Method and apparatus for charging rechargeable cells

Abstract

A method for charging rechargeable cells, in particular lithium ion cells. An apparatus for charging such cells. In order to specify a method for charging a lithium-based cell and an apparatus for charging a lithium-based cell, in which the capacitance of the cell is optimally used, the charging time is drastically shortened, the shelf life of the cell is extended and/or it is possible to increase the capacitance of the cell, a method is stated which includes the following steps, pulsed charging of the cell, wherein the charging current I.sub.L exceeds the nominal charging current I.sub.Lmax of the cell during the charging pulses; and the cell is discharged between the charging pulses using load pulses.

Claims

1. A method for charging a lithium-ion based rechargeable cell, comprising: pulsed charging the cell, wherein a charging current I.sub.L, during the charging pulses, exceeds a maximal nominal charging current I.sub.Lmax of the cell by up to 5-times; and discharging the cell between the charging pulses by load pulses, wherein the load pulses are shorter than the charging pulses, and measuring a voltage U.sub.z of the cell during each load pulse; checking whether the measured voltage U.sub.z of the cell is equal or greater than an end-of-charge voltage U.sub.Lmax of the cell; finishing charging of the cell after the measured voltage U.sub.z of the cell at each of a predefined number of load pulses (n) corresponds to an end-of-charge voltage U.sub.Lmax of the cell, wherein n is greater than or equal to 1, wherein a level of the charging current I.sub.L varies in consecutive charging pulses.

2. The method according to claim 1, wherein the charging current I.sub.L, during the charging pulses, is more than 1.5 times the maximal nominal charging current I.sub.Lmax of the cell.

3. The method according to claim 2, wherein the charging current I.sub.L is twice the maximal nominal charging current I.sub.Lmax or greater.

4. The method according to claim 1, wherein a level of the charging current I.sub.L, during the charging pulses and/or a height of the load pulses is dependent on a state of the cell.

5. The method according to claim 4, wherein the level of the current during the charging pulses and/or the load pulses is set depending on an internal resistance of the cell and/or a temperature of the cell.

6. The method according to claim 1, wherein a discharging current I.sub.Last is a maximum of 20-25 % of the maximal nominal charging current I.sub.Lmax flows during a load pulse.

7. The method according to claim 1, a level of a discharging current I.sub.Last varies in consecutive load pulses.

8. The method according to claim 1, wherein on exceeding a predetermined voltage of the cell, the charging operation is terminated and/or wherein on exceeding a predetermined temperature (T.sub.max) of the cell the charging operation is terminated.

9. The method according to claim 1, wherein depending on the measured voltage, a level of the charging current I.sub.L is set for the subsequent charging pulse, wherein if the measured voltage U.sub.z of the cell lies above a predefined value during the load pulse, the charging current I.sub.L is reduced in the next charging pulse.

10. The method according to claim 1, wherein depending on the measured voltage, the level of a discharging current I.sub.Last is set for the subsequent load pulse, wherein if the measured voltage U.sub.z of the cell lies above a predefined value during the load pulse, the discharging current I.sub.Last is reduced in the next load pulse.

11. The method according to claim 1, where a length of a load pulse corresponds to about one-half of a length of a charging pulse.

12. The method according to claim 1, wherein if the measured voltage U.sub.z of the cell reaches the end-of-charge voltage U.sub.Lmax during a load pulse, the charging current I.sub.L is reduced in the next charging pulse.

13. A device for charging a lithium-ion based rechargeable cell, comprising a controller configured to: pulse charge the cell, wherein a charging current I.sub.L, during the charging pulses, exceeds a maximal nominal charging current I.sub.Lmax of the cell by up to 5-times: and discharge the cell between the charging pulses by load pulses, wherein the load pulses are shorter than the charging pulses, and finish charging of the cell after a measured voltage UZ at each of a predefined number of load pulses (n) corresponds to an end of-charge voltage U.sub.Lmax of the cell, wherein n is greater than or equal to 1, wherein a level of the charging current I.sub.L varies in consective charging pulses.

