GB2310089A

GB2310089A - Battery charging; back-up power supply

Info

Publication number: GB2310089A
Application number: GB9702282A
Authority: GB
Inventors: Goesta Baarman; Eero Sirola
Original assignee: Nokia Telecommunications Oy
Current assignee: Nokia Oyj
Priority date: 1996-02-08
Filing date: 1997-02-05
Publication date: 1997-08-13
Also published as: FI960591A0; FI960591A; GB9702282D0; DE19706058A1

Description

1 BATTERY CHARGE CIRCUIT 2310089 The present invention relates to a

battery charge circuit including: a power supply whose output voltage is responsive to a control signal to be supplied to a control input of the power supply, an electrical equipment connected to an output of the power supply, to which equipment the power supply generates operating voltage, and a battery connected in parallel with the electrical equipment, which battery the power supply charges.

The term electrical equipment refers in this application generally to any load to which a power supply supplies operating voltage. The invention especially relates to adjusting the charging current of accumulator battery in telecommunication equipments although the charge circuit of the invention can also be employed in other connections.

It is a requirement for accomplishing an economically preferable charge circuit that the battery can be charged by the same power supply that is utilized for generating operating voltage for the actual electrical equipment, i.e. the load. A problem with most known battery charge circuits is precisely that a specific separate power supply has been reserved for charging the battery, in which case the price of the battery charging circuit will rise considerably. Previously in small back-up systems, batteries have been connected directly in parallel with the load, whereby they can be charged with the same power supply with which the load is supplied operating voltage. Direct connection is, however, damaging to the battery as there is no restriction of charging current, in which case the charging current supplied to the battery may grow too great, even so great that it will cause permanent damage to the battery.

It is also previously known to utilize linear adjusters for charging a battery connected in parallel with the load. The problem with linear adjusters is their weak coefficient of efficiency which is primarily caused by thermal losses produced when the linear adjuster lowers the operating voltage into a range suitable for charging the battery.

At least in the preferred embodiment the present invention solves the problem explained above and provides a battery charge circuit which is low in price and by means of which the battery can be charged fast and in a controlled manner. Tlis is attained by the charge circuit of the invention that is characterized in that the circuit includes a first error amplifier for comparing a 2 current to be supplied to the battery with a first reference value and for generating a control signal lowering the output voltage of the power supply and for supplying it to the control input of the power supply when the current to be supplied to the battery exceeds the first reference value.

The invention is based on the idea that when an error amplifier is employed together with a battery connected in parallel with an electrical equipment, which error amplifier constantly observes the strength of the current to be supplied to the battery and supplies to the control input of the power supply a control signal that lowers the output voltage of the power supply if the strength of the current to be supplied to the battery attains a predetermined maximum value, a very simple and low-priced circuit is achieved by means of which the strength of the current to be supplied to the battery will remain at a suitable level regardless of variations caused by the load, that is, the electrical equipment. The most significant advantages of the charge circuit of the invention are thus that the strength of the current to be supplied to the battery can be limited to such a level where it will certainly cause no damages to the battery, whereby the operating life of the battery will be extended, the charge circuit can be achieved by means of simple, small and low-priced components in which case the battery charge circuit will be small in size and inexpensive, and the coefficient of efficiency in the charge circuit will be good.

In one preferred embodiment of the charge circuit of the invention, the circuit includes a second error amplifier for comparing the voltage of the battery with a second reference value and for generating a control signal lowering the output voltage of the power supply and for supplying it to the control input of the power supply when the voltage of the battery attains the second reference value. In that case, overcharging the battery will be avoided as the second error amplifier constantly supervises the voltage of the battery and interrupts the charging of the battery by lowering the output voltage of the power supply when the voltage of the battery has attained the determined upper limit, that is, when the battery is fully charged.

The preferred embodiments of the charge circuit of the invention will be apparent from appended dependent claims 2 to 6.

In the following, the invention will be explained in more detail by way of example by means of a preferred embodiment of the invention with reference to the appended figures, in which 3 Figure 1 illustrates a first preferred embodiment of the charge circuit of the invention, Figure 2 illustrates currents present in the circuit of Figure 1.

