DE9004061U1 - Charger for nickel/cadmium batteries - Google Patents

Description

* ""'"' ^: ""Patent Attorney

Graduate Physicist Reinffrled Frhr. v. Schorlemer

D-35OO Kassel Brüder-Grimm-Platz 4· Telephone (O561) 1 5335

Landwehr + Schultz Electronics GmbH Calden, D-3527 Calden

Charger for nickel / cadmium batteries

The invention relates to a charger of the type specified in the generic term of claim i

Like many other batteries, cadmium batteries can be recharged after being discharged. In contrast to lead batteries, for example, it is not possible to determine the end of the charging phase by simply specifying a voltage or a period of time. This is because, on the one hand, the charging characteristic of a nickel/cadmium battery is strongly dependent on its temperature, its age, its current, usually unknown state of charge, the strength of the selected charging current and other variables, and therefore neither the charging voltage nor the charging time can be a measure of the state of charge achieved by the battery. On the other hand, depending on the strength of the charging current and the current state of charge, harmful gases develop in nickel/cadmium batteries, which, particularly in the case of closed, gas-tight batteries, which is the main aim of the invention, can lead to such high internal pressure in the battery that the battery or individual cells are damaged or even permanently destroyed. Therefore, charging a nickel/cadmium battery must take place under controlled conditions in order to avoid harmful internal pressures, which occur particularly when the battery is overcharged with high charging currents.

Safe charging of nickel/cadmium batteries is possible by using a sufficiently small charging current, e.g. a current that does not exceed a value of 1/20 I. I is the current that would flow if the nominal capacity of the battery were discharged for one hour and therefore at a nominal capacity of

eg 4.5 Ah would be exactly 4.5 A. In this example, the battery could be charged with a current of 0.225 &lgr; unlimitedly and, in particular, even beyond the state of full charge without the occurrence of gas evolution that is harmful to the battery's function. Of course, the value i/20 I is not an absolute value, but depends on the type, temperature and other properties of the batteries and must therefore be determined individually.

Charging with a current of 1/20 I is harmless, but very time-consuming. As a nickel/cadmium battery usually has to be charged to around 140% of its nominal capacity in order to compensate for the heat losses and the like, a 4.5 Ah battery would have to be charged for around 28 hours with a constant current of 0.225 Ω. This is unreasonable for many applications.

In order to safely charge a nickel/cadmium battery, it is also known to discharge the battery almost completely, i.e. to discharge voltages of 0.9 to 1.0 volts, before charging, because at these voltages all possible different charging characteristics converge and therefore a defined initial state is created from which charging can be carried out safely with a preselected current strength for a preselected period of time. However, this method is also time-consuming due to the required discharge time and should be rejected due to the energy losses caused.

It would also be possible to equip the battery or its cells with sensors to determine the temperature, internal pressure or the like and to stop the charging process as soon as one of the sensors emits a signal with a critical value. However, this would increase the design effort and lead to batteries with more than two connections, which is undesirable.

To avoid the problems described, special charging devices have become known that make it possible to determine selected points of the charging characteristic and to stop the charging process when a certain

The problem with these charging rates is that they only end the charging process, which takes place with the current intensity selected in the case at a point which is at the known maximum of the charging curve or even behind this maximum. This does indeed make it possible for the battery to be charged up to its nominal capacity and for overcharging to be avoided at high charging currents. However, even when using these known methods, it cannot be prevented that considerable gases develop and, for safety reasons, reduced charging currents must be provided.

Finally, it is already known (e.g. Nickel-Cadmium Application Handbook from General Electric, Third Edition 1986, pp. 3 - 59, "Inflection Point Cutoff") that the charging curves of conventional nickel-cadmium batteries have a characteristic course. They have two turning points and a maximum that is just before the point at which a nickel-cadmium battery is considered fully charged (namely after charging to approx. 140% of the nominal capacity). As far as gas development is concerned, this begins when the second turning point of the charging characteristic is reached. It can also become so great that the charging process should be stopped as a precaution when the second turning point is reached, at least when using large charging currents. Chargers that could achieve this are not yet known, perhaps because if the charging process is stopped prematurely, the battery's full nominal capacity would not be reached.