14. The device according to claim 13, further comprising a device for providing a sink in order to discharge the cell during the load pulses, wherein a size of the load pulse can be set.

15. The device according to claim 13, further comprising at least one capacitor to provide the load pulse and/or the charging pulse.

Description

(1) Examples of the invention will now be described with reference to the figures, in which

(2) FIG. 1 shows the construction of a commonly used lithium-ion cell;

(3) FIG. 2 shows a lithium-ion cell in a wound state;

(4) FIG. 3 shows a schematic current signal characteristic of a charging method according to the invention for a high-energy cell;

(5) FIG. 4 shows a current characteristic of a pulse-charging method according to the invention for a high-current cell;

(6) FIG. 5 shows a current, voltage and temperature characteristic of a further embodiment of the charging method according to the invention;

(7) FIG. 6 shows a current, voltage and temperature characteristic of a further embodiment of the charging method according to the invention;

(8) FIG. 7 shows a section of the current, voltage and temperature characteristic according to FIG. 5;

(9) FIG. 8a, 8b show a flow diagram for a charging method according to the invention;

(10) FIG. 9 shows an embodiment of a signal characteristic during the charge-preparing phase according to another embodiment;

(11) FIG. 10 schematically shows the construction of a charging device for applying the pulse-charging method according to the invention.

(12) FIG. 1 schematically shows the construction of a lithium-ion cell comprising a cathode and an anode. During the charging operation lithium-ions migrate from the positive electrode to the negative electrode which for example is coated with lithium graphite. During the discharging operation the lithium-ions migrate from the negative electrode back to the positive electrode. The two electrodes are separated from each other by a separator, wherein the lithium-ions migrate through this separator.

(13) Lithium-ion cells compared to other rechargeable cells are characterised in that they have no memory effect and self-discharge is very low. The usual end-of-charge voltage of lithium-ion cells is approx. 4.2V, based on a nominal voltage of 3.6V. Lithium-ion cells, for example, include lithium polymer cells, lithium iron sulphate cells, lithium graphite cells and lithium cobalt cells.

(14) FIG. 2 shows a lithium-ion cell in a wound state. The anode 21 and the cathode 22 lie opposite each other and are separated from each other by a separator 23. The terminal lugs 24 and 25 on the electrodes 21 and 22 lie diagonally opposite each other. That is, the electrical resistance in the electrodes increases as the line length increases. Thus the electrical resistance in the electrodes grows as the distance to the terminal lug increases. Therefore the lithium-ions endeavour to take the path of the least electrical resistance as they migrate from the positive to the negative electrode, which resistance, however, is not formed by the electrode directly opposite, but is located through the cell between the electrodes (indicated with 27). Due to the short charging pulses with a charging current I.sub.L, which is higher than the nominal charging current I.sub.Lmax of a cell, the lithium-ions are urged to the other electrode without having time to look for a path with the least electrical resistance. As a result the separator 23 is formed up thus permitting a uniform ion exchange between the two opposite electrodes 21 and 22. In addition due to the charging pulse as well as the load pulse being limited over time, the temperature of the electrodes 21, 22 is prevented from rising which otherwise would cause an increase in the internal resistance of the electrodes which in turn would lead to an uneven resistance distribution causing a further rise in the temperature of the electrodes on the one hand and a change in the lithium distribution within the cell on the other, leading to an uneven distribution of the lithium deposition. An uneven deposition of lithium would lead to no longer having a complete chemical reaction surface available between the electrodes, thereby reducing the maximum possible charging cycles. On the other hand, if the lithium deposition were to grow unevenly on one of the electrodes, the separator would be reached at some time and be punctuated causing a short circuit. The short charging pulses or load pulses have the effect of counteracting this, wherein prevention of an excessive rise in temperature is especially important. The reference symbol 27 shows a path of the lithium-ions which try to take the path of the least electrical resistance. If the cell were not charged/discharged with the short high charging pulses or load pulses, the lithium-ions would try to take the path represented by the reference symbol 27, which would lead to an uneven distribution of the lithium deposition on the electrodes.

