GB2155709A - Battery charging - Google Patents

Abstract

The voltage across a battery B to be recharged is compared by a voltage control circuit (105) (Fig. 6), with a reference voltage from a reference voltage circuit (107), and if below the reference level, causes circuitry to allow charging of the battery B through a low resistor Ro and thyristors TH1 and TH2 in series between a secondary centre tap and the secondary ends. Firing of the thyristors TH1 and TH2 occurs in alternation, the firing point being controlled by a phase shifted signal from a phase shifter circuit 101 and a bias signal from a current control circuit (104) and subsequently the voltage control circuit (105) to a thyristor drive circuit 102. The current control circuit (104) senses the charging current through the resistor Ro and holds its level constant until the battery voltage reaches a predetermined level at which the voltage control circuit (105) overrides the current control circuit (104) and so controls the charging current that the battery voltage remains constant until the charging current falls to a predetermined low level at which a low level current trip circuit (106) alters the voltage sensed by the voltage control circuit (105). Thereupon the voltage control circuit (105) holds the charging current at a low "care and maintenance" level. An alternative embodiment (Figs 10, 11), charges the battery via a full wave rectifier and one thyristor and has means which, if the battery voltage is initially below a predetermined flat level, controls the thyristor to supply trains of charging current pulses to the battery, the pulses increasing in current amplitude within each train. When the battery voltage exceeds the flat level, the charging is thereafter effected according to the above constant current and constant voltage stages. <IMAGE>

Description

SPECIFICATION Battery charging The present invention relates to the charging of rechargeable batteries, such as lead-acid batteries.

It is known to charge such a battery in a three stage method in which the battery is first charged to a predetermined battery cell voltage and then, in the second stage, charging is carried out for a time proportional to the duration of the first stage. It is also known to give the battery a subsequent "care and maintenance" charge; this is intended to prevent the battery becoming discharged once it is already fully charged.

Such methods require the use of a battery charger with a timer to determine the durations of the first and second stages and circuitry to relate the duration of the first stage to that of the second stage. It is a defect of such methods that the timer is prone to malfunction causing the battery to be charged insufficiently, or for too long with the risk of damage to the battery. The necessity for a timer also increases the complexity of the charging circuitry and thereby the cost of the battery charger.

The present invention seeks to overcome these problems by eliminating the timer and using only current and voltage parameters to determine the lengths of the changing stages. Thus the present invention proposes a charging method having a first stage in which the battery is charged at a constant current until a predetermined battery voltage is attained, and a second stage in which the battery is maintained at that predetermined voltage until the current has fallen to a predetermined value.

Once the current has fallen below the predetermined value, the battery is deemed to be fully charged. It is then possible to carry out a "care and maintenance" charge to prevent the battery self-discharging. The "care and maintenance" charge will be carried out at a volage less than that which determines the transition between the first and second stages.

Since the durations of the charging stages are determined by battery characteristics, there is no need for a timer. Furthermore a more accurate charge may be given than in the known charging methods because the duration of the charge is determined by the battery itself, and not by external factors.

The invention will now be described in more detail, solely by way of example, with reference to the accompanying drawings, in which: Figure lisa graph showing the current characteristics of a battery charging method according to the present invention; Figure 2 is a graph showing the voltage characteristic corresponding to the current characteristic of Figure 1; Figures 3 and 4 are block circuit diagrams of two different embodiments of apparatus according to the present invention; Figures 5 and 6 are circuit diagrams of parts of an embodiment of apparatus according to the present invention shown in more detail than Figure 3; Figures 7(a) and (b) are circuit diagrams of parts shown in block form in Figures 5 and 6; Figures 8(a), (b), (c) and (d) are further circuit diagrams of parts shown in block form in Figure 6;; Figure 9 is a graphical representation of two waveforms occurring in the circuit of Figures 5 and 6; Figure 10 is a detailed circuit diagram of a further embodiment of apparatus according to the invention; Figure 11 is a graphical representation of a waveform occurring in the circuit of Figure 10; and Figure 12 is a circuit diagram of an apparatus embodying the invention showing adjustable resistors added to allow adjustment of parameters of the circuit.