The invention is therefore based on the object of creating a charging device of the type described at the outset, in which the charging process is essentially interrupted when the second turning point is reached. In order to nevertheless be able to achieve the state of full charge that is not achieved, the charging device should preferably be designed in such a way that the charging of the nickel/cadmium battery is not completely terminated after the second turning point is reached, but is continued under other conditions that are not harmful to the battery.

The characterizing features of claim 1 serve to solve this problem.

The invention has the advantage that it provides a charging device with simple design means that operates with a high charging current and stops the charging process near the second turning point. In a particularly preferred embodiment of the invention, it is additionally provided that the charging process is continued with a reduced, harmless charging current when the second turning point is reached.

Further advantageous features of the invention emerge from the subclaims.

The invention is explained in more detail below in conjunction with the accompanying drawing using a preferred and currently considered best embodiment. They show:

Fig. 1 schematically shows an embodiment of the charger according to the invention; and

Fig. 2 different charging and pressure characteristics of common nickel/cadmium batteries.

Fig. 2 shows typical charging characteristics 1 and 2 for nickel/cadmium batteries. Characteristic curve 1 is obtained, for example, with a charging current of I and characteristic curve 2 with a charging current of 1/5 1. In the coordinate system of Fig. 2, the charged current quantity in % of the nominal capacity (% K) is plotted on the abscissa and the charging voltage (U) is plotted along the ordinate. For a 4.5 Ah battery, the charging current I would therefore be 4.5 A, the charging current 1/5 I would be 0.9 A and the current quantity leading to full charge of 140% would be approximately 6.3 Ah. Each characteristic curve 1 or 2 has a first turning point WP1, a second turning point WP2 and a maximum MP in a manner that is characteristic of nickel/cadmium batteries.

In Fig. 2, a curve 3 is also shown, which shows the course of the internal pressure (P) of a closed nickel/cadmium battery, also plotted along the ordinate, for charging according to characteristic curve 1. From this it can be seen that the internal pressure P is comparatively small up to the second turning point WP2, but then increases more strongly and, in particular, rises steeply after reaching the maximum MP. A similar course would result for characteristic curve 2. If, on the other hand, the same nickel/cadmium battery is charged with a charging current that only has a strength corresponding to the value 1/20 I, then the internal pressure would follow a curve 4 in Fig. 2, i.e. initially it would only increase very slightly, then remain practically constant and thus not reach critical values.

According to the embodiment of the invention that has been considered to be the best so far, both in terms of the method and the design of the charger, the nickel/cadmium batteries are charged up to the turning point WP2 with a high current of e.g. I or 1/5 1 and after reaching the turning point WP2 only with a harmless current of e.g. 1/20 I. This ensures that the internal pressure increases up to the second turning point WP2 according to curve 3, which is still tolerable, and after the second turning point WP2 increases only very little or not at all according to curve 4. Therefore, on the one hand, after a quick charge, which was carried out e.g. with the current I, the nickel/cadmium battery can remain connected to the charger for as long as desired without there being any risk of damage due to excessive overpressure. On the other hand, the battery can be charged with the highest permissible current until the second turning point WP2 is reached, i.e. in the shortest possible time, and after the second turning point WP2 is reached with a harmless current (e.g. 1/20 I) until the value 140% K is reached. The charging time from the turning point WP2 to the value 140% K may be longer than the time until the second turning point WP2 is reached, but overall, i.e. also taking into account the development of gas, optimal conditions are achieved.

In Fig. 1, a charging device S according to the invention is outlined with a dotted line. It has connections 6 for connection to a conventional alternating voltage network of e.g. 220 V and connections 7 to which a nickel/cadmium battery 8 to be charged is connected with the polarity shown. The direction of the flowing charging direct current is indicated by double arrows.

