GB2292024A - A battery charging circuit - Google Patents

Abstract

A circuit for charging batteries B1 - B4 comprises means for supplying a charging current to each battery, load application circuits (400) for applying a load to each battery and control means comprising a main control unit (200), charging control circuit (300) and discharging control circuit (400) for connecting the charging current supply means and load applying circuits (400) alternately to the batteries. The circuit enables alkaline and nickel-cadmium batteries to be recharged. The main control unit initially measures the internal resistance of the battery and charging is prevented if the measured value exceeds a preset value. The circuit also monitors the load voltage to determine if the battery requires charging, charging being terminated when the charging voltage exceeds a preset value or when the rate of increase of the charging voltage is less than a preset value. <IMAGE>

Description

A BATTERY CHARGING CIRCUIT AND A BATTERY CHARGER INCORPORATING SUCH A CIRCUIT This invention relates to a battery charging circuit and technology for use more particularly but not exclusively with a battery charger.

Battery chargers are known, particularly for use with nickel cadmium batteries which can be recharged many times prolonging their useful life. However, such batteries are more expensive than carbon zinc/chloride and alkaline batteries which have generally been regarded as disposable, that is to say once they have been discharged once, they have no remaining useful life.

Americans spend on average US$3.4 billion per year on disposable batteries, using each battery and then discarding it, without concern for the damage done to the environment. The scale of battery dumping has forced several European countries to set up expensive battery disposal plants as the chemicals in batteries are hazardous to the environment. The cost of such plant is expensive and is effectively paid for by battery users.

It is an object to the invention to provide a charging circuit suitable for recharging alkaline and carbon zinc/chloride batteries, thus extending their useful life.

According to the invention there is provided a charging circuit for charging a battery comprising means for supplying a charging current to a said battery, means for applying a load to a said battery and control means for connecting the charging current supply means and load applying means alternately to the said battery.

Alternate charging and discharging of the battery through a load using the apparatus of the invention agitates and provides a flow of chemicals in an alkaline battery, thus increasing the ability of the battery to accept charge.

Preferably sensing means are further provided for sensing at least one voltage across the said battery and most preferably the sensing means senses the voltage across the battery in an open circuit state, in a load state when the battery is connected to the load application means and in a charging state when the battery is connected to the charging means. Such sensed voltages are usable to calculate the optimum charging time and battery state before charging.

Most preferably, the control means controls the current supply means to turn off shortly before the load application means is connected. With neither connected to the battery, the sensing means is then able to measure the open circuit voltage of the battery at that time.

The charging circuit of the described embodiment of the invention in particular calculates the internal resistance of the battery to determine if the condition of the battery is poor and thus if attempts to recharge the battery are futile.

In a preferred embodiment, the circuit is adapted for use in a charger for AA, AAA, C and D type alkaline batteries and the sensing means the invention determines if the open circuit voltages of the battery is below or equal to 1.0 volt or if the internal resistance of the battery is greater than 2 ohm, indicating that the battery is in too poor a condition for recharging. Furthermore, the sensing means can also determine if the battery does not in fact require charging by sensing the loaded circuit voltage, in particular if this is greater than or equal to 1.4 volts.

The charging means preferably applies a constant current charge to the battery at a voltage of below 10 volts, preferably 4 volts through a charging resistor between 10 to 100 ohms, preferably 39 ohms for AA and AAA batteries and 27.3 ohms for size C and D. Preferably, the charging intervals should be between 10 seconds and 10 minutes in duration most preferably 60.9 seconds with the intervening discharge time duration being between 1 milliseconds and 2 seconds, preferably 200 milliseconds.

The invention is principally directed to a battery charging circuit which may be incorporated into a battery charger specifically dedicated to charging of one or more batteries. However, the charging circuit may equally be incorporated directly into appliances in a similar manner to the way nickel cadmium battery charging circuits are currently included in some rechargeable products. In such a case, in a similar manner to such known charging circuits, the charging circuit of the present invention will be actuatable to charge alkaline batteries directly, taking power from a mains transformer.

An embodiment of the invention will now be described by way of example with reference to the accompanying drawings in which: Figure 1 is a perspective view of the battery charger incorporating an embodiment of a battery charging circuit in accordance with the invention.

