GB1590917A - Electric battery charging apparatus - Google Patents

Description

(54) ELECTRIC BATTERY CHARGING APPARATUS (71) We, CHLORIDE GROUP LIMITED, a Company registered under the laws of England, of 52 Grosvenor Gardens, London SW1W OAU, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to electric battery charging apparatus including means arranged automatically to terminate the charge, or a phase of the charge. More particularly, it relates to battery charging apparatus in which such termination occurs when the rate of rise of a control signal dependent on the battery voltage falls below a predetermined value, indicating that the battery is practically fully charged.Our British patent specification No. 1,097,451 describes one form of such apparatus, and a number of other arrangements of this type have since been proposed.

According to the present invention, the charge terminating means includes means arranged to combine a signal dependent on battery voltage and a ramp signal, the signal combining means being arranged to produce a combination signal which changes in one sense in response to a rise in battery voltage, and in the opposite sense in response to the rise of the ramp signal, and the charge terminating means further comprising means arranged to store the extreme value which the combination signal has so far reached in the said one sense, and means arranged to compare the stored extreme value with the actual value of the combination signal, and to terminate the charge, or a phase of it, when the combination signal changes in the said opposite sense by more than a predetermined amount from the stored extreme value.

Any change of the combination signal in the said opposite sense indicates that the rate of change of the battery voltage signal, which change results from the rise in the battery voltage, has fallen below the rate of rise of the ramp signal. In the case of a lead-acid battery, a normal criterion for terminating the charge is that the cell voltage should be rising at less than 11 mV in 45 minutes. When using charge terminating means according to the present invention, the ramp signal may therefore have a slope equivalent to 11 mV per cell in 45 minutes, so that the combination signal begins to change in the opposite sense as soon as the criterion for charge termination has been met.

The term 'charge termination', as used above, is intended to cover both complete switching off of the charging current, and reduction of the average value of the charging current to a much. lower value; for example, after termination of the main phase of the charge, the battery might receive a continuous trickle charge, or periods of normal charging at intervals of, say, 12 hours, to refresh the battery.

The store may be either an analogue or a digital store; in the case of a digital store, the output of the store would normally be coupled directly to a digital-analogue converter, so that the comparing step can be performed on an analogue signal. It should be understood, however, that this does not exclude the possibility of comparing signals in digital form.

In any battery charging arrangement in which charge termination depends upon the rate of rise of battery voltage, the difficulty arises that the battery voltage will vary with fluctuation of supply voltage as well as with the state of charge of the battery. To overcome this difficulty, the present applicants have proposed in British Patent Specification No. 1,495, 523 to provide means for generating a control signal which constitutes the difference between a signal dependent on battery voltage and a signal dependent on a.c.

supply voltage. The proportion may be such that the resultant control signal is substantially independent of fluctuations of a.c.

supply voltage, at least when the battery is approaching the fully charged condition.

The signal dependent on battery voltage which is combined with the ramp signal by the combining means may constitute a control signal which is derived in a manner similar to that described in the abovementioned specification No. 1,495,523; how ever, in the preferred embodiment, the combining means also performs the task of combining a signal which is dependent only on battery voltage with a signal which is dependent on a.c. supply voltage. Thus, in addition to the signal dependent only on battery voltage and the ramp signal, the combining means may receive a signal dependent on the a.c. supply voltage.

During the early stages of a charge, the battery voltage may rise only slowly, possibly at less than 11 mV per cell in 45 minutes, and therefore the combination signal monitoring means may be prevented from terminating the charge until the battery voltage has risen above about 2 35 volts per cell, in the case of a lead-acid battery. It may also be necessary to prevent termination of the charge, even after the battery voltage has risen above 2 35 volts per cell, since the mains voltage compensation described above may not give strictly accurate compensation until the battery is quite close to the fully charged condition.This approach to the fully charged condition may be indicated by the fact that the rate of rise of battery cell voltage has fallen below a predetermined value, greater than the value at which the battery is regarded as being fully charged; this greater value can be so chosen that, provided the battery voltage has risen above 2 35 volts per cell, the approach to the fully charged condition is reliably indicated, despite fluctuations in the mains supply voltage and imperfections in the mains voltage compensation arrangements.

The invention may be carried into practice in various ways, but one specific embodiment will now be described by way of example, with reference to the accompanying drawings, of which: Figure 1 is a schematic diagram, partly in block form, showing a battery charger embodying the invention, and intended for charging lead-acid batteries; Figure 2 is a graph illustrating the principle of operation of the battery charger; and Figure 3 is a simplified diagram of a circuit used in the charger of Figure 1 for determining when a battery has reached a fully charged condition.

