the terminals of the battery and determining the amount of charge being transferred to the battery by integrating the measured current or by multiplying a constant current value and a time value, the time value corresponding to a duration during which the constant current has been transferred to the battery.

8. Method of claim 1, further comprising a first current charging step followed by a first voltage charging step followed by a second current charging step, the first current charging step being terminated and the first voltage charging step being started upon reaching at least one of the following limits: a predefined charging current limit, a pre- defined battery temperature change rate limit, a predefined battery's internal resistance limit, a predefined battery's internal resistance change rate limit, a predefined amount of charge limit, the charge being transferred to the battery during the first constant voltage step and/or an estimated rate of internal heat generation limit, an estimated electrode/electrolyte potential drop limit at the anode or an estimated electrode/electrolyte potential drop limit at the cathode, at least one of the limits being linked with degradation mechanisms according to a battery model or a diagnostic technique.

9. Method of claim 1, further comprising terminating the voltage charging step and starting a subsequent current charging step upon reaching or approaching at least one of the following limits: a time limit referring to the duration of the constant voltage step, a predefined voltage limit, a predefined battery temperature limit, a predefined battery temperature change rate limit, a predefined battery's internal resistance limit, a predefined battery's internal resistance change rate limit and/or a predefined amount of charge limit, the charge being transferred to the battery during the constant voltage step or since the start of the charging step.