Figure 2 is a schematic block diagram of the charging circuit of the embodiment of Figure 1; Figure 3 is a circuit diagram equivalent of the block diagram of Figure 2; Figure 4 is an illustration of the charging wave form of the circuit to Figure 3; Figure 5 is a schematic diagram illustrating an equivalent circuit for calculation of the internal resistance of the battery; Figure 6 is a graph of charging voltage against time; Figure 7 is a flow diagram illustrating the battery screening algorithm of the charging circuit to Figures 2 and 3; and Figure 8 is a flow diagram illustrating the battery charging algorithms of the charging circuit of the embodiment of Figures 2 and 3.

With regard to Figure 1 a general perspective view of the battery charger incorporating a battery charging circuit of the invention is shown.

The charger, generally designated 100 includes a casing 114 provided with an openable lid 112 normally biassed in an open position as shown but held in closed position by a catch connected to release button 110. The charger 100 includes 4 cradles, 120(1-4) each for receiving a battery B1-B4. The batteries B1-B4 are held in a respective cradle 120(1-4) between clips 122(1), 124(1), for recess 120(1) and likewise for the others. The position of clip 124(1) is adjustable in dependence upon the size of the battery as is well-known in the art. In particular, clip 124(1) is movable between two positions, the first position for charging AA and AAA batteries and a second position for charging C & D size. All such batteries due to their battery chemistry have the same notional battery open circuit voltage, namely 1.5 volts.

Insertions of battery into cradle 120(1-4) causes a corresponding switch 126(1) (not shown) to move from an open position for AA and AAA size batteries to a closed position for C and D size batteries for the purpose to be described hereinafter.

The charger is connectable to a mains supply via a separate transformer providing 6V d.c. (not shown) and is also provided with several user-actuatable button/switches 130, 132, 134, and a liquid crystal display 140.

Switch 130 selects between charging of alkaline and nickel cadmium batteries, switch 132 selects for which battery B1 B4 charging information is presented on liquid crystal display 140 and switch 134 selects the size of the battery inserted in any particular cradle 120 (1-4).

The battery charger of Figure 1 incorporates an alkaline battery charging circuit shown schematically in Figure 2 which comprises, generally, a main control unit 200 which receives a voltage supply from a power supply 250 to input VDD and a reset circuit 290 to input RES. A voltage reference is provided by voltage reference circuit 270 to input AV REF. A visual output is provided by means of LCD 140 controlled through segment bus 142 and common bus 144 via resistance network 146 and an audible output is provided by buzzer 150.

The charging circuitry comprises a charging control unit circuit 300 connected to MCU 200 via bus 310, a discharge control circuit 400 connected to main control unit 200 via bus 410 and an analog voltage input circuit 500 connected to MCU 200 via input bus 510. Charging control circuit 300, discharging control circuit 400 and analog input circuit 500 are connected directly to batteries B1-B4.

As the charging of alkaline batteries require precise monitoring of each battery individually, the charging control unit 300, discharging control circuit 400 and analog input circuit 500 each in fact comprise one separate circuit for each battery B1-B4 to be charged. This is shown more clearly in Figure 3 in which it can be seen that each battery B1-B4 is provided with an identical circuit of which charging control circuit 300 (1), discharging control circuit 400 (1) and analog input circuit 500 (1) for connection to battery B1 are indicated by dotted lines and specific reference will be made to these circuits hereinafter, it being apparent that the circuits for the other batteries B2-B4 as shown are the same and operate in the same way.

The main control unit is provided with random access memory 210, read only memory 220 containing operating programs and look-up tables, oscillator 222 connected to crystal 224 and in/out ports 226 connected to buses 310, 410, 510.

With reference to Figure 3, the blocks of Figure 2 are shown in more detail.

Main control unit 200 is preferably a microcontroller unit with A-D conversion capability or discrete components with similar functions. Power supply 250 is of standard construction receiving a 6V 500 milliamp input through jack socket 252. Two regulator circuits generally designated 254 and 256 are provided, regulator circuit 254 being based around an adjustable regulator LM 317 providing a four volt output for feeding to the charging circuit 300. Regulator circuit 256 is based about transistor 257 and provides a 3.5 volt regulated output for powering the main control unit 100 and sensing circuitry.