The first stage of a method according to the present invention is to charge the battery at a constant current. A suitable charging circuit, described hereinafter, is connected to the battery and currrent I (see Figure 1) is applied, commencing at a time to. The magnitude of the current 11 is not critical. In general a higher current will result in a shorter charging time, but it is desirable that the current be not too high or damage to the battery may occur. It has been generally found that a charging current of about a fifth to a tenth of the battery capacity gives suitably rapid results without risk of damage but even greater currents can be safely applied.

Whilst the constant current is fed to the battery the terminal voltage on the charging circuit will rise with time, as shown in Figure 2.

If the constant current were to be maintained indefinitely, the voltage would continue to rise and the battery would be damaged. Therefore in accordance with the present invention this first charging stage is terminated when a predetermined voltage V, is reached. The predetermined voltage V is chosen to be preferably approximately equal to maximum battery terminal voltage. Thus, if Vpc is the voltage per cell of the battery when charging, V, is desirably 2.6 Vp.

When the predetermined voltage V1 is reached, at time t1, the charging circuit changes from delivering a constant current to delivering a constant voltage.

This constant voltage stage is the second stage of the charging method. During the second stage, the current falls as the battery approaches its fully charge state. When the current is sufficiently low, for example about a twenty-fifth to a fiftieth of the capacity of the battery, the battery is deemed to be fully charged and the second stage is terminated at time t2. The current at this time is 12 as shown in Figure 1.

When the battery is fully charged, the terminal voltage on the charging circuit will be approximately equal to the battery terminal voltage, any slight differences being due to voltage drops across the leads of the charging circuit. Hence the current flowing when the battery is fully charged is that used to "gas" the battery, i.e. the current at which gassing occurs. If the second charging stage is terminated when the current 12 is equal to or only slightly greater than the "gassing" current, it can be said with confidence that the battery is fully charged. The "gassing" current may be determined by applying a voltge of 2.6 Vpc to a ful charged battery and noting the current that flows once equilibrium is reached.

Thus, once the current has fallen to a suitably low value, the battery is deemed to be fully charged and the second stage of charging is completed. The charger may have indicators which show when this occurs, triggered by a signal from the current detector.

If the fully charged battery is not to be used immediately, a "care and maintenance" charge may be applied. Thus as shown in Figure 2, a constant voltage V2 is applied after time t2, the end of the second charging stage. Voltage V2 is typically 2.25 Vp, which voltage may be maintained indefinitely without gassing or other deterimental effects on the battery. As can be seen from Figure 1, a small current is drawn from the charging circuit to compensate for battery self-discharge, and the battery is maintained in the fully charged state.

The charging ciruit may be designed so that, once the "care and maintenance" charge has been established, it is not possible to return to the charging state without first removing the battery.

However, if the current required by the battery in "care and maintenance" exceeds the value 12 which is the gassing current of V, then the charging circuit re-commences charging by re-applying the constant current I,. Thus a rechargeable battery is recharged by a three step charging method in which the time of transition from the first to the second stage, and the time of termination of the second stage, are determined by the battery parameters, from measurement of the voltage and current applied to the battery.

Figure 3 shows one example of a charging circuit for carrying out a method of charging in accordance with the present invention. A battery B to be recharged is connected in series with a low-value resistor R,, (e.g. less than 01 ohms), the positive terminal of the battery B being conected to a centretap output terminal of a transformer secondary winding S, the primary winding P being intended to be connected across a 240 volts, 50 Hz mains supply. The end of the resistor R0 remote from the battery B is coupled through first and second thyristors TH1 and TH2 to the ends of the secondary winding S.

The battry B is connected as shown to pins 2 and 3 of a charge controller circuit 100. The resistor R0 is connected pins 3 and 7 of the controller 100 to enable a voltage proportional to battery current to be sensed by the controller 100 at pin 3, pin 7 being a ground/return pin.

A red emitting LED designated RLE is connected across pins 4 and 5 of the controller 100 and is energised when charging of the battery is taking place. A green emitting LED designated GLE is connected across pins 6 and 7 of the controller 100 and is energised when the battery has reached the charged state.