A power supply unit 9 (shown only schematically) is connected to the connections 6, which converts the input AC voltage into a DC voltage U, which in turn is converted in an additional device 10 of the power supply unit 9 or in a separate constant current source into a constant direct current, which flows via a switch 11 and through the battery 8 to the ground pole.

Parallel to the connections 7 there is an adaptation network 12, which consists, for example, of a voltage divider formed by resistors and is connected to the two inputs of a digital circuit arrangement 13, which is surrounded by a dashed line. This consists of a plurality of components which are generally known in digital electronics and are controlled by a clock generator 14, which controls the processes taking place in the circuit arrangement 13 in a manner known to those skilled in the art. The individual components are only explained schematically below, since their function and application are assumed to be generally known.

An analog/digital converter 15 is connected to the inputs of the digital circuit arrangement Γ, i-u. This outputs a binary word at a first output ISa, which indicates the size of the charging voltage at the terminals 7 in binary coded form and is fed to a digital memory 16. In fact, the output 15a, as shown schematically in Fig. 1, consists of a plurality of lines so that each binary word can be formed from a corresponding plurality of parallel appearing bits (logical low "L" or high "H"). All lines through which binary words, numbers or the like must be transmitted are designed accordingly* The output of the memory

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chers 16 is connected to the input of another digital memory 17 and to one input of a comparator 18, the other input of which is connected to the output of the memory 17 and the output of which is connected to one input of an OR gate 19.

A second output 20 of the analog/digital converter 15 is connected to the reset input of a digital counter 21. A signal appears at this output 20 whenever the binary word at the other output has changed by one bit. This change therefore corresponds, depending on the setting of the adaptation network 12, to a specific change in the charging voltage, for example a change of 50 mV each time. Alternatively, a different voltage change per bit can also be provided or the analog/digital converter 15 can be set up so that the said signal appears after a change in the binary word at the output 15a by two or more bits.

The input of the counter 21 is fed with the signals of the clock generator 14. Its output indicating the counting state is connected to the input of a digital memory 22, whose output is connected to the input of another digital memory 23 and the input of a comparator 24, whose second input is connected to the output of the memory 23. The comparator 24 has two outputs, one of which is connected to the set input of a standard RS flip-flop 25 and the other to one input of an AND gate 26. The other input of the AND gate 26 is connected to the Q output of the flip-flop 25, while its output is at the second input of the OR gate 19, whose output is connected to the switch 11. This switch 11 is, for example, an electronic switch which is opened when an "H" signal is present at the output of the OR gate 19 and closed when an "L" signal is present at this output.

In the circuit arrangement 1.3 described, blocks 14, 15 and 21 - 2 form: «a means for distinguishing the turning points, the AND-1 led 26 a means for generating a switching signal for the switch 11 when the second turning point WP2 is reached, the output 20 a means for

Emitting reset signals which correspond to constant changes in the charging voltage, the flip-flop 25 is a means which generates a flag signal at the Q output when the first turning point WPl is reached, and the blocks 16 - 18 are a means which generates a trip signal for the switch 11 when the maximum inflection point MP &dgr;3?2&eegr;£3&idiagr;^�Iacgr;.

The described circuit arrangement works as follows:

After the discharged Ni Val/Cadmium battery is connected to the charging device 5, the latter is switched on by means of a switch (not shown), whereby a constant charging current is made available at the output of the device 10 and the supply voltages for the circuit arrangement 13 are made available at an output 27 of the power supply unit 9, so that the battery 8 is gradually charged. Since the battery 8 is initially in a state of charge which in Fig. 2 is before the first end point WP1, the binary word U located in the memory 16 at any time and corresponding to the current charging voltage is always greater than the