On initial application of power from power supply 250, reset circuit 290 will send a pulse to RES thus resetting the MCU 200 to a default condition.

Since precise voltage measurement is required, a voltage reference circuit 270 is provided. The circuit 270 is based on a reference diode 272 which effectively provides a reference voltage across resistor 274 and potentiometer 276. Adjustment of potentiometer 276 will cause an adjustment in the reference voltage generated by diode 272 thus accurately adjusting AV REF.

A battery screening algorithm (Figure 7) and the battery charging algorithm (Figure 8) will now be described are controlled by a main control routine which accesses the routines for each battery in sequence, as required, so that each battery is connected to the charging control circuit, discharging control circuit and the measurements of the various voltages taken at the correct times. Then signal AV REF is then fed to input AV REF of MCU 200 with capacitors 278 being provided to ground any high frequency noise.

Two earth paths are provided in the control unit, the first being a signal earth (illustrated by a downwardly pointing triangle) and the second being a power earth (indicating by the conventional earth sign). Separate earth paths are provided so that any noise in the power earth path is kept separate from the signal earth path. Earth connections for the MCU 200 to both earths are provided via inputs AVSS and VSS.

Turning to the battery charging circuitry, charging control circuit 300 includes 4 charging circuits one for each battery of which charging circuit 300 (1) will be described.

Control line C1-C4 of MCU 200, being part of bus 310 is connected via a resistor 312 to the base of transistor 314 which in turn is connected by means of biasing resistors 316, 318 to PNP transistor 320. The emitter of transistor 320 is connected to a 4 volt power supply from power supply circuit 254 with the connector being connected to current limiting transistors 322, 324. Resistor 322 is permanently in circuit and is preferably of 39 ohms resistance.

Resistor 324 is of 91 ohms resistance and is arranged to be switched in circuit independence upon the state of switch 126 (1). With switch 126 (1) closed, the total resistance of resistors 322, 324 is 27.3 ohms thus giving a higher charging current for C and D size batteries. Resistors 322, 324 are connected by means of line 328 to battery connection 122 (1).

Load application circuit 400 comprises 4 load application circuit portions 400 (1-4) which are connected by means of bus 410 to control signal ports L1-L4 of main control unit 200.

Each load circuit, of which load circuit 400 (1) will now be described comprises a control transistor 410 connected to control line L1 via resistor 412, the connector of transistor 410 being connected to a load resistor 414 preferably of value 4.7 ohms to 15 ohms but most preferably 5.1 ohms having a 0.5 watt capacity.

Load resistor 414 is also connected to battery terminal to 122(1).

Analog input circuit 500 comprises 4 circuit portions 500 (1-4) each connected to an analog input AD1-4 of main control unit 200. Circuit 500 (1) basically connects terminal 122 (1) to analog input AD1 of main control unit 200 and provides protection circuitry for MCU 200, in particular a pull high resistor 520 in parallel with capacitor 522, connected to VDD, to prevent the voltage at terminal AD1 from floating if a battery is not connected and a voltage swing protection circuit in the form of diode 530 connected to signal ground and resistor 510 for limiting current if the polarity of battery B1 reverses.

The operation of the charging circuit of Figures 2 and 3 will now be described with reference to Figures 4-6 and operational Flow Charts of Figures 7 and 8.

The main control unit 200 controls the charge of each battery B1-B4 independently by controlling, independently, the charging circuits 300 (1-4) and discharging control circuits 400 (1-4). Reference will be made only to the charging of battery B1 but will be understood that the main control unit 200 controls tho charge control and discharge control circuits for the other batteries 2-4 independently but in generally the same mannor.

The form of the charge control wave form is shown in Figure 4. Constant charging current at voltage V@ is applied to battery B1 by turning on transistors 314, 210 by applying a signal on line C1 for a period C, causinq current to flow through resistor 322 (and optionally 324) at voltage v.

The charging current is applied in a plurality of charging intervals C@ which may be in the range 10 seconds to 2 minutes but most preferably 60.7 seconde.

After cach charging interval C@, instead of being connected to the charging circuit 300, a signal is provided on Line L1 to turn on transistor 410 and connect terminal 122(1) and battery B1 to load resistor 414 for a discharge interval D@. The discharge interval should be in the range @ millisocond to 2 seconde and most proferably 200 milliseconds. The charging and discharging circuits are thus connected to the battery alternately.

