WO2007048386A2

WO2007048386A2 - Method for charging an energy accumulator

Info

Publication number: WO2007048386A2
Application number: PCT/DE2006/001839
Authority: WO
Inventors: Peter Birke; Michael Keller; Manfred Malik
Original assignee: Temic Automotive Electric Motors Gmbh
Priority date: 2005-10-28
Filing date: 2006-10-17
Publication date: 2007-05-03
Also published as: DE102006004737A1; DE112006001189A5; WO2007048386A3

Abstract

The invention relates to a method for charging an energy accumulator, consisting of several cells connected in series and/or in parallel. According to said method, a plurality of charging voltage values are detected at constant intervals, a first charging voltage value being compared with at least one second charging voltage value. The charging of the energy accumulator ceases when the first charging voltage value is greater than or equal to the second charging voltage value.

Description

Method for charging an energy store

description

The present invention relates to a method for charging an energy store, in particular an energy store for use in a motor vehicle with hybrid drive, consisting of several serially and / or parallel connected cells, in particular energy storage cells.

A hybrid drive is the combination of different drive principles or different energy sources for a drive task within an application. Hybrid-powered vehicles, also called hybrid vehicles, include, for example, an internal combustion engine and an electric machine. The electric machine is usually designed as a starter / generator and / or electric drive. As a starter / generator it replaces the normally existing starter and alternator. In an embodiment as an electric drive, an additional torque, d. H. an acceleration torque, to propel the vehicle to be contributed by the electric machine. As a generator, it enables a recuperation of braking energy. Furthermore, hybrid vehicles have at least one energy store. The energy from the energy storage can be used for starting the internal combustion engine, for the electrical consumers in the vehicle and for acceleration processes. The energy storage is powered while driving on the generator from the engine. In addition, the energy storage is charged by energy recovery during braking. A control electronics ("hybrid controller"), the various modes are controlled, among other things, whether the energy storage energy to be removed or supplied.

Energy storage consist mostly of individual energy storage cells, such as nickel-metal hydride cells, hereinafter called NiMH cells. Since these cells are connected in series in a typically high number (eg 168 units for a 202 V NiMH-based energy store), balancing after a certain period of operation is essential, ie bringing all cells back to the same state of charge of charge, SoC). For example, this can be accomplished by achieving 100% SoC for all cells.

Because in the continuous operation of such energy storage is due to the switch-in principle to unbalanced states of charge within the energy storage, d. that is, the cells have different charge states and voltage levels over time. In order to maintain the functionality and the longevity of the energy storage, the cells must be brought at regular intervals to almost the same voltage level within the energy storage, d. H. the charging voltage of the individual cells must be adjusted again, d. H. be balanced or balanced.

If such balancing or equalizing charge is not carried out, it can even lead to negative voltages (polarity reversal) to individual energy storage cells in extreme cases, which would lead to these energy storage cells would be destroyed. Furthermore, this would possibly lead to a drifting apart of the energy storage cells in their. Voltage levels to overload or degeneration of individual energy storage cells, could even lead to a security risk.

An equalizing charge should always be made when the individual energy storage cells are operationally drifted into their state of charge in such a way that it can lead to a possible deep discharge or even reversal of individual energy storage cells, which in the sequence to a irreversible damage or even acute danger to the operation of the energy store.

In contrast to, for example, lead acid and lithium ion cells, the conditional overcharge tolerance can be exploited in NiMH cells, hydrogen and oxygen being developed, which in turn are absorbed in the electrodes and / or recombined into water.

For hybrid energy storage, it is also highly desirable that such a process of equalizing charge is completed as quickly as possible, since the memory is not available for the operating modes described above during this compensation charge.

A very special challenge is when the cells are neither individually monitored in the voltage nor can be individually balanced, but in units (modules) of typically more than 10 cells are monitored in the voltage, and only the entire Memory can be charged and discharged. This is usually the case with NiMH.

In particular, NiMH cells have only a conditional overcharge tolerance, which can effectively be used very well for equalizing, but at the same time means avoiding long-term overcharge arrest, possible cell damage from gassing, overheating and corrosion of the negative To avoid electrode.