(15) FIG. 3 shows a signal characteristic over time for a charging method according to the invention with a charge-preparing phase and a pulse-charging phase. The charging method shown here is exemplary for a high-energy cell with a capacity of 2.3 Ah.

(16) With this charging method the cell is charged during the charge-preparing phase comprising a first rising phase 33 with a charging current rising from 0 to 1 A within one minute. After this one minute the charging operation is stopped for a duration of 2 s, i.e. the cell is no longer supplied with a charging current, wherein the voltage of the cell is measured at first without and then with a predefined load. After 2 seconds have passed and a voltage above the end-of-discharge voltage of 3.0V has been measured, the charge-preparing phase is finished and the pulse-charging process can begin.

(17) In the pulse-charging phase the pulse duration of the positive charging pulses 31 is initially 1 s, wherein the duration of the load pulses 32 is 0.5 s. During the load pulses 32 the cell is subjected to a load of 300 mA, wherein the voltage U.sub.Z of the cell is measured within a load pulse 32. If the voltage during this load is more than 4.2V the charging operation is finished.

(18) Within the cell the following happens during the charging operation according to the invention: the crystals being created inside the cell during the charging pulses 31 damage the separator 23 of the cell, whereby this would lose both charge and capacity. Moreover the crystals obstruct the movement of ions between the electrodes 21, 22, resulting in a distinct reduction of the lifespan of the cell. However, since in the load pulses 32 according to the invention which lie between the charging pulses 31 these crystals are again immediately reduced due to the load, the negative effect of the crystals is cancelled.

(19) This constitutes a major advantage of the charging method according to the invention. According to the inventive charging method as per FIG. 3 a charging pulse 31 of 5 A is employed during the pulse-charging phase, which may be approx. twice as great as the nominal charging current of 2.3 A for high-energy cells.

(20) Apart from the raised current values during the charging pulses 31, a voltage is also applied or is admissible in the charging method according to the invention which is higher than the specified end-of-charge voltage U.sub.Lmax, which is predefined for the respective cell and in the present case is specified to be 4.2V for the high energy cell. In this way the high current can be maintained up to the last charging pulse 31, which in comparison to the conventional charging method makes it possible for the cell to be charged to 100% or more within a very short time.

(21) With other conventional charging methods the charging current used is kept constant, but this is lowered when the end-of-charge voltage U.sub.Lmax is reached. Due to the current sinking when the end-of-charge voltage U.sub.Lmax is reached, a distinctly higher charging time is required, in particular for charging the remaining 20% of capacity of a cell. With traditional charging methods the voltage moreover is measured during the interruption of the charging pulses. Because thus no load pulses are applied the crystals or dendrites formed during charging, which damage the separator 23, are not removed. Due to the fact that these crystals are not removed again, commonly used charging methods must never use a constant raised charging current and a voltage, which lies above the end-of-charge voltage U.sub.Lmax.

(22) There are also charging methods which use a continuously rising current for charging, wherein however a continuously rising charging current I.sub.L results in a degeneration of the cell, in particular if the cell is to be charged to 100%. Besides, a considerable rise in temperature has been observed.

(23) Due to the charging method according to the invention, where a defined sink is used during the load pulse 32 in order to remove the crystals or dendrites and to counteract a temperature increase, it is possible, even when the end-of-charge voltage U.sub.Lmax is reached, to maintain the charging current I.sub.L during the charging pulses 31 and to charge the cell, even for a constant charging current I.sub.L and a voltage higher than the end-of-charge voltage U.sub.Lmax. Due to the continuously removed crystals or dendrites during the load pulses a higher voltage and uniform current pulses can be employed, resulting in a drastic reduction of the charging time. Due to the short pulses a rise in temperature is avoided and the cell is charged in a very careful manner so that the lifespan of the cell is not impaired in any way, despite the higher voltage and current values.