The gates of the thyristors TH1 and TH2 are connected respectively to pins 10 and 9 of the controller 100 to receive drive signals, and the controller 100 has a pin 8 connected to one end of the secondary winding S to receive a timing signal from the mains frequency.

Pin 2 of the controller 100 is the supply input pin for the circuitry of the controller.

The embodiment of Figure 4 differs from that of Figure 3 only in having a transformer secondary winding S' which is not centre tapped but is connected to a full wave bridge rectifier, represented by a block in Figure 4, whose positive and negative output terminals are connected respectively to pin 2 of the controller 100 and, through a single thyristor TH, to pin 7. The output of the full wave rectifier is not smoothed, so that a timing signal can still be picked up by the connection of pin 8 to the rectifier negative output terminal.

The charge controller 100 controls the level of current and voltage in the various charging stages by adjusting the firing angle of the thyristor or thyristors. This is carried out by utilizing a phase shifted version of the rectified mains supply. A phase-shifted signal is produced in the controller 100 and is used to ensure that the firing point of the or each thyristor occurs towards the trailing edge of a rectified supply voltage pulse.

Figure 5 shows part of an embodiment similar to that of Figure 3, and having the same charge controller 100. Parts of the controller 100 connected to its pins 2, 7, 8, 9, 10 and 11 are shown. The controller 100 receives its supply via pin 2 and a diode D3 which is included to prevent damage to the controller if it is accidentally connected incorrectly, i.e. with the reverse polarity, to the battery B.

During charging, current flow from the transformer to the battery B is from the centre tap of the secondary winding S to the positive terminal of the battery, through the battery B and the resistor R,, and back to the transformer secondary winding S via the two thyristors TH1 and TH2. Two Zener diodes ZDa and ZD2 are connected cathode to cathode between the cathodes of the thyristors to suppress spikes. Resistors R1 and R2 connect the gates to the cathodes of the thyristors respectively to prevent spurious firing.

A timing signal is picked up by connection of the pins 8 and 11 through diodes D1 and D2 to a phase shifter circuit 101 which produces a shallow smoothed sine pulse train, shifted relative to the mains frequency waveform at the secondary winding S, at an output terminal 99. A d.c. bias level is supplied to the terminal 99 through a conductor 98, and the biased, phase-shifted smoothed sine pulse train at the terminal 99 is supplied to a thyristor drive circuit 102 having two output terminals which are coupled respectively through diodes D4 and D5 to the gates of the thyristors TH2 and TH1 via pins 9 and 10. Figure 9 illustrates the biased, phase-shifted smoothed sine pulse train signal which occurs in operation at the terminal 99.

Figure 6 shows the remainder of the controller 100, including itspins3,4,5,6,13, 14, and 20. It will be understood that where a circuit element is shown grounded in Figure 6, a connection to pin 7 (Figure 5) is implied.

The phase shifter circuit 101 and the thyristor drive circuit 102 are shown in detail in Figure 7(a).

The phase shifter circuit 101 consists of a parallel combination of a resistor R4 and a capacitor C1 connected in series with a resistor R3 between the cathode of the supply diode D3 and the coupling diodes D1 and D2 of Figure 5. The voltage on the capacitor C1 is used to control the thyristor drive circuit 102, which consists of a PNP transistor TR1 having its emitter coupled through a Zener diode D6 to the supply diode D3, its base coupled through a resistor R5 to the terminal 99, and its collector coupled through a resistor R6 to the drive coupling diodes D4 and D5 of Figure 5.

It will be seen that the diodes D1, D2 and D3 are arranged to pass full-wave rectified current from the secondary winding S through the series resistors R4 and R3 of the phase shifter circuit 101. The capacitor C1 across R4 delays and smooths the voltage across R4, and is also charged to a bias level determined by the bias signal on the conductor 98.

The transistor TR1 of the thyristor drive circuit 102 conducts, during charging of the battery, for a predetermined time during each half cycle of the mains frequency supply provided through the diode D3. Figure 9 illustrates the relationship between the resultant conducting of the thyristors TH 1 and TH2 and the phase shifted smoothed sine pulse train at the terminal 99, the voltages represented being substantially those across the thyristors TH 1 and TH2 (rectified ac supply) and the voltage at the terminal 99 relative to the cathode of the supply diode D3. In Figure 9, the voltage is of increasing negative magnitude relative to the cathode of the diode D3 in the vertically upwards direction.

