GB2167617A

GB2167617A - Battery charger

Info

Publication number: GB2167617A
Application number: GB08429866A
Authority: GB
Inventors: Arthur Raymond Bailey
Original assignee: ELECTRONIC RESEARCH ASS LIMITE
Current assignee: ELECTRONIC RESEARCH ASS LIMITE
Priority date: 1984-11-27
Filing date: 1984-11-27
Publication date: 1986-05-29
Also published as: GB8429866D0

Abstract

The charger includes a differentiator circuit 4,5 for sensing the rate of change of voltage across a battery 12 to be charged. At the approximate position of full charge of the battery, the rate of change of voltage becomes negative. The circuit 4, 5, 6 senses this change and actuates a transistor switch 2 which cuts off the source of charging current so as to prevent over charging of the battery. A diode 9 and a filter 10, 11 act to prevent overcharging and premature charge termination due to effects of surge voltages. Charging current is limited by a resistor 3 which may be an incandescent lamp which then acts as a charging indicator and also as a current stabiliser. <IMAGE>

Description

SPECIFICATION Improvement in or relating to the rapid recharging of storage batteries The invention to be described applies particularly but not exclusively to the recharging of nickel-cadmium batteries.

Nickel cadmium batteries can be recharged fully in shortly over one hour, but normally a slow recharging rate is used. In this latter case the battery will reach full charge in about fourteen hours. This slow rate is used because the charging current is then sufficiently low not to damage the battery even if it is left on continuously. More rapid recharging necessitates the charge being terminated when the battery is fully charged.

In the case of nickel-cadmium batteries it is very difficult to detect the fully charged condition as there is little voltage rise at the fully charged state.

Many complicated and therefore expensive methods have been used to determine the correct point to terminate the charging process. The invention to be described uses, not the voltage as in conventional systems, but the rate-of-rise of battery voltage to determine the correct point to terminate the charge.

Figure 1 shows the typical curve of voltage against time for a nickel-cadmium cell being charged at the maximum permissible rate. It will be seen that there is a more or less uniform rate of rise of battery voltage during the recharging process. When the cell is fully charged the voltage ceases to rise and indeed begins to fall due to the internal temperature rise within the cell. It is this change of slope of the rate-of-change of cell voltage that is utilised by this invention to terminate the charge of the cell.

For purposes of description only a suitable circuit will be outlined. This is shown in Figure 2.

A suitable d.c. supply (1) is used to provide the recharging energy to the cell(s) 12 to be recharged.

This is controlled by the switching transistor 2. The charging current is limited to the maximum safe value by resistor 3. This can, with advantage, be an incandescent lamp as it will then act as a charging indicator and also act as a current stabiliser. A capacitor 4 and resistor 5 are used as an electrical differentiator so that a voltage will appear across the resistor that is proportional to the rate of rise of battery voltage. This will be positive with respect to the common battery negative line so long as the voltage across the battery is still rising, that is the battery is not fully charged. This voltage is used to activate an electronic amplifier 6 such that it will provide an output voltage sufficient to turn on transistor 2 so long as the voltage across the battery is rising at a rate that indicates it is still charging.The diode 7 and capacitor 8 are included so that the start of charge is automatic when the supply voltage 1 is applied. On switching on the supply 1, the capacitor 8 is initially uncharged. This means that as the collector of transistor 1 rises in potential so does its base, turning the transistor 1.

This charging current gives an increase in battery voltage which turns on the amplifier 6. This locks on the charge. The diode 7 is necessary if the amplifier can "sink" as well as "source" current, this would tend to make the circuit less likely to turn on reliably and also more prone to drop out prematurely if there was a disturbance in the electrical supply.

To obtain a sufficiently high voltage for reliable operation of the amplifier the time-constant of C (4) and R (5) will need to be long. This could give rise to some overcharging of a previously fullycharged cell due to the voltage jump at the start of charging. To reduce this effect a diode 9 will clip this voltage surge and thus limit the overcharge time. If there are difficulties due to supply voltage surges, then a simple filter of C and R 10 and 11 can remove these effects and prevent premature termination of the charge.

The basis of the claim is therefore a means of rapid recharging of rechargeable cells that terminates the charging period by determining the rateof-rise of battery voltage. When this falls below that of a battery accepting charge, the charge is then automatically terminated. This termination value measured in volts per unit time, may be positive, zero or negative depending on the termination point required on the charging curve. This is shown diagrammatically in Figure 3.

The embodiment is described above by way of example only.

According to the invention in a first aspect, there is provided a circuit of controlling a charging operation of a battery, including means for sensing the rate of charge of battery voltage and for terminating the battery charging operation in response to the sensed rate of charge.

According to the invention in a second aspect, there is provided a circuit for controlling the charging operation of a battery, the circuit including differentiating means for generating a signal indicative of the rate of change of potential difference across a battery being charged and control means for controlling the application of charging current to the battery, in response to the differentiating means.

1. A charging circuit connectable to a rechargeable battery for applying a charging current thereto comprising first means for sensing a rate of change of voltage across the battery and second means for controlling the application of the charging current to the battery in response to the output of the first means so that the charging current ceases to be applied to the battery when the rate of change of voltage ceases to be positive.