(24) Moreover there is almost no self-discharge due to the non-existent crystals, with the effect that a cell charged to 100% does not discharge when idle or when decoupled, and thus does not degenerate and even after years of storage can still develop its full capacity.

(25) The signal characteristic according to FIG. 3 relates to a high-energy cell with a capacity of 2.3 Ah, an end-of-charge voltage U.sub.Lmax of 4.2V, a nominal voltage of 3.7V, an end-of-discharge voltage U.sub.EL of 3.0V, a nominal charging current I.sub.Lmax of 2.3 A and a maximum discharging current of 4.2 A and a continuous discharging current of 3.5 A.

(26) In FIG. 4 a charging method according to the invention for a high-current cell is shown which comprises a capacity of 2.3 Ah. Further characteristic data of the high-current cell is an end-of-charge voltage U.sub.Lmax of 4.1V, a nominal voltage of 3.3V, an end-of-discharge voltage U.sub.EL of 2.0V, a nominal charging current I.sub.Lmax of 10 A, a discharging current of max. 50 A and a continuous discharging current of 25 A.

(27) In contrast to a charging method according to FIG. 3, with a high-current cell charging is carried out with a distinctly higher current of 20 A in absolute terms, during the charging pulses 41 in the pulse-charging phase. The load pulses 42 too between the charging pulses 41 are distinctly greater (2 A) in absolute terms.

(28) FIG. 5 shows a signal characteristic of voltage, current, capacity and temperature for a charging method according to the invention. According to FIG. 5 a lithium cell is fully charged during a time of approx. 18 minutes without the temperature of the cell rising. The temperature is shown in the lower part of FIG. 5 and remains in a region below 35° C. from start to finish of the charging operation. The current/voltage characteristic shows pulsing the cell with charging pulses of 5 A, wherein a charging pulse is followed by a load pulse which is smaller than 1 A, preferably the load pulses used are max. 300 mA. In the upper part of FIG. 5 the voltage is shown. At the start of the charging process the voltage present at the cell is below 3.7V, wherein during the first charging pulses the voltage U.sub.Z is initially below 4.2V. When looking at the voltage characteristic over the total time of the charging process, however, it can be seen that the voltage U.sub.Z at the cell reaches 4.25V after a relatively short time, approx. 2.5 min, which is above the end-of-charge voltage U.sub.Lmax of a lithium-ion cell.

(29) With commonly used charging methods the current level during the charging pulses would be reduced. Due to the charging method according to the invention, in which extremely short charging pulses with amperages above the nominal charging current I.sub.Lmax are used, the voltage U.sub.Z at the cell can rise further even when exceeding the end-of-charge voltage U.sub.Lmax without the amperage in the charging pulses having to be reduced. It is thus possible to drastically reduce the charging time of the cell without the temperature rising or the cell degrading in some form or another. When reaching a voltage U.sub.Z of approx. 4.5V the current during the charging pulses start to reduce because a voltage U.sub.Z of approx. 4.1V was measured in several consecutive load pulses, which approx. corresponds to the max. end-of-charge voltage U.sub.Lmax.

(30) Further it can be recognised that both the height of the charging pulses and the height of the load pulses varies depending on the measured voltage U.sub.Z of the cell. One can see that for a rise in voltage during the load pulses the stress or the load current during the load pulses is successively reduced until the voltage during the load pulse is again within a specified range which follows the lower trend of the voltages. It is thus avoided that the temperature at the electrodes rises.

