GB2236917A

GB2236917A - Battery charging

Info

Publication number: GB2236917A
Application number: GB8922957A
Authority: GB
Inventors: Peter Richard Stott
Original assignee: Individual
Current assignee: Individual
Priority date: 1989-10-12
Filing date: 1989-10-12
Publication date: 1991-04-17
Also published as: GB8922957D0

Abstract

A battery 5 is charged, e.g. by the alternator 1 of a vehicle engine, at a first predetermined voltage, which may be dependent on the temperature of the battery as measured at 5, for a first predetermined period of, for example, three minutes. The battery is then charged at a second, higher predetermined voltage, for example about 0.5 volts higher than the first predetermined voltage, for a second predetermined period which may be about five to eight minutes. The second predetermined period and/or voltage may be dependent on the temperature of the battery, appropriate values being stored in a ROM 8. Charging voltage changes may be effected by varying the output of a voltage regulator 2 thereby controlling the field coil 6 of the alternator 1. <IMAGE>

Description

METHOD OF CHARGING A CHARGEABLE BATTERY The present invention relates to a method of charging a chargeable battery by controlling the chargeable voltage.

Chargeable batteries are generally charged at a predetermined constant voltage. The voltage must be sufficient to cause charging, but excessive voltages may be ineffective and may cause damage to the battery. On the other hand, if the battery can be charged at a relatively high voltage without causing damage, the battery can indeed be charged at a higher rate. With increasing demands on automobile electrical systems, for example, there is a requirement for a simple, economical charging method for such batteries to maintain the state of charge as high as possible.

It is therefore an object of the present invention to provide a method of charging a battery in such a manner.

According to the present invention there is provided a method of charging a chargeable battery which comprises the steps of: charging the battery at a first predetermined voltage for a first predetermined period; and subsequently charging the battery at a second predetermined voltage, higher than the first predetermined voltage, for a second predetermined period.

The first predetermined period may be about three minutes.

The second predetermined period may be about five minutes to eight minutes. The second predetermined period may be dependent on the temperature of the battery. The second predetermined period may be determined by reference to stored data relating to the temperature of the battery and the period.

The second predetermined voltage may be about 0.5 volts higher than the first predetermined voltage.

Alternatively, the second predetermined voltage may be determined by reference to stored data relating to the battery charging current and the voltage.

The second predetermined voltage may be determined by controlling the magnetic intensity of a field coil of a power source.

The first predetermined voltage may be dependent on the temperature of the battery. The first predetermined voltage may be determined by reference to stored data relating to the temperature of the battery and the voltage.

For a better understanding of the present invention and to show more clearly how it may be carried into effect reference will now be made, by way of example, to the accompanying drawings in which: Figure 1 is a diagrammatic illustration of one embodiment of an apparatus for carrying out the method according to the present invention; Figure 2 is a graph showing the relationship between current in a feedback loop and voltage drop in a voltage modifier; Figure 3 is a diagrammatic illustration of a second embodiment of an apparatus for carrying out the method according to the present invention; Figure 4 is a graph showing the relationship between battery temperature and time at increased charging voltage; Figure 5 is a diagrammatic illustration of a third embodiment of an apparatus for carrying out the method according to the present invention; and Figure 6 is a graph showing the relationship between charging voltage and battery temperature.

Figure 1 shows a power source 1, such as an alternator of a vehicle engine, which produces a voltage across the terminals of a battery 5 and across a voltage regulator 2.

The voltage regulator 2 controls the voltage at a nominal value, for example by increasing or decreasing the magnetic intensity of a field coil 6 of the power source 1 through a feedback loop.

A conventional battery charging arrangement on a vehicle or the like has a voltage output from the current source controlled at a nominal value, usually 14.0 volts. This voltage represents a compromise between an ideal charging voltage when the battery charge level is low and an over-charge voltage when the battery is fully charged or at an elevated temperature.

I have found that it is possible to control the battery charging voltage in a manner which enables a battery to be charged more quickly and to maintain the battery in a fully charged state without subjecting the battery to over charging. It is becoming more important to maintain a battery fully charged with vehicles and boats being provided with an increasing number of electrical and electronic devices, such as telephones, intruder alarms and engine management units, which put ever increasing demands on the battery.

I have found that an existing battery charging arrangement can be modified in a simple and cost-effective manner by incorporating a voltage modifier 3 in the feedback loop of the voltage regulator 2 to a field coil 6 of the power source 1. This is because under normal conditions the output current in the main cable to the battery 5 is proportional, at any given charging voltage, to the current in the field coil 6.

In its most basic form the voltage modifier 3 is switched in by a pulse width modulator 4 for a predetermined time of say 5 to 8 minutes whereupon the voltage modifier 3 operates to decrease the output voltage in the feedback loop of the voltage regulator 2 so as to increase the charging voltage by a predetermined amount, say about 0.5 volts. After the predetermined time has elapsed the pulse with modulator 4 switches out the voltage modifier for a further predetermined time of say about 3 minutes.