Figure 1 illustrates a first preferred embodiment of the charge circuit of the invention. An electrical equipment 1 shown in Figure 1 derives its operating voltage from an AC/DC switched-mode power supply 2. In the case of Figure 1, it is assumed by way of example that the switched-mode power supply 2 is a switched-mPde power supply utilizing pulse-width modulation where the input voltage may be 220 VAC and the nominal output voltage is 48 VDC, for example. The charge circuit of the invention can employ any power supply whose output voltage can be changed by means of a control signal to be supplied to the power supply.

A battery 3 is connected in series with the electrical equipment 1, the battery being possibly a lead accumulator known as such in which the nominal value of the voltage is 48 V, for example. The power supply 2 thus charges the battery at the same time as it supplies operating voltage to the electrical equipment. If the power supply 2 for some reason loses the operating voltage, because of a power failure, for example, the battery 3 starts to discharge in such a manner that it supplies operating voltage to the electrical equipment 1. When the power failure has terminated, the power supply 2 recharges the battery 3 so that it will be fully charged again.

In order that the current fa to be supplied to the battery would not grow too great, the charge circuit of Figure 1 includes means for lowering the output voltage of the power supply 2 when the current fa attains the maximum value Imax determined for it. This takes place so that the current fa to be supplied to the battery 3 is measured by a resistor Rl or a Hall element, for example. The measured voltage signal is passed via a resistor R2 to a first input 5 of an error amplifier 4 connected in parallel with a capacitor Cl. A reference voltage obtained from an adjusting element P1 is directed to a second input 6 of the error amplifier 4, which adjusting element can be e. g. a potentiometer with which a zener diode D1 is connected in parallel. By means of the potentiometer D1 the user may determine the desired maximum value Imax, e.g. 1 A, for the current]a to be supplied to the battery 3. When the error amplifier 4 detects that the voltage of the signal supplied to its first input 5 is the same as or greater than the reference voltage supplied to its second input 6, it supplies a control signal to the control input 10 of the power supply 2 by 4 utilizing the diode D2 and a resistor R4, the control signal controlling the power supply to lower its output voltage to such a level that the current la to be supplied to the battery again falls below the maximum value Imax. A voltage of +10 V to +12 V, for example, may be connected to the second connection of resistors R3 and R4.

In case the power supply 2 is a switched-mode power supply utilizing pulse-width modulation, the control signal supplied by the error amplifier 4 preferably controls the pulse-width modulator of the power supply. Thus a smaller current la than the maximum value Imax flows constantly through the battery at the same time as the current lk flowing through the electrical equipment 1 may vary 0 A --> to the greatest possible output current of the power supply.

The charge circuit of Figure 1 also comprises means for supervising the voltage of the battery. When the battery 3 is about to be fully charged, the battery voltage will rise above its nominal value (about 53... 54 V in a lead accumulator of 48 V) in which case the charging will be switched off. For accomplishing this, the charge circuit includes resistors R5 and R6, a capacitor C2 and a second error amplifier 7 whose first input 8 is supplied with a signal representing the voltage of the battery at that moment. In addition to this, the charge circuit includes a second adjustment element P2, that is, a potentiometer, for example, by means of which the user may determine the maximum value Umax for the voltage Ua of the battery. Reference voltage is thus passed from the potentiometer P2 to a second input 9 of the second error amplifier.

In case the voltage to be supplied to the first input 8 of the error amplifier is greater than the reference voltage to be supplied to the second input 9, the voltage Ua of the battery exceeds the greatest allowed voltage Umax determined for it. In this case the error amplifier 7 supplies via the diode D3 a control signal to the control input 10 of the power supply 2. This control signal induces the power supply to lower the output voltage to such a level that the current to be supplied to the battery 3 will be almost non- existent (cf. Figure 2).