η previous word U because the corresponding charging voltage until reaching

n-1,
of the maximum MP increases continuously and is therefore always greater at any time t than at the previous time t. The corresponding

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Output U < U of comparator 18 is therefore at "L". &eegr; n-1

With the signal appearing at the output 20 of the analog/digital converter 15, on the one hand the word in the memory 16 is pushed into the memory 17 and the new word is transferred into the memory 16 and on the other hand the counter 21 is reset, the achieved counting result is pushed into the memory 22 and thereby the previous counting result in the memory is transferred into the memory 23. Therefore the number T

η in the memory 22 each - expressed in pulses - which lead to the change of the

binary word by one bit or the time period T required to change the charging voltage by e.g. 50 mV, whereas the time in the memory

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The number in the table indicates the time period T corresponding to the ^preceding

n-1

Change by one bit or 50 mV. Since, according to Fig. 2, the characteristic curves 1 and 2 are curved to the right up to the first turning point WPl, in this area T ^T is always present, so that the corresponding output of the

n-1 η

Comparator 24 is at "H" and the output of the OND element 25 is at "L". From the first turning point WPl, the charging characteristic curve is curved to the left.

Therefore, from this point onwards T CT is with the result that now the assigned

nn-1
The corresponding output of the comparator 24 is at "H" and therefore the flip-flop 25 is set to "H" at its Q output (at the beginning of the bathing process the flip-flop 25 is reset again at its R input automatically or by means of a special switch). As a result, however, the output of the AND gate 26 remains in the "L" state.

After reaching the z*eV -.n turning point WP2, the conditions i-äi: change in such a way that T < T <-^.it um* the corresponding output of the

nl &eegr;
Compare gate 24 is at "H". Since ^both inputs of AND gate 26 are at "H", an "H" signal corresponding to the logical state High also appears at its output, which is passed on via ODfiiv gate 19 to switch 11 and opens it, thereby completing the charging process.

If the charging process in Fig. 2 is started, for example, at a point corresponding to the value 90% K, then essentially the same conditions arise. This is because regardless of how strongly the nickel/cadmium battery is charged at the start of the charging process, the charging characteristic curve, which then begins at a different charge level, always passes through two turning points, the first of which is a right/left turning point and the second a left/right turning point.

If the charging process is started at a charging state in which the charging voltage depending on the selected charging current is already between the second turning point WP2 and the maximum MP, no "H" signal can appear at the output of the AND gate 26 because the flip-flop 25 is not set at its Q output and can no longer be set. In order to nevertheless terminate the charging process in good time, the condition U <U is set at the output of the comparator 18 in this case.

n nl
uses the Maxiiriuiri MP at the corresponding output of the Vet <j Ic sicherers 18

an "H" state and thus an opening of switch 11.

&psgr; If the charging of the battery 8 is finally started when it is still in a practically fully charged state and therefore the charging voltage has a value that corresponds to a point behind the maximum MP of the charging characteristic, then the first two measured values already fulfill the condition U^U, and the charging characteristic lies behind the

n-1
Maximum MP drops and no longer has an increasing range. This means that the charging process is aborted immediately after the charger is switched on.

The invention therefore makes it possible to terminate the charging process as a rule at the second turning point WP2 and in special cases at the maximum MP. The charging process is of course not terminated exactly at the second turning point WP2 or exactly at the maximum MP because the measured values (binary words at output 15a) are processed step by step and not quasi-continuously. In addition, the exact switching time depends on the selected clock frequency, which should preferably be at least 1 MHz. The generation of the switching signal "when the second turning point WP2 or maximum MP is reached" therefore means that the charging process is terminated as soon as the circuit arrangement 13 described has recognized that the charging voltage in the associated charging characteristic curve has reached the second turning point WP2 or the maximum MP.