The apparatus shown in Figure 1 can be supplemented in a number of ways. For example, the ideal charging voltage varies with the characteristics of the battery and the charging current.

The field current and thus the charging current can be determined by a semiconductor or known voltage modifier resistor (not shown) within the voltage modifier 3 and the voltage charge can be dependent on the charging current.

Figure 2 is a graph showing the voltage drop across a voltage modifier 3 incorporating a semiconductor in dependence upon the current flowing through the modifier.

The voltage drop across the modifier 3 results in a corresponding increase in voltage by the regulator 2. For a small current flowing through the voltage modifier 3 the voltage drop is small, but as the current increases the voltage drop increases, initially in a non-linear manner, but subsequently in a linear mLnner although this is not essential. Thus for a current flow of 1000mA the voltage drop is about 0.4 volts. In practice for a low antimony battery the current flowing in the feedback loop is about 1000mA which is about 30% of maximum field current. This current flow results in an increased voltage at the battery of about 0.4 volts, that is about 14.4 volts. For higher charging currents the voltage increases further and for lower charging currents the voltage decreases, the voltage changes being in an analogue manner.

As an alternative to a semiconductor, a resistor would result in a straight-line graph. As a further alternative, a desired relationship between input voltage to the modifier 3 and output voltage could be stored, for example in a read-only memory, and the voltage modifier could be arranged to modify the voltage in accordance with the desired relationship.

It is also known that ideal charging conditions vary with battery temperature. For this purpose, Figure 3 shows a modified apparatus which incorporates a temperature sensor 7 for determining the temperature of the battery and a read-only memory (ROM) 8 which stores data representing a predetermined charging time in dependence on the battery temperature determined by the temperature sensor 7.

Instead of increasing the voltage for a fixed predetermined time, for any particular temperature measured by the sensor 7 it is possible to look up in the ROM 8 a more suitable time for example as illustrated in Figure 3. Thus, for example as shown in Figure 4, although the predetermined time may be about 12.5 minutes in the illustrated graph for a battery temperature of 200C, the time may fall to about 4 minutes at 300C and rise to about 30 minutes at OOC. The ideal time/temperature relationship for any particular battery can be determined by experiment and the required data for that battery can be stored in ROM. Different batteries will merely require the installation of a different ROM.In addition to varying the duration of the pulse of increased voltage in dependence upon battery temperature, it may also be desirable to vary the nominal charging voltage. Figure 5 shows a modified apparatus according to which the voltage modifier 3 is able to modify the voltage in the feedback loop in dependence on the temperature measured by the temperature sensor 7 and upon data stored in the ROM 8. Figure 6 is a graph showing for one particular battery the variation in ideal charging voltage with battery temperature. For any type of battery this relationship can be determined and stored in the ROM 8. As the temperature detected by the temperature sensor 7 increases the voltage modifier decreases the voltage in the feedback loop so as to reduce the charging voltage at the battery.

By way of example the ideal charging voltage may be about 14.5 volts at OOC, but may decrease to about 13.8 volts at 40cm.

Further, the actual power supplied to the battery 5 may be determined by storing in the ROM 8 the relationship between power, charging voltage and current in the field coil.

Although the apparatus described hereinabove is designed to modify an existing electrical system of regulator and power source, it would be a simple matter to manufacture a new regulator incorporating such an apparatus. In such a case it would be possible to incorporate into the ROM complete data relating to the desired performance of the system.

This would apply in respect of original equipment on vehicles with predetermined battery type, alternator size and manufacturer.

When high losses occur in the main charging loom from the alternator to the battery, a further semiconductor may be connected in series with the voltage modifier. This is particularly useful when there is a voltage drop of about 0.7 to 1 volt, for example where split charge systems are used on boats and commercial vehicles which employ twin batteries.

Claims

1. A method of charging a chargeable battery which comprises the steps of: charging the battery at a first predetermined voltage for a first predetermined period;and subsequently charging the battery at a second predetermined voltage, higher than the first predetermined voltage, for a second predetermined period.

2. A method according to claim 1, wherein the first predetermined period is about three minutes.

3. A method according to claim 1 or 2, wherein the second predetermined period is about five minutes to eight minutes.

4. A method according to claim 1, 2 or 3, wherein the second predetermined period is dependent on the temperature of the battery.

5. A method according to any preceding claim, wherein the second predetermined period is determined by reference to stored data relating to the temperature of the battery and the period.

6. A method according to any preceding claim, wherein the second predetermined voltage is about 0.5 volts higher than the first predetermined voltage.

7. A method according to any one of claims 1 to 6, wherein the second predetermined voltage is determined by reference to stored data relating to the battery charging current and the voltage.

8. A method according to any preceding claim, wherein the second predetermined voltage is determined by controlling the magnetic intensity of a field coil of a power source

9. A method according to any preceding claim, wherein the first predetermined voltage is dependent on the temperature of the battery.

10. A method according to claim 9, wherein the first predetermined voltage is determined by reference to stored data relating to the temperature of the battery and the voltage.

11. A method of charging a chargeable battery substantially as hereinbefore described with reference to the accompanying drawings.