So far, simple ways of charging, in particular, NiMH cells have been proposed, such as the lump-sum introduction of 1.5 times the charge, which corresponds to the capacity, and a simple monitoring over the temperature. It relies solely on the overload tolerance of NiMH cells. Likewise, slow charging is possible until, for example, a charge plateau is reached. However, one runs the risk of damaging the cells via gas loss (usually via the activation of the gas valve) and thus dehydration, as it effectively loses water with hydrogen and oxygen. This loss can quickly become critical due to the amount of electrolyte minimized for gas exchange through the separator. Further, the oxygen can oxidize the surface of the metal hydride alloy of the negative electrode, which can lead to passivation and thus reduced power output.

The object of the invention is therefore to provide the fastest possible charging method while avoiding prolonged overcharging of the energy store.

This object is solved by the features of patent claim 1. Advantageous embodiments of the invention will become apparent from the dependent claims and the further description.

During charging of the energy store, at least two charge voltage values are detected at timed intervals, wherein a first charge voltage value is compared with at least a second charge voltage value, and charging of the energy store is terminated when the first charge voltage value is greater than or equal to the second charge voltage value.

Preferably, the second charging voltage value is a currently detected charge voltage value and the first is an immediately previously detected charging voltage value.

After the start of the charging process, the voltage of the individual energy storage cells usually first increases. After a certain time the tension remains despite charging the cells at the same level or even decreases, ie there is a voltage drop observed. The already mentioned recombination region, in which charge flows but is not stored, has been reached. One sees a "voltage hump", which the cells pass through, and thus advantageously the end of the charging process is recorded in time.

The charging of the energy store is preferably carried out at least a second time. The charging process is completed when, in the second charging process, the detected time until completion of the charging corresponds to the detected time until completion of the charging of the first charging operation. It is thus checked whether all cells pass through the "voltage hump" a second time in the same time, ie whether the time until the voltage drop is constant or not shorter, if this is the case, all cells and thus the energy storage are charged If some of the cells are not yet fully charged, the time for the previous charging process would be longer, since charge is additionally shifted from the positive to the negative electrode before the recombination cycle for these cells has been reached.

Since it can be determined from individual loads when, d. H. after which time a voltage drop takes place and thus the voltage hump is reached, a period of time is advantageously defined for the charging process and the charging process ends after the time has elapsed.

In order to prevent overloading of the electrodes, the charging process should be interrupted in particular if the energy store is outside a defined temperature range.

Advantageously, the temperature range is between 10 ⁰ C and 4O ⁰ C. At too high temperatures, the charge acceptance of the negative electrode decreases, that is, the metal hydride, as the back pressure increases the hydrogen. At too low temperatures, the kinetics are bad and there is a threat of overloading the Solid state lattice of both electrodes.

In order to complete the charging process as quickly as possible, it has proved to be advantageous, in addition to the monitoring of the voltage hump, as described above, in addition a charge current control as a function of the state of charge of the energy storage, also called SoC ("State of charge") to provide.

Up to about 80% and 90% to 95% of the SoC in subsequent increments, the energy store is charged with higher currents (but decreasing to higher SoCs) to effectively minimize the total charge time. Here, the knowledge for the required amount of charge from the knowledge of the SoC is obtained before the start of the charge, for example, extrapolated to avoid a further interrogation of the SoC with previous relaxation time of the cell.

After exceeding 90% of the SoC, a compromise has to be made between overloading due to excessively high currents and damage due to excessively long dwell time in the transfer area, so that a reduction of the current is carried out at the latest when 95% of the SoC is reached.

In the range of the state of charge of the energy storage of less than 95% is preferably worked with pulse and continuous currents of 1 to 25 C, at the same time a limitation must be made by a temperature monitoring. In the case of the reduced current, however, the preferred range is 0.5 to 1.5 C, usually as a continuous current. Especially in these areas, the temperature monitoring and the resulting possible current limitation are of central importance. In this case, it is again to check whether the voltage hump is present, in this case, the further charge of the energy storage ^' to end. Preferably, the charge balance of the energy storage cells is made operationally.

Preferably, the charge equalization is carried out as a function of the state of charge of the energy store.