According to a particularly preferred embodiment of the invention, the switch 11 is a two-stage switch, the movable contact 28 of which, shown schematically in Fig. 1, is switched from a first fixed contact 29 to a second fixed contact 30 by the "H" signals at the output of the OR gate 19. The first fixed contact 29 is connected to an output of the device 10 which can deliver a high charging current intended for rapid charging, whereas the second fixed contact 30 is connected to an output of the device 10 which provides a comparatively small charging current which is, however, harmless in terms of gas development. As a result, the phase of the chord charge which runs up to approximately the second turning point WP2 is followed by a phase of the slow charge which runs until full charging is achieved , which is indicated in Fig. 2, for example, by a dashed line for the characteristic curve 1.

This ensures that charging can be continued up to the K point and beyond without damaging the battery 8.

The invention is not limited to the described embodiment, which can be modified in many ways. For example, it would be possible to design the entire circuit arrangement 13 as a microprocessor with an integrated analog/digital converter 15 and to control the described function with the aid of suitable software, which can be stored in an external memory (EPROM, EEPROM or the like). In this case, the function of the flip-flop 25 could be implemented, for example, by setting a "flag". Instead of the described use of digital components, it would also be possible to achieve the desired function with analog components. It is also possible to carry out the charging process with a direct current that changes in strength instead of with a constant direct current. Accordingly, the reduced current used after reaching the second turning point WP2 can be a current that changes over time, in particular a pulsed direct current, instead of a direct current. In this case, it is only necessary to use the output of the OR gate 19 to control a device which switches the switch 11 on and off in the desired ratio. The reduced current is expediently so large that it maintains the full charge of the battery even if it remains connected to the charger 5 for a longer period of time after charging.

Finally, it is understood that the charger described can, if required, also be equipped with all the necessary displays for the various charging phases and with means that enable the charging current to be adapted to the type of battery used, and that the charger can be designed either for individual cells and/or for accumulators that consist of a plurality of individual cells connected in series and/or parallel. In this case, an indicator for the end of the rapid charging phase would be particularly useful. Finally, the charger could also be designed in such a way that it is suitable for charging several batteries at the same time.

Claims (8)

Expectations 1. Charger for nickel/cadmium batteries whose charging characteristics have two turning points and a maximum, with a power supply for generating a charging current, with electrical connections intended for connection to the battery, with a switch connected between the power supply and the connections and with a circuit arrangement connected to the connections and the switch and generating a switching signal for the switch depending on a selected property of the charging characteristic of the battery, characterized in that the circuit arrangement (13) has means (14, 15, 21-25) for distinguishing between the two turning points (WP1, WP2) and means (26) for generating the switching signal when the second turning point (WP2) is reached. 2. Charger according to claim 1, characterized in that the means contain a counter (21), a clock generator (14) connected to its input, two memories (22, 23) connected by a comparator (24) and connected to the output of the counter (21) and a means (15) connected to the reset input of the counter (21) for emitting reset signals corresponding to constant changes in the charging voltage. 3. Charger according to claim 2, characterized in that a means (25) generating a warning signal when the first turning point (WPl) is reached is connected to an output of the comparator (24). 4. Charger according to claim 3, characterized in that the means for generating the switching signal is an AND gate (26) which is connected to another output of the comparator (24) and the output of the means (25) generating the reminder signal. 5. Charger according to one of claims 1 to 4, characterized in that the circuit arrangement (13) is realized by a microcomputer with integrated analog/digital converter (15) and an external memory intended for storing the software. 6. Charger according to one of claims 1 to 5, characterized in that the circuit arrangement (13) has a second means (16, 17, 18) which generates a switching signal when the maximum (MP) is reached and is also connected to the switch (11). 7. Charger according to one of claims 1 to 6, characterized in that the switch (11) has two inputs (29, 30) and the power supply unit (9, 10) has two outputs intended for different charging currents and connected to these inputs (29, 30). 8. Charger according to one of claims 1 to 6, characterized in that a means for periodically switching the switch (11) on and off is connected between the switch (11) and the means (16 - 18 or 26) generating the switching signal.