Once every 3 charging intervals, the signal on line C1 ceases and for a short interval O@ of length in the range 1 millisecond to 2 seconds but most preferably 200 milliseconds, the battery B1 is connected either to circuit 800 nor circuit 400. This 200 millisecond time interval reduces the relevant period C, accordingly. The voltage at terminal 122(1) during this interval drops to the open circuit vol@ag@ of the battery @@@@ly vol@age V@.

As will be appreciated, due to the three time intervals Ct, o, and D1, it is possible for sensing circuit 500 (1) to provide to the analog terminal AD1 of MCU 200 in any cycle the charging voltage V, the open circuit voltage V,, and the load voltage Vd of the battery.

The charging cycle is generally of the same form as that shown in Figure 4 and is repeated continuously until a full battery charge condition, or a bad battery charge condition is detected.

With reference first to Figure 7, when a battery is initially inserted in the battery charger, and switches 130, 134 and, automatically, 126 (1) are switched according to the size and type of battery, the main control unit 200 firstly measures the open circuit voltage V0 of the battery B1 at step 601, then connects load resistor 414 by applying a signal on line L1 and measures the load voltage VL at step 602.

The MCU 200 then calculates the internal resistance (r) of the battery and this calculation is explained with reference to Figure 7 which illustrates the connection of battery B1 to load resistor 414.

By Kirchhoff's Laws: VO = ILr + IL RL -- 1 V@ = = 1L RL ... 2 Solving 1 and 2: Internal Resistance, r = RL (VO - VL) / VL.

The following tests are then performed: 1. At step 604, a check is made to ensure that the internal resistance (r) of the battery is less than a pre-determined figure. If the internal resistance is greater than 2 ohms and the condition of the battery can be taken to be poor.

2. At step 605 it is determined if the open circuit voltage is greater than one volt. If not, the battery cannot be charged.

If both steps 604-605 have indicated bad battery, a display signal indicating the same is sent to LCD 140 and no charging is performed.

3. It is then determined if the load voltage Vl is greater than or equal to 1.4 volts. If so, it indicates that the battery does not require charging and in such case a display indicating that the battery is full is sent to LCD 140.

If the battery satisfactorily passes the three tests 604606 calculations are then made of the capacity that is to say the amount of charge which the battery can take up and the approximate charging time and these are displayed on LCD 140. The available capacity and recharging time are a non-linear function of the load voltage VL, the open circuit voltage of the battery V0 and the size of the battery.This relationship is determined by experiment, that is to say by conditioning a battery so that in its discharge cycle it is at a particular value of load voltage and, a particular value of open circuit voltage and then discharging the battery completely, measuring the energy released and, independently, charging a battery from the same state and then discharging it to indicate the approximate charging time to full charge and the available full capacity. The results are stored in a look-up in table in ROM 220 of MCU 200.

Assuming the battery requires charging and can be charged, control then passes to the battery charging algorithm of Figure 8.

In Figure 8, the MCU 200 through sensing circuitry 500 (1) measures the charging voltage Vc every three cycles of charging current, measures the open circuit voltage V0 at the same frequency and measures the loaded voltage VL every charging cycle during period D,.

The battery internal resistance (r) is then calculated at step 704 every three minutes, as is the cumulated energy, which is simply the charging current since the start of charge multiplied by the elapsed time.

Several tests are then performed as follows: 1. At step 705 the charging voltage Vc is measured. If this is greater than 1.7 volts, this indicates and fully charged state and charging is terminated and a "full" display symbol is sent to LCD display 140.

2. At step 706, the charging voltage Vc gradient is measured. In particular, under the reference to Figure 6 a charging voltage against time characteristic is shown. In general, the charging voltage will rise as charging increases and will then fall. If the charging voltage starts to increase at greater than 0.4 volts per hour and then decrease at greater than 0.4 volts per hour, this indicates a fully charged or bad battery and charging is stopped.

This is calculated by taking three consecutive measurements and measuring a first slope, M1, between the first two points and a second slope, M2, between the second two points as shown in Figure 5. A further test is taken to measure the internal resistance at step 707 and, if the rate of change of charging voltage exceeds the values indicated at steps 706 and if the internal resistance (r) is greater than two, in a similar manner to the battery screening algorithm, the battery is rejected and the charging stopped and a "bad" battery symbol displayed in LCD 140.