The energy storage cells used are preferably nickel-metal hydride cells (NiMH cells).

In the following description, further features and details of the invention in conjunction with the accompanying drawings will be explained in more detail by means of exemplary embodiments. In this case, features and relationships described in individual variants can in principle be applied to all exemplary embodiments. In the drawings show:

1 shows the typical charging curve of NiMH cells,

FIG. 2 is a flow chart or flow chart of the method according to the invention; and FIG

Fig. 3 is a charging current control as a function of the state of charge of the energy storage.

However, the specific embodiment, which does not limit the application to this embodiment, describes a charging method for an energy store with nickel-metal hydride cells. Typically, in this case, a number of 150 to 200 energy storage cells connected in series Energy storage package summarized, the individual voltages of each energy storage cell are approximately 1, 2 volts. However, it is also possible to combine significantly more or fewer cells. Furthermore, it is possible to interconnect individual energy storage packets in parallel and / or in series to form an energy store.

FIG. 1 shows the typical charging curve 1, 2, 3 of a nickel-metal hydride energy storage cell. The voltage curve of the energy storage cell during charging is plotted over time, wherein the voltage curve is shown in for three different temperatures, namely O ⁰ C, 20 ⁰ C and 4O ⁰ C. It turns out that charging acceptance drops at a higher temperature. Here it is clear that the energy storage cell charges up to a voltage of about 1, 6 volts at 0 ⁰ C, then the voltage of the energy storage cell does not increase further, even decreases. The voltage hump 4 is detected by the fact that the voltage applied to the cell is sampled equidistantly and the individual sampled values are compared with previous values. At the moment in which no voltage increase occurs and a voltage decrease is measurable, it can be assumed that the maximum of the voltage bump 4 is reached and passed. In this case, another charge is terminated. The end of the charging process is detected at a constant small transit time in the voltage hump 4, which is typical for the charge profile of nickel-metal hydride cells.

2 shows a flowchart or flow chart of the method according to the invention. At the beginning of the process (start), a value for U _m j _{n is} to be stored, where U _m represents a minimum voltage difference value. Furthermore, a value for t _ma χ is to be stored, where t _{max represents} a maximum time value. Subsequently, the temperature T of the energy store is checked and the charging current I determined as a function of the temperature of the energy store. After starting the charging process, first a first charging voltage value Ui and then a second charging voltage value U _{2 are} measured. It should be noted that all measurements of U ₁ and U ₂ always occur at an equidistant distance. After measuring Ui and U ₂ , the charging voltage difference U ₂ -U _{1 is} formed and it is checked whether this value is greater than the value initially stored for U _m m. If this is the case (yes), it is further checked whether the state of charge SoC of the energy storage is already at 100%. If the state of charge SoC is less than 100%, the energy storage continues to be charged. On the other hand, if the state of charge SoC has reached 100%, the energy store is now temporarily overcharged, since the energy store has a conditional overcharge tolerance that can be used in particular for the formation of equalizing charge of the energy store. These steps also take place when the charging voltage difference value U ₂ -U _{1 is} not greater than the charging voltage difference value U _m j _n . The energy store is thus charged further, with the time ti being measured in parallel therewith. Again, two charging voltage values Ui and U _{2 are} measured and then charge and time recording ti are terminated, whereby the value for ti is documented. Now it is checked whether the charging voltage difference U ₂ -U _{1 is} less than zero, ie the value for U ₁ is therefore greater than the value for U ₂ and the difference value is negative. This is an indication that the "voltage hump" shown in Fig. 1 has been crossed, since the charging voltage now drops again.

Once the "voltage hump" has been passed, a second charging process of the energy store is carried out, ie the energy store is again briefly overcharged, again recording a time t ₂ and two charging voltage values Ui and U _2. When charging is completed, the time recording t _{2 is} stopped and it is again checked whether the "voltage hump" has been passed. If the "voltage hump" has been crossed, the time values for U and t _{2 are} compared, and if the times ti and t _{2 are} identical until the voltage drop, the Charging be completed because it is now ensured that the energy storage is fully charged to form equalization charges. However, if no voltage drop was detected after measuring the values Ui and U2, ie the difference value U ₂ -Ui is greater than zero, it is further checked whether the time value for t _max set at the beginning of the method has already been exceeded. If this is not the case, the energy store continues to be charged. T _max already passed, time recording for ti and / or t ₂ is cleared and the store in the dock area is performed again.