The bias signal on conductor 98 is controlled by an over-current control circuit 103, a current control circuit 104, and a two-level voltage control circuit 105 represented by blocks in Figure 6. The voltage across the resistor R0 is supplied from the pin 3 to the input terminals of the over-current control circuit 103, the current control current 104 and a low current trip circuit 106. Reference voltages are supplied to second input terminals of the circuits 103,104 and 106, and to an input terminal of the two-level voltage control circuit 105, which receives at its other input terminal a voltage from a pin 14 at the junction of two series resistors RV2 and R16 connected between the cathode of the supply diode D3 and pin 7 (ground).Respective output terminals of the circuits 103, 104 and 105 are coupled together as shown by series resistors R1 4 and R15, so that a high positive signal at the output of the over-current control circuit 103 overrides low signals at the circuits 104 and 105, a high positive signal at the output of the current control circuit 104 overrides low signals at the circuits 103 and 105, and a high positive signal at the output of the two-level voltage control 105 overrides low signals at the outputs of the circuits 103 and 104, the over-current control circuit 103 and the current control circuit 104 each having a suitably poled output diode (not shown in Figure 6).

When the circuit is used, the battery B is initially connected in with the mains supply off. If the battery voltage is low, but not lower than substantially 6 volts, assuming that the battery is a lead-acid battery, the voltage at pin 14 is lower than the reference voltage supplied by a reference voltage circuit 107 to the two-level voltage control circuit 105. As a result, a low voltage appears at the output terminal of the voltage control circuit 105 and is applied to the end if the resistor R14. The voltage across the resistor R0 is also low in these conditions, and therefore the circuits 103, 104 and 106 produce low output voltages. Consequently the terminal 99 is at a low voltage and the transistor TR is biased to conduct.If the mains is now switched on, the transistorTR1 conducts in each half cycle, and the thyristors THI and TH2 are fired accordingly so that current is passed through the battery B.

If an excessive current pulse passes through the battery B, the voltage across the resistor R0 goes high and the over-current control circuit 103 responds by producing a high output voltage on the conductor 98 which biases the transistor TR1 off, thereby preventing firing of the thyristors TH1 and TH2.

During the initial stage of charging of the battery, the current control circuit 104, in response to the voltage at pin 3, which it compares with the reference voltage developed across resistors R19 and R7 in series, produces an output voltage that rises if the voltage at pin 3 is higher than a predetermined value, and that falls if the voltage at pin 3 is lower than this predetermined value, thereby setting a constant current level, the current level 1, of Figure 1, through the resistor R,, by feedback action on the firing points of the thyristors TH1 and TH2. The level of the charging current is held by this arrangement until the rise in the voltage across the battery B causes the voltage at pin 14 to exceed the reference voltage being applied to the two-level voltage control circuit 105.When this happens, the circuit 105 produces a high output voltage which overrides the output of the current control circuit 104 and so controls, by feedback action on the firing points of the two thyristors, the firing of the thyristors that the battery voltage is held constant at the value V1 of Figure 2. This voltage control is maintained until the charging current level falls to the level 12 of Figure 1,which is sensed by the low current trip circuit 106 which responds by producing a high output voltage. This high output voltage alters the voltage at the pin 14 by reverse biasing a diode D9 which couples pin 14 to ground through an adjustable resistor RV3 in series with a resistor R1 1. The voltage at pin 14 becomes higher when the diode D9 is reverse biased, so that the two-level voltage control circuit 105 now sets the voltage to which the battery B is held at a lower value, the float voltage, which is the voltage V2 of Figure 2. The whole circuit will now remain in this stage until, if it should occur, a high current through the resistor R0 causes the low current trip circuit 106 to revert to its original condition and produce a low output voltage, thereby forward biasing the diode D9 so that the whole circuit returns to its initial condition.

A Zener diode D8 is included at the output terminal of the low current trip circuit 106 to ensure a positive switching action. Capacitors C4 and C5 are included to suppress noise.