If the battery 3 is fully discharged and its voltage is to be increased fast, the charge circuit may include a third error amplifier and means for determining the second greatest allowed current value to be supplied to the battery. Therefore the charging current may be two times greater, for example, when the accumulator voltage is very low. When the voltage of the battery after this attains 80 % of the nominal voltage, for example, the charging current will fall to the nominal value because of the second error amplifier.

Figure 1 illustrates with broken lines the parts which are according to the invention advantageous to be integrated and enclosed with the power supply. Integration of these parts provides a compact and efficient power supply comprising the charge circuit of the invention.

Figure 2 illustrates currents present in the circuit of Figure 1. The vertical axle 1 in Figure 2 shows the strength of the current and the horizontal axle shows time t.

It can be seen in Figure 2 that once the charging of the battery has started, the current la to be supplied to the battery will remain constant regardless of variations in the current lk to be supplied to the electrical equipment. The strength of the total current Itot supplied by the power supply corresponds to the sum of the currents la and lk. The strength of the current la to be supplied to the battery corresponds to the maximum current Imax determined by the user, the maximum current being 1 A by way of example in the case of Figure 2.

When the charging of the battery has proceeded so far that the voltage Ua of the battery attains the maximum current Umax (vertical line in Figure 2) determined for it, the second error amplifier of the charge circuit will be activated, whereby the output voltage supplied by the power supply will lower to such a level that the current la to be supplied to the battery will fall almost to zero.

It is to be understood that the explanation above and the figures related thereto are only intended to illustrate the present invention. Different variations and modifications of the invention will be evident to those skilled in the art without deviating from the scope of the invention disclosed in the appended claims.

6 66032.340

Claims

1. A battery charge circuit including: a power supply whose output voltage is responsive to a control signal to be supplied to a control input of the power supply, an electrical equipment connected to an output of the power supply, to which equipment the power supply generates operating voltage, and a battery connected in parallel with the electrical equipment, which battery the power supply charges, wherein the circuit includes a first error amplifier for comparing a current to be supplied to the battery with a first reference value and for generating a control signal lowering the output voltage of the power supply and for supplying it to the control input of the power supply when the current to be supplied to the battery exceeds the first reference value.

2. The charge circuit according to claim 1, wherein the circuit includes a second error amplifier for comparing the voltage of the battery with a second reference value and for generating a control signal lowering the output voltage of the power supply and for supplying it to the control input of the power supply when the voltage of the battery attains the second reference value.

3. The charge circuit according to claim 1 or 2, wherein the circuit includes means for generating a signal responsive to the current to be supplied to the battery and for supplying it to a first input of the first error amplifier, and means for generating a signal representing the first reference value and for supplying it to a second input of the first error amplifier, whereby the first error amplifier is arranged to generate and supply a control signal to the control 7 input of the power supply, which signal lowers the output voltage of the power supply if the voltage level of the signal to be supplied to its first input is the same as or greater than the voltage level of the signal to be supplied to its second input.

4. The charge circuit according to claim 2 or 3, wherein the circuit includes means for supplying a signal responsive to the voltage of the battery to a first input of the second error amplifier, and means for generating a signal representing the second reference value and for supplying it to a second input of the second error amplifier, whereby the second error amplifier is arranged to generate and supply a control signal to the control input of the power supply, which signal lowers the output voltage of the power supply if the voltage level of the signal supplied to its first input is the same as or greater than the voltage level of the signal supplied to its second input.

5. The charge circuit according to claim 3 or 4, wherein the means for generating the signal representing the first reference value and correspondingly, the second reference value comprise first and second adjusting elements which are preferably potentiometers or the like, and whose setting determines the first reference value, that is, the greatest possible current to be supplied to the battery, and correspondingly, the second reference value, that is, the greatest possible voltage of the battery.

6. The charge circuit according to claim 1, wherein said power supply is a switched-mode power supply comprising means for adjusting the output voltage of the power supply by utilizing pulse-width modulation in response to the voltage level of the control signal to be supplied to the control input of the power supply.

7. The charge circuit substantially as hereinbefore described with reference to the accompanying drawings.