Fig. 3 shows a charging current control as a function of the state of charge of the energy storage. Up to about 80% and in further steps 90% to 95% of the state of charge SoC of the energy store, the charging of the energy store is done with high currents in order to effectively minimize the total time of charging. In this case, the knowledge for the required charge quantity is obtained from the knowledge of the state of charge SoC before the start of charging, for example extrapolated, in order to avoid a further interrogation of the state of charge SoC with preceding relaxation time of the cell.

In the range of the state of charge of the energy storage of less than 95% is preferably worked with pulse and continuous currents of 1 to 25 C, at the same time a limitation must be made by a temperature monitoring. In the case of the reduction of the current, however, the preferred range is 0.5 to 1.5 C, usually as a continuous current. Especially in these areas are the temperature monitoring and the resulting possible current limit central. In this case, it is again to check whether the voltage hump is present, in this case, the further charge of the energy storage is to end.

Furthermore, it should be noted that nickel-metal hydride cells are not only damaged by deep discharge and polarity reversal, but cobalt metal, cobalt suboxide or cobalt oxide are often used in the positive electrode, which are so electrochemically reacted during the first charging, the so-called forming, that an effective conductivity scaffold is formed, which contacts the extremely poorly electronically conductive nickel hydroxide particles well. With prolonged reaching discharge voltages around 0.8 volts, this scaffold can be gradually destroyed, resulting in a drastic deterioration of "health status", also called SoH "State of Health", the energy storage cells and thus the energy storage, in terms of maximum power , leads.

An equalizing charge carried out in good time, without simultaneously causing damage by improper selection of the current ranges or times in which cells are in the overcharge area, is thus a very decisive life-prolonging step for an energy store, in particular if it consists of a series connection and / or series connection of Nickel-metal hydride cells.

Claims

claims

1. A method for charging an energy storage device, consisting of several serially and / or parallel connected cells, characterized in that during charging of the energy storage at least two

Charge voltage values are sequentially detected in time-constant intervals, wherein a first charging voltage value with at least a second

Charging voltage value is compared, and the charging of the energy storage is terminated when the first

Charging voltage value is greater than or equal to the second charging voltage value.

2. The method according to claim 1, characterized in that the second charging voltage value is a currently detected charge voltage value and the first is an immediately previously detected charging voltage value.

3. The method according to claim 1 or 2, characterized in that the charging process is detected in time.

4. The method according to any one of claims 1 to 3, characterized in that the charging of the energy store is performed at least a second time.

5. The method according to any one of claims 1 to 4, characterized in that the energy store is fully charged when the second charging process, the detected time until completion of the charging of the detected time until completion of the loading of the first charging corresponds.

6. The method according to one or more of the preceding claims, characterized in that the charging process is terminated after a defined period of time.

7. The method according to one or more of the preceding claims, characterized in that the charging process is interrupted when the energy storage is outside a defined temperature range.

8. The method according to claim 7, characterized in that the temperature range is defined as the range between 10 ⁰ C and 40 ⁰ C.

9. The method according to one or more of the preceding claims, characterized in that the charging is carried out up to a state of charge (SoC) of 80% of the energy store with high charging current and preferably pulsed charging current.

10. The method according to one or more of the preceding claims, characterized in that the charging process in a state of charge (SoC) range from 90% to 95% of Energy storage is performed with reduced charging current.

11. The method according to one or more of the preceding claims, characterized in that the charging in a state of charge range of 95% to 100% of the energy storage is made with a nearly 90% reduced charging current based on the preceding claim.

12. The method according to one or more of the preceding claims, characterized in that are used as energy storage cells nickel-metal hydride cells (NiMH cells).

13. The method according to one or more of the preceding claims, characterized in that, depending on the state of charge of the individual energy storage cells or sub-circuits (modules) of these a compensation of the charges is made of the same.

14. The method according to claim 13, characterized in that the compensation is made operationally.