The low current trip circuit 106 also controls the energising of the green and red LEDs GLE and RLE.

Figure 7(b) shows the reference voltage circuit 107 in detail. The reference voltage for the two-ievel voltage control circuit 105 is taken from the junction of two series resistors R24 and R25 connected in parallel with two series Zener diodes D13 and D14 connected between pin 7 (ground) and a resistor R17 to the cathode of the supply diode D3 (Figure 5).

The junction of the resistors R24 and R25 is also coupled through an adjustable resistor RV1 to the reference input terminals of the circuits 103 and 104 and one end of the resistor R19.

The circuits 105,103,104 and 106 are shown in detail in Figures 8(a), (b), (c) and (d).

The two-level voltage control circuit 105 consists of an operational amplifier OP1 provided with a feedback capacitor C6 and input resistors R1 8 and R21 arranged to form a differential integrator circuit.

The over-current control circuit 103 consists of an operational amplifier OP2 with an output diode D11 arranged to decouple the amplifier OP2 from high positive signals at the circuit output terminal.

The current control circuit 104 consists of an operational amplifier OP3 with a feedback capacitor C3 and input resistors R20 and R22 arranged to form a differential integrator circuit. An output diode D10 is provided to decouple the amplifier OP3 from high positive signals at the circuit output terminal.

The low current trip circuit 106 consists of an operational amplifier OP4 with a feedback capacitor C2 and input resistors R8 and R23 arranged to form a differential integrator circuit. However, a diode D7 is connected between the output terminal of the amplifier OP4 and the capacitor C2 so that the integrator action only occurs for a positive amplifier output, the circuit 106 being effectively not present when the amplifier OP4 produces a low output voltage since diode D7 isolates the amplifier from the output terminal of the circuit 106. The cathode of the diode D7 serves as the output terminal for the circuit 106, and a connection from the output terminal of the amplifier OP4 is taken to a resistor R10, Figure 6, in series with the red LED RLE between pins 5 and 4, so that the red LED is energised while the amplifier output is low.

Figure 12 shows an embodiment similar to that of Figures 3,5 and 6 in which the same controller 100 is shown with adjustable resistors R200, R300, R400 and R500 connected for adjusting various circuit parameters. The resistor R200 adjusts the differences between the voltages V, and V2 of Figure 2. The resistor R300 adjusts the value of the low current at which the low-current trip circuit 106 operates. The resistor R400 adjusts the voltage at the pin 14, being in parallel with the resistor RV2 and raising the values of V2 and V, by the same amount.

The resistor R500 is in parallel with the resistor R16 and lowers the values of V2 and V1 by the same amount.

Figure 10 shows an embodiment similar to that of Figure 4 but including further circuitry which allows a very flat battery to be recharged. The controller 100 is again used and is represented in Figure 10 by the block 100.

The controller 100 will only operate if the battery voltage is above approximately 6 or 7 volts. For a very deeply discharged battery charging is therefore unable to start.

The circuit of Figure 10 allows the charging to commence from any voltage. Once the battery and mains supply are connected the thyristor firing angle is phased forward over several cycles providing the battery voltage is below 8 volts. An operational amplifier OP30 determines the battery voltage and if low enough its output goes high, which charges a capacitor C20,. A network R101, R201, C,Ot provides a phase shift which ensures that the firing angle is always on the rear edge of the mains signal.

The battery at this point may be in either a low impedance or a high impedance state (i.e. with sulphated plates). If the latter, the battery voltage will rise above 8 volts and the flat start circuitry turns off. The controller 100 will operate alone at this stage and continue with the standard charge cycle.

If the battery is in a low impedance state the current through R0 will rise rapidly. An operational amplifier OP10 produces a smoothed amplified current signal. This is monitored by an operational amplifier OP20 and if maximum allowed current from the amplifier OP10 is exceeded, the output of the amplifier OP20 goes low turning off the Thyristor Drive transistor Tor 10. The circuitry will now return to its starting condition and will continue to cycle in this state pushing pulses of energy into the battery B until the battery voltage rises to the value at which the controller 100 can operate. If a short circuit exists across the battery terminals then the flat start circuitry will continue to operate ad infinitum.