Referring first to Figure 1, it can be seen that the charger comprises a transformer 10, ballast choke 12 and bridge rectifier 14 which serve to supply a charging current to terminals 16, to which the battery 18 being charged is connected. The mains power supply to the primary of the transformer 10 is controlled by the contacts 20A of a contactor, whose coil 20B is in turn energised by the contacts 22A of a relay. This relay, whose coil is shown at 22B, forms part of a charge controller, which is shown in Figure 1 as a block 24; in addition to the connections to the relay contacts 22A, the charge controller 24 also has two connections 26 which lead to the terminals 16, to allow it to sense the voltage of the battery 18, and is supplied with power from the mains.

In operation, the charging of the battery is started by first connecting the charger to the mains supply, so that the charge controller is energised, and then connecting the battery 18 to the terminals 16. After a delay of about 5 seconds, the relay closes its contacts 22A, and therefore the contactor also closes its contacts 22A, and therefore the contactor also closes its contacts 20A, energising the transformer 10. Charging current now flows into the battery 18; the current is controlled by the choke 12, and decreases as the battery voltage rises, giving a taper charging characteristic. When the battery voltage reaches about 2 35 volts per cell, the charge controller begins to monitor the rate of rise of battery voltage, in order to determine when the battery is fully charged.When the rate of rise of voltage falls below 11 mV per cell in 45 minutes, the battery is considered to be fully charged, and the charge is terminated.

Figure 2 illustrates the broad principle which is used to ascertain when the rate of rise of the battery voltage has fallen below this value. The curve A represents a typical rise in cell voltage, plotted against time, as the battery approaches the fully charged state.

The curve B represents a ramp signal, which changes steadily at a rate equivalent to a cell voltage change of 11 mV in 45 minutes, in the opposite direction to the change in the cell voltage signal; this ramp signal is generated within the charge controller 24. The curve C represents the signal obtained by combining the signals A and B, and it will be seen that this signal C reaches a maximum at the point where the slope of curve A is 11 mV per 45 minutes, and then decreases again. The charge controller 24 monitors the signal C, and terminates the charge as soon as the signal has decreased by more than a small amount from its maximum previously attained value, indicating that the peak of the curve C has been passed.

The charge controller 24 also includes means arranged to modify the cell voltage signal A in response to variations in mains voltage, in order that the signal A should be dependent only on the state of charge of the battery, and not on the mains supply voltage, but these signal modifying means cannot ensure that the signal A will be totally independent of mains voltage fluctuations over the whole of the charging period. Accordingly, the signal modifying means is arranged to perform correctly when the battery is approaching its fully charged state, and termination of the charge is inhibited until this state is approached. This avoids premature termination of the charge which might occur owing to fluctuations in mains supply voltage in the early stages of the charge.Also, it is quite normal for the battery voltage to rise slowly before the gassing voltage of about 2 35 volts per cell is reached, and the inhibition of termination mentioned above also ensures that the charge will not be prematurely terminated if the rate of rise of cell voltage should be less than 11 mV in 45 minutes in the early stages of a charge.

Figure 3 illustrates, in simplified form, the circuit which is used in the charge controller 24 to detect when the rate of rise of battery voltage falls below 11 mV in 45 minutes. In this circuit, the battery voltage signal A, ramp signal B, and mains compensation signal are generated by circuits 200, 202 and 204 respectively (not shown in detail), and are combined by a summing amplifier 206. The amplifier output is connected to a negative peak rectifying circuit comprising a diode 208 and a capacitor 210; the amplifier 206 is connected as an inverting amplifier, so that the voltage on the capacitor 210 is an indication of the greatest value so far reached by the signal C of Figure 2. The voltage on the capacitor 210 is also fed back, through a negative feedback resistor 212, to the summing node of the amplifier 206.Thus, as long as the changes in the inputs applied to the amplifier are only such as will produce a negative shift in the amplifier output, the feedback resistor 212 will be effective to limit the closed-loop gain of the amplifier. However, a change in the opposite sense in the inputs applied to the amplifier, which represents a decrease in the signal C of Figure 2, tends to reverse bias the diode 208, so that the feedback resistor 212 is no longer effective, and a large positive swing appears at the output of the amplifier 206. This change is detected by a comparator 214, which monitors the voltage across the diode 208, and terminates the charge when it detects the change from forward bias to strong reverse bias on the diode. In this way, the charge is terminated when the rate of rise of battery cell voltage falls below 11 mV in 45 minutes.

WHAT WE CLAIM IS: 1. Means for terminating the charging of an electric storage battery, or a phase of such a charge, the terminating means including means arranged to combine a signal dependent on battery voltage and a ramp signal, the signal combining means being arranged to produce a combination signal which changes in one sense in response to a rise in battery voltage, and in the opposite sense in response to the rise of the ramp signal, and the charge terminating means further comprising means arranged to store the extreme value which the combination signal has so far reached in the said one sense, and means arranged to compare the stored extreme value with the actual value of the combination signal, and to terminate the charge, or a phase of it, when the combination signal changes in the said opposite sense by more than a predetermined amount from the stored extreme value.