(31) FIG. 6 shows a further signal characteristic of the charging method according to the invention. In this charging method a 100% charge of the cell is achieved over 25 minutes. Similar to the charging method according in FIG. 5 it can be recognised that the voltage rises relatively quickly above the end-of-charge voltage of 4.2V without then having to use a reduction of the current during the charging pulses. The charging pulses are not reduced until after approx. 17 minutes when the voltage U.sub.Z at the cell has reached 4.5V. After reaching a voltage of 4.5V a voltage of more than 4V is already present at the cell in the load pulses. I.e. the cell is almost fully charged. In order to avoid a further rise in temperature a start is made at a predefined voltage of 4.5V to reduce the amperage during the charging pulses wherein, however, the height of the load pulses is not changed. It can be clearly seen that the voltage U.sub.Z is rising further during the load pulses and reaches a value of 4.2V, which corresponds to the end-of-charge voltage U.sub.Lmax of the cell. With this signal characteristic also it can be recognised that the temperature of the cell hardly changes and, at any rate, does not rise beyond 35° C.

(32) FIG. 7 shows a signal characteristic of current, voltage, capacity and temperature during a charging process. At the beginning of the pulse-charging phase a charging pulse of 5 A is applied, which in a high-energy cell corresponds to somewhat more than double that of the I.sub.Lmax. The voltage U.sub.Z is measured during both the load pulses and the charging pulses. In the first charging pulse with 5 A a voltage U.sub.Z of approx. 4.2V is reached at the cell. The first charging pulse is followed by a first load pulse of less than 500 mA, preferably 300 mA, in which the cell is discharged. The current of the load pulses is approx. 3%-6% of the current during the charging pulses.

(33) The view of FIG. 7 clearly shows that the height of the load pulses varies. In the middle part of the view it can be recognised that during a load pulse the voltage abruptly rises to initially 3.8V or just below 4V. This abrupt voltage rise is counteracted in that the subsequent load pulses are reduced in their amperage. Initially the load pulses are halved from 300 mA to 150 mA. Should the voltage rise further, it is also possible to apply a load pulse with amperages of less than 50 mA. After the height of the load pulses has been reduced, it can be recognised that the voltage U.sub.Z returns to the voltage trend during the load pulses resulting in the voltage during a load pulse lying again within a range of below 3.75V.

(34) The view in FIG. 7 represents s section of time of a charging process according to FIG. 5 or 6, which however represents only 1 minute approx. Therefore no change can be detected either in the temperature or in the capacity.

(35) FIGS. 8a and 8b represent a flow diagram for a charging process according to the invention where both a charge-preparing phase and pulse-charging phase are carried out. After the charging process has been started in step S301, the voltage U.sub.Z at the cell is initially measured (S302). When the voltage U.sub.Z is greater than the end-of-charge voltage U.sub.Lmax, i.e. when in the case of a high-energy cell more than 4.2V are present at the cell, the cell is fully charged, and the charging process is finished.

(36) If the cell voltage U.sub.Z is less than the end-of-charge voltage U.sub.Lmax, a check is carried out in step S303 to see whether the cell voltage is greater than an end-of-discharge voltage U.sub.EL. The end-of-discharge voltage U.sub.EL of a high-energy cell is about 3V, that of a high-current cell about 2V. If the cell voltage U.sub.Z lies above the end-of-discharge voltage U.sub.EL, the pulse-charging process according to FIG. 8b can be immediately continued. If however the cell has a voltage U.sub.Z which is less than the end-of-discharge voltage U.sub.EL, a charge-preparing phase for activating the cell must be carried out.