A supply is obtained via a diode D101 for the flat start circuitry, the supply rail thus providing a positive supply voltage referred to as V+. An earthy return is provided via a diode D90 to the full wave rectifier negative. The supply is thus obtained from the rectifier. The operational amplifier OP30 is used as a comparator which compares the voltage obtained via the diode D101 and a Zener diode D77, with the battery voltage applied via a resistor R90.

Voltages are thus measured with respect to the battery positive terminal.

If the battery voltage is less than 8 volts the output from the amplifier OP30 goes high and charges the capacitor C201 via a resistor R301. As this voltage on capacitor C201 rises, a second comparator, formed by an operational amplifier OP40, gradually brings the firing point on the thyristor forward. A gate drive signal is supplied to the thyristor TH by a drive circuit consisting of the PNP transistorTR10, a resistor R70 and a diode D40. The resistor R70 is a current limiting resistor, and the diode D40 ensures that the gate drive from the flat start circuitry and controller 100 are diode OR'd together-such that either control mechanism can fire the thyristor TH.A resistor R50 controls the base current supplied to the transistor TRiO, and a series combination of a diode D30 and a resistor R40 provides hysteresis to ensure a distinct switching action. A resistor R60 couples the base of the transistor TR1 0 to its emitter to ensure that the transistor is off in the absence of a low output from the amplifier OP40. A phase shift network, similar in operation to the phase shifter circuit in the controller 100, is formed by a capacitor C101, resistors R80 and R101, and diodes D20 and D110.An input resistor R201 couples this network to the comparator amplifier OP40.

As the gate drive to the thyristor TH phased forward, the current through the battery B rises, is detected across the resistor R,, and amplified and smoothed by a differential operational amplifier OP10, and components R160, R180, C30 and R170, R240, C50. The values of the capacitors C30, C50 ensure good filtering of the 100 Hz current waveform. The voltage across the resistor R0 is fed to the amplifier OP10 via resistors R140, R110, and resistors R150, R120with a small DCvoltageapplied via the diodes D101, D110 and a resistor R130 to ensure that the input voltages to the amplifier OP10 are not at the level of the negative supply rail to the amplifier OP10.

The output from the amplifier OP10 is compared by a further comparator amplifier OP20 with a reference voltage set by voltage divider formed by resistors R190, R202, R130. When the output voltage of the amplifier OP10 exceeds this reference voltage, i.e. the battery current has exceeded a predetermined value, then the output voltage of the amplifier OP20 goes low. This discharges the capacitor C20 via a diode D50, thereby preventing the amplifier OP40 and the transistor TR10 from firing the thyristor TH.

A series combination of an LED D80 and current limit resistor R220 form an indicator which shows when the current pulsing is occurring, i.e. the LED D80 is energised when the output voltage from the amplifier OP20 is high.

When the output of the amplifier OP30 is high, showing that the battery voltage is less than 8V, then a series diode D60 and resistor R230 provide a positive voltage at the junction of D70, D120, R230, D60 such that the diode D120 is reversed biased and does not conduct.

When the battery voltage is greater than 8 volts, the output of the amplifier OP30 is low, the voltage at the cathode of the diode D60 is correspondingly low, and the diode D120 now provides hysteresis action to latch the amplifier OP20 low, and thus the output of the amplifier OP30 is held low via the diode D60, and the transistor TR10 is prevented, by the resultant condition of the amplifier OP40, from firing the thyristor TH.

At this point, with the battery voltage greater than 8 volts, the controller 100 resumes normal operation.

Thus the flat start circuitry allows gradually increasing current into the battery B. The form of the charging current supplied by the flat start circuitry is illustrated in Figure 11.The flat start circuitry operates until the battery voltage is greater than 8 volts, and then latches up and ceases operation, allowing the controller 100 to operate alone.

It will be seen from Figure 11 that the charging current supplied by the flat start circuitry to the battery B is in the form of trains of 100 Hz current pulses of increasing amplitude within each train, the envelope of batch train being a right angled saw tooth having a peak at a maximum current level IMAM. The sensing of current at the level 1MAX through the voltage across R0 by the amplifier OP10 causes the drive to the thyristor TH to be inhibited for a short time, as can be seen from Figure 11.