2. Charge terminating means as claimed in Claim 1 in which the signal storing means is arranged to store the signal in analogue form.

3. Charge terminating means as claimed in Claim 1 or Claim 2 which includes means arranged to disable the ramp signal generator until the battery voltage reaches a predetermined value.

4. Charge terminating means as claimed in Claim 3, in combination with a charger for lead-acid batteries, which terminating means is so arranged that the said predetermined value is 2 35 volts per cell.

5. Charge terminating means as claimed in any of the preceding claims which includes means arranged to prevent termination of the charge by the signal comparing means before the battery voltage reaches a predetermined value.

6. Charge terminating means as claimed in any of the preceding claims, in which the signal combining means also has an input for signal dependent on mains supply voltage, and is arranged to produce a combination signal which at least when the battery is nearly fully charged, is independent of variations in mains supply voltage.

7. Charge terminating means as claimed in Claim 6, which also includes termination preventing means arranged to detect when the battery is approaching the fully charged condition, and to prevent termination of the charge by the signal comparing means until the said approach is detected.

8. Charge terminating means substantially as herein described, with reference to Figure 3 of the accompanying drawings.

9. Electric battery charging apparatus incorporating charge terminating means as claimed in any of the preceding claims.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (9)

**WARNING** start of CLMS field may overlap end of DESC **. normal for the battery voltage to rise slowly before the gassing voltage of about 2 35 volts per cell is reached, and the inhibition of termination mentioned above also ensures that the charge will not be prematurely terminated if the rate of rise of cell voltage should be less than 11 mV in 45 minutes in the early stages of a charge. Figure 3 illustrates, in simplified form, the circuit which is used in the charge controller 24 to detect when the rate of rise of battery voltage falls below 11 mV in 45 minutes. In this circuit, the battery voltage signal A, ramp signal B, and mains compensation signal are generated by circuits 200, 202 and 204 respectively (not shown in detail), and are combined by a summing amplifier 206. The amplifier output is connected to a negative peak rectifying circuit comprising a diode 208 and a capacitor 210; the amplifier 206 is connected as an inverting amplifier, so that the voltage on the capacitor 210 is an indication of the greatest value so far reached by the signal C of Figure 2. The voltage on the capacitor 210 is also fed back, through a negative feedback resistor 212, to the summing node of the amplifier 206.Thus, as long as the changes in the inputs applied to the amplifier are only such as will produce a negative shift in the amplifier output, the feedback resistor 212 will be effective to limit the closed-loop gain of the amplifier. However, a change in the opposite sense in the inputs applied to the amplifier, which represents a decrease in the signal C of Figure 2, tends to reverse bias the diode 208, so that the feedback resistor 212 is no longer effective, and a large positive swing appears at the output of the amplifier 206. This change is detected by a comparator 214, which monitors the voltage across the diode 208, and terminates the charge when it detects the change from forward bias to strong reverse bias on the diode. In this way, the charge is terminated when the rate of rise of battery cell voltage falls below 11 mV in 45 minutes. WHAT WE CLAIM IS:

1. Means for terminating the charging of an electric storage battery, or a phase of such a charge, the terminating means including means arranged to combine a signal dependent on battery voltage and a ramp signal, the signal combining means being arranged to produce a combination signal which changes in one sense in response to a rise in battery voltage, and in the opposite sense in response to the rise of the ramp signal, and the charge terminating means further comprising means arranged to store the extreme value which the combination signal has so far reached in the said one sense, and means arranged to compare the stored extreme value with the actual value of the combination signal, and to terminate the charge, or a phase of it, when the combination signal changes in the said opposite sense by more than a predetermined amount from the stored extreme value.

2. Charge terminating means as claimed in Claim 1 in which the signal storing means is arranged to store the signal in analogue form.

3. Charge terminating means as claimed in Claim 1 or Claim 2 which includes means arranged to disable the ramp signal generator until the battery voltage reaches a predetermined value.

4. Charge terminating means as claimed in Claim 3, in combination with a charger for lead-acid batteries, which terminating means is so arranged that the said predetermined value is 2 35 volts per cell.

5. Charge terminating means as claimed in any of the preceding claims which includes means arranged to prevent termination of the charge by the signal comparing means before the battery voltage reaches a predetermined value.

6. Charge terminating means as claimed in any of the preceding claims, in which the signal combining means also has an input for signal dependent on mains supply voltage, and is arranged to produce a combination signal which at least when the battery is nearly fully charged, is independent of variations in mains supply voltage.

7. Charge terminating means as claimed in Claim 6, which also includes termination preventing means arranged to detect when the battery is approaching the fully charged condition, and to prevent termination of the charge by the signal comparing means until the said approach is detected.

8. Charge terminating means substantially as herein described, with reference to Figure 3 of the accompanying drawings.

9. Electric battery charging apparatus incorporating charge terminating means as claimed in any of the preceding claims.