(37) Thus a first rising phase is performed in step 304. After the cell has been charged during the first rising phase, the cell voltage U.sub.Z is measured under load. In other words, the level of the voltage U.sub.Z at the cell under load is checked. If the voltage U.sub.Z is now greater than the end-of-discharge voltage U.sub.EL of 2V or 3V, depending on the cell used, the pulse-charging phase can be started. Otherwise a first rising phase is repeated in steps S306 or S307, wherein the voltage measurement is also repeated. Should, after repeating the first rising phase, the cell voltage still be below the end-of-discharge voltage U.sub.EL, a second rising phase is carried out using a charging current I.sub.L of more than the nominal charging current I.sub.Lmax (S308). Even although not shown in FIG. 8a, it is checked after the end of the second rising phase, whether the cell voltage U.sub.Z has reached the end-of-discharge voltage U.sub.EL. If the cell voltage U.sub.Z, even after the second rising phase has still not reached the end-of-discharge voltage U.sub.EL, the cell is defective and cannot be charged any further. The pulse-charging phase, which is shown in FIG. 8b, can only be started on condition that the end-of-discharge voltage U.sub.EL is reached. After the pulse-charging phase has been started, a charging pulse is initially applied for a time duration t1 with a charging current I.sub.L, which is greater than the nominal charging current I.sub.Lmax. Following the charging pulse a load pulse is applied which preferably is only half as long as the charging pulse and in which the cell is loaded with a discharging current I.sub.Last which is approx. 25% of the nominal charging current I.sub.Lmax. During the load pulse the cell voltage U.sub.Z is measured and it is checked, whether the cell voltage U.sub.Z is greater than the end-of-charge voltage U.sub.Lmax. Should the voltage U.sub.Z of the cell be already above the end-of-charge voltage U.sub.Lmax, it is checked in steps S315 and S316/S317, whether the end-of-charge voltage has been reached three times. Should this be the case, the cell is fully charged. If in step S313 the cell voltage U.sub.Z is smaller than the end-of-charge voltage, the height of the next charging pulse/load pulse is set in step S314 based on the measured voltage U.sub.Z during the load pulses, and then the process is continued with steps S311/S312, as is evident in the signal characteristics of FIGS. 5, 6 and 7.

(38) FIG. 9 shows a detailed charge-preparing phase. In the upper part of FIG. 9 it can be recognised that the cell is initially charged with a linearly rising current up to an amperage of 1 A, wherein the voltage at the cell rises from approx. 3.5V to 3.7V during this time. During subsequent load application a voltage measurement is again carried out. After the first rising phase it can be recognised that the voltage U.sub.Z at the cell is below 2.0V which is less than the end-of-discharge voltage U.sub.EL for a high-current cell, so that a further first rising phase has to be performed because the pulse-charging phase can only be started on the basis of the end-of-discharge voltage U.sub.EL. After repeating the first rising phase, another voltage measurement is carried out, which indicates that the cell, at the end of the repeat of the first rising phase, comprises a voltage of 2.1V, which is above the end-of-discharge voltage U.sub.EL. Now, depending on the respective embodiment, a second rising phase can be carried out during which the cell is charged up to an amperage which lies above the nominal charging current I.sub.Lmax. Alternatively it is possible to start the pulse-charging phase immediately.

(39) FIG. 10 shows a device for performing the charging method. Normally the device for performing the charging method is called a charging device. In contrast to conventional charging devices a charging device for performing the charging method is able to apply a defined sink/a defined load pulse to the cell. The charging device 100 is connected with the cell 140. The cell 140 is connected with a temperature sensor 160, which is coupled to the charging device 100 for continuous or periodic temperature monitoring. The charging device 100 comprises a CPU 110 which performs the charging method according to the invention. The CPU 110 is connected with a memory 120 and with a display 130 for outputting measured values. Further the charging device comprises an input unit 150 via which the charging method can be influenced. The memory 120 has various parameters stored in it for the charging process. For a certain cell, for example, this may be its characteristic data such as capacity, end-of-charge voltage, nominal voltage, end-of-discharge voltage, maximum charging current, maximum discharging current and continuous discharging current or only some of these. Based on these values the height of the charging pulses/load pulses is calculated. Also critical temperature values may be stored in the memory 120, related to the respective cell. The charging device may preferably be equipped with a detection device in order to identify the cell to be charged.

(40) Similarly it is possible that the type of cell is entered via input means 150. The CPU 110 of the charging device, depending on the respective charging method, measures the voltage and/or the current in the charging/load pulses. Preferably the charging device 100 comprises at least one capacitor which is used for providing the charge for the charging pulse. Similarly it is possible to use the at least one capacitor for discharge during the load pulse, wherein the stored charged is then discharged via a resistance.