GB2155257A - Battery charger - Google Patents

Abstract

When a control module 3 senses that the voltage of a battery 2 has fallen to a first low level (Lo 1), it causes a control unit 5 to connect a battery charger 1 to an A.C. supply 6, controls 3, 5 operating to disconnect the charger 1 from the supply 6 when the battery voltage rises to a higher level (Hi 1). An LED display and/or an audible alarm in an alarm unit 4 operates when either of the two above voltage levels (Lo 1, Hi 1) is passed and also if the battery voltage falls to a second low level (Lo 2) below the first low level, eg. in the event of failure of the A.C. supply 6. Testing units 7, 8 may cause a test load to be applied across the battery 2, eg. for 5 seconds once every 24 hours. The Lo 1 alarm is inhibited during the test and if the Lo 2 set point is stripped, a flashing LED is latched on in the unit 7. Module 3 may indicate the state of charge of the battery 2 on a voltmeter which measures voltage only over a desired range above zero. Circuits are described (Figure 2,3) to substract the required voltage offset from the battery voltage before application to the meter. <IMAGE>

Description

SPECIFICATION Battery charging system This invention relates to systems for controlling the charging of batteries and is particularly suited to use in "stand-by" power systems.

The high reliability and long working life obtained from modern rechargeable batteries used in standby power, emergency power and central battery systems tends to obscure the fact that unless properly charged and maintained they can be the weak link in a stand-by power system. If the battery has been properly specified for a particular application, experience has shown that unsuitable or faulty charging systems account for the majority of battery failures and inadequate maintenance for a large proportion of the remainder.

The extent, and therefore the cost of maintenance is very much related to the quality performance and reliability of the charger. For example, the most time consuming battery maintenance operation is the topping up of the battery cells with water. But, other than through cell breakage, electrolyte is only lost through incorrect charging. A battery voltage of 10% (typically 0.22 volts per cell) above the recommended level can dissipate the electrolyte within a few days, leaving the battery useless and possibly with only a fraction of its normal capacity recoverable. Maintenance requirements in this circumstance would be substantial, requiring a daily frequency.

A system which indicates an unexpected rise in charger voltage would enable the user to identify the problem and take apprpriate action to rectify the fault.

Whilst a smal percentage overcharge voltage will render the battery unserviceable through water loss, a charger voltage of only 10% below the recommened level will not maintain the battery in a charged condition and an operational failure is probable.

If the charger is poorly regulated, overcharge and water loss will occur during periods of high mains voltage. In the event of a minor charger malfunction caused by component failure or simply old age, over or under charge is probable. Commonly battery failure is caused by accidental, but undetected, switch-off or fusing of the battery charger AC power supply, resulting in the battery system receiving no charge and gradual loss of battery capacity.

Battery and chargers are usually located in unattended battery rooms, generator outbuildings or remote switching locations and are subject only to periodic inspection and test.

To test and verify proper charger output and battery condition, an accurate high resolution DC voltmeter is needed, an item not normally available to the maintenance personnel, and expert knowledge of battery and charger operating characteristics is required to make proper adjustments.

The usual procedure is to employ local or in-house maintenance personnel to top-up batteries and clean terminals and to rely on six or twelve monthly visits by the battery or charger Service Engineer to test and adjust, if required.

There is a problem with the present system that if malfunction occurs between these visits, the first indication of the malfunction may be the failure of the battery system to work when its operation in a power cut may be of critical importance.

In accordance with one aspect of the invention, there is provided a system for controlling the charging of batteries including control means for sensing the high or low charge of the batteries and means for altering the charge of the batteries in response to the sensing.

Preferably, the state of charge is indicated on a meter. The problem with known meters suitable for such application is that they operate over a wide range and resolution is low.

In accordance with another aspect of the invention, there is provided a voltmeter which measures voltage only over a desired range above zero voltage.

A system for controlling the charging of batteries will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a diagram illustrating the system, Figure 2 is a general circuit diagram for use in providing a high resolution on a meter of the system, and Figure 3 is a diagram of a specific circuit of Figure 2 for use in the system.

Referring to Figure 1,the system is arranged to operate a battery charger 1 connected to the batteries 2, which may be for example a stand-by power supply. The batteries 2 provide power for the system.

The system comprises a control module 3 electrically connected to the batteries 2 and having a voltage indicator (not shown) which divides the input voltage from the batteries by the number of battery cells to provide a high resolution, as will be described. The control module 3 provides an output signal dependent upon a switching arrangement with three alarm set points, as follows: Hi1 -the switching arrangement provides an output signal voltage when the battery voltage rises above a predetermined value.

Lol and Lo2 -the switching arrangement provides a different output signal voltage when the battery voltage falls below respective predetermined values.

Electrically connected to the module 3 is an alarm 4which receives all three outputs from the module 3.

The alarm 4, which may have a LED display and/or an audible alarm, operates when any of the thee alarm set points are passed.

Also connected to the module 3 is a charger control unit 5 which has an input connection to an AC power supply 6 and an AC output to the battery charger 1. The unit 5 operates in response to the signals from the module 3. For example, with the Lo1 voltage set at 2.2 volts per cell (VPC) and Hi1 set at 2.3 VPC, when the battery terminal voltage falls to 2.2 VPC, the Lo1 is generated and causes the unit 5 to connect the AC supply 6 to the charger 1, whereupon the battery charge will rise until the battery output is 2.3 VPC. At that voltage the module 3 will generate the Hil signal to switch off unit 5 and disconnect the AC supply from the charger 1. That cycle will automatically repeat itself indefinitely. In the event of failure in the AC supply on the charger 1 the further fall in battery terminal voltage can be indicated by the alarm 4 by setting Lo2 below Lo1.

To provide a test facility the system may comprise testing units 7,8. Testing unit 7 has an input connection from Lo1 and Hi1 switches of module 3 and is operated by the DC voltage from module 3. Testing unit 8 has an input connection with an output from testing unit 7, an output connected across the batteries 2 and a feed-back information connection to unit 7.

The unit7 is wall mounted adjacent module 3 and connected to unit 8 which is mounted close to the battery or normal load terminals. The unit 7 causes the unit 8 test load to be switched in circuit across the battery terminals so as to cause a suitable test current to be drawn from the battery.

The test load is automatically switched in circuit at a frequency and for a time determined by the particular application. Typically this may be a test of 5 seconds duration applied once every 24 hours. The application of the test load to the battery causes the terminal voltage to fall. The Lo2 set point is set to alarm an excessive fall in voltage during the test and Lo1 alarm is inhibited whilst the test is in progress.

If the Lo2 set point is tripped