3. At step 708 a similar internal resistance r test is applied to all batteries passing the step 706 test.

4. In step 709 the charging voltage Vc is continuously measured and stored. This will generally rise and then fall after obtaining a peak Vp as shown in Figure 6. When such peak is attained, this is stored and subsequent charging voltage measurements taken. When the difference between Vp and the current charging voltage Vc exceed 0.5 volts, this indicates a fully charged state and charging is stopped with a full symbol displayed on LCD 140.

5. At step 710 and 711, tests based on the capacity calculations made in the battery screening algorithm are performed. In particular, the cumulated energy calculated in step 704 is compared to the estimated capacity of the battery calculated in step 607. If the energy is found to be equal to the estimated value, charging is stopped since the battery is assumed to have reached its fully charged state.

Likewise at step 711, if the actual charging time since charging was initiated is found to be greater than the estimated charging time calculated at step 607, then charging is stopped.

At the end to these tests, the control algorithm returns to the main control routine.

Although an embodiment of the battery charging circuit of the invention has been described for use in a battery charger, this is not to be construed as limitative. For example, the battery charging circuit may be incorporated into a device using alkaline batteries in a similar manner to the provision of a charging circuit for a nickel cadmium battery incorporated in an appliance as at present, for example, in a notebook computer or rechargeable lamp.

The battery charger 100 is adapted to be suitable for charging both alkaline and nickel cadmium batteries. The battery charging circuit of the embodiment of the invention to be described is suitable for charging both regular and nickel cadmium batteries although a different control of the charging current is required. The charging algorithms for nickel cadmium batteries are well-known and, as it will be apparent to one skilled in the art, that the charging circuit of the invention is equally applicable for use of a charging nickel cadmium battery using a known nickel cadmium battery charging algorithm and can be configured for alkaline or nickel cadmium charging by actuation of button 130 in a manner known in the art.

Claims (14)

1. A charging circuit for charging a battery comprising means for supplying a charging current to a said battery, means for applying a load to a said battery and control means for connecting the charging current supply means and load applying means alternately to the said battery.

2. A charging circuit according to claim 1 wherein sensing means are further provided for sensing at least one voltage across the said battery.

3. A charging circuit according to claim 2 wherein the sensing means are adapted to sense the voltage across the battery in an open circuit state, in a load state when the battery is connected to the load application means and in a charging state when the battery is connected to the charging means.

4. A charging circuit according to any preceding claim wherein the control means is arranged to control the current supply means to turn off shortly before the load application means is connected.

5. A charging circuit according to any preceding claim adapted to calculate the internal resistance of the battery to determine the condition of the battery.

6. A charging circuit according to any preceding claim adapted for use in a charger for AA, AAA, C and D type alkaline batteries.

7. A charging circuit according to claim 3 wherein the sensing means is arranged to determine if the open circuit voltages of the battery is below or equal to

1.0 volt or if the internal resistance of the battery is greater than 2 ohm.

8. A charging circuit according to any preceding wherein the sensing means is arranged to determine if the loaded circuit voltage is greater than or equal to 1.4 volts.

9. A charging circuit according to any preceding claim wherein the charging means applies a constant current charge to the battery at a voltage of below 10 volts through a charging resistor between 10 to 100 ohms.

10. A charging circuit according to claim 9 wherein the charging means applies a constant current charge to the battery at a voltage of about 4 volts through a charging resistor of about 39 ohms for AA and AAA batteries and 27.3 ohms for size C and D.

11. A charging circuit according to any preceding claim wherein the charging intervals are between 10 seconds and 10 minutes in duration with the intervening discharge time duration being between 1 milliseconds and 2 seconds.

12. A charging circuit according to claim 11 wherein the charging interval is about 60.9 seconds with an intervening discharge time of about 200 milliseconds.

13. A battery charger comprising a battery charging circuit as claimed in any preceding claim.

14. A charging circuit substantially as hereinbefore described and as illustrated in Figures 2 and/or 3, optionally in combination with Figures 1 and 4 to 8 of the accompanying drawings.