Whether or not the flat start circuitry operates to charge the battery B is determined by the voltage across the battery B as sensed by the operational amplifier OP30. The output of the operational amplifier OP30 is used to enable or disable the thyristor drive circuitry including the amplifier OP40 and transistorTR10 and unlatch or latch the amplifier OP20 accordingly.

Both the flat start circuitry and the controller 100 operate if the battery voltage is below substantially 8 volts but above approximately 7 volts. The controller 100 will charge a battery having a voltage of between substantially 7 volts and 16 volts, the preferred value of V, for a nominal 12 volt battery being 15.8 volts. The flat start circuitry will charge a battery having a voltage of 0 to substantially 8 volts.

The preferred value of V2 for a nominal 12 volt battery is 13.8 volts. These values of voltage are for the circuit constructed with suitable component values.

Claims (11)

1.A A method of charging a rechargeable battery, the method comprising a first stage in which the battery is charged at a constant current until a predetermined battery voltage is attained, and a second stage in which the battery is maintained at that predetermined voltage until the current has fallen to a predetermined value.

2. A method according to claim 1, wherein after the said first and second stages, a care and maintenance charge is carried out at a voltage less than that which determines the transition between the first and second stages.

3. Apparatus for charging a rechargeable battery, the apparatus comprising means for charging the battery at a constant current until a predetermined voltage is attained, and means for maintaining the battery at the said predetermined voltage until the current has fallen to a predetermined value.

4. Apparatus according to claim 3, wherein means are provided for supply to the battery a care and maintenance charge at a voltage less than the said predetermined voltage.

5. Apparatus for charging a rechargeable battery, the apparatus comprising means for supplying charging current to the battery, means for sensing the magnitude of the charging current, means for sensing the battery voltage, and means for controlling the magnitude of the charging current supplied to the battery by the charging current supplying means, the controlling means being so coupled to the current and voltage sensing means to maintain a constant level of charging current until the battery voltage reaches a first predetermined level, and then to so decrease the level of the charging current as to maintain the battery voltage at the first predetermined level until the charging current level has fallen to a predetermined value.

6. Apparatus according to claim 5, wherein the means for supplying charging current to the battery includes at least one thyristor arranged to be connected in series with the battery, and the means for controlling the magnitude of the charging current supplied includes a thyristor drive circuit coupled to the gate or gates of the thyristor or thyristors to supply a firing signal thereto, and means for supplying a firing time control signal to the thyristor drive circuit which determines the timing of the firing signal.

7. Apparatus according to claim 6, wherein the means for supplying a firing control signal includes a phase shifter circuit coupled to the means for supplying charging current and adapted to shift the phase of a signal derived from the source of the charging current and to supply the phase shifted signal to the thyristor drive circuit.

8. Apparatus according to claim 7, wherein the means for sensing current and voltage are so coupled to the thyristor drive circuit as to supply biasing signals thereto in combination with the said phase shifted signal.

9. Apparatus according to any one of claims 5 to 8, wherein the current sensing means include means for sensing that the charging current has fallen to the said predetermined value and for adapting the means for controlling the current so as to cause the current supplying means to supply a care and maintenance current at a voltage less than the said first predetermined level.

10. Apparatus according to any one of claims 6 to 9, wherein the means for sensing the battery voltage includes means for activating a further means for controlling the magnitude of the charging current when the battery voltage is less than a predetermined flat level, the said further means including a further thyristor drive circuit coupled to the gate or gates of the thyristor or thyristors to supply a firing signals thereto, and second means for supplying a firing timing control signal to the further thyristor drive circuit to determine the timing of the firing signal and including a further phase shifter circuit coupled to the means for supplying charging current and adapted to shift the phase of a signal derived from the source of the charging current and to supply the phase shifted signal to the furtherthyristor drive circuit.

11. Apparatus for charging a rechargeable battery, substantially as described herein before with reference to Figure 3 or 4 or to Figures 5,6,7 and 8, orto Figure 10 of Figure 12 ofthe accompanying drawings.