2. A circuit as claimed in claim 1 wherein the second means comprises a switch for connecting a power source to the battery, the state of the switch being dependent upon the output of the first means.

3. A circuit as claimed in claim 2 wherein the switch means comprises a transistor.

4. A circuit as claimed in any one of the preced

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

Claims

**WARNING** start of CLMS field may overlap end of DESC **. SPECIFICATION Improvement in or relating to the rapid recharging of storage batteries The invention to be described applies particularly but not exclusively to the recharging of nickel-cadmium batteries. Nickel cadmium batteries can be recharged fully in shortly over one hour, but normally a slow recharging rate is used. In this latter case the battery will reach full charge in about fourteen hours. This slow rate is used because the charging current is then sufficiently low not to damage the battery even if it is left on continuously. More rapid recharging necessitates the charge being terminated when the battery is fully charged. In the case of nickel-cadmium batteries it is very difficult to detect the fully charged condition as there is little voltage rise at the fully charged state. Many complicated and therefore expensive methods have been used to determine the correct point to terminate the charging process. The invention to be described uses, not the voltage as in conventional systems, but the rate-of-rise of battery voltage to determine the correct point to terminate the charge. Figure 1 shows the typical curve of voltage against time for a nickel-cadmium cell being charged at the maximum permissible rate. It will be seen that there is a more or less uniform rate of rise of battery voltage during the recharging process. When the cell is fully charged the voltage ceases to rise and indeed begins to fall due to the internal temperature rise within the cell. It is this change of slope of the rate-of-change of cell voltage that is utilised by this invention to terminate the charge of the cell. For purposes of description only a suitable circuit will be outlined. This is shown in Figure 2. A suitable d.c. supply (1) is used to provide the recharging energy to the cell(s) 12 to be recharged. This is controlled by the switching transistor 2. The charging current is limited to the maximum safe value by resistor 3. This can, with advantage, be an incandescent lamp as it will then act as a charging indicator and also act as a current stabiliser. A capacitor 4 and resistor 5 are used as an electrical differentiator so that a voltage will appear across the resistor that is proportional to the rate of rise of battery voltage. This will be positive with respect to the common battery negative line so long as the voltage across the battery is still rising, that is the battery is not fully charged. This voltage is used to activate an electronic amplifier 6 such that it will provide an output voltage sufficient to turn on transistor 2 so long as the voltage across the battery is rising at a rate that indicates it is still charging.The diode 7 and capacitor 8 are included so that the start of charge is automatic when the supply voltage 1 is applied. On switching on the supply 1, the capacitor 8 is initially uncharged. This means that as the collector of transistor 1 rises in potential so does its base, turning the transistor 1. This charging current gives an increase in battery voltage which turns on the amplifier 6. This locks on the charge. The diode 7 is necessary if the amplifier can "sink" as well as "source" current, this would tend to make the circuit less likely to turn on reliably and also more prone to drop out prematurely if there was a disturbance in the electrical supply. To obtain a sufficiently high voltage for reliable operation of the amplifier the time-constant of C (4) and R (5) will need to be long. This could give rise to some overcharging of a previously fullycharged cell due to the voltage jump at the start of charging. To reduce this effect a diode 9 will clip this voltage surge and thus limit the overcharge time. If there are difficulties due to supply voltage surges, then a simple filter of C and R 10 and 11 can remove these effects and prevent premature termination of the charge. The basis of the claim is therefore a means of rapid recharging of rechargeable cells that terminates the charging period by determining the rateof-rise of battery voltage. When this falls below that of a battery accepting charge, the charge is then automatically terminated. This termination value measured in volts per unit time, may be positive, zero or negative depending on the termination point required on the charging curve. This is shown diagrammatically in Figure 3. The embodiment is described above by way of example only. According to the invention in a first aspect, there is provided a circuit of controlling a charging operation of a battery, including means for sensing the rate of charge of battery voltage and for terminating the battery charging operation in response to the sensed rate of charge. According to the invention in a second aspect, there is provided a circuit for controlling the charging operation of a battery, the circuit including differentiating means for generating a signal indicative of the rate of change of potential difference across a battery being charged and control means for controlling the application of charging current to the battery, in response to the differentiating means. CLAIMS

4. A circuit as claimed in any one of the preced ing claims wherein the first means comprises differentiator means connectable across the battery for differentiating the voltage across battery with respect to time so as to provide said output.

5. A circuit as claimed in claim 4 wherein the differentiator means comprises an R.C. circuit.

6. A circuit as claimed in any one of the preceding claims further comprising a limiting resistor for limiting the charging current applicable to the battery.

7. A circuit as claimed in claim 5 wherein the limiting resistor comprises an incandescent lamp.

8. A circuit as claimed in any one of the preceding claims further comprising voltage surge limiting means associated with said first means for reducing the effect of voltage surges upon said first means.

9. A circuit as claimed in claim 8 wherein the voltage surge limiting means includes a diode.

10. A circuit as claimed in claim 8 or claim 9 wherein said means includes a filter.

11. A charging circuit substantially as hereinbefore described with reference to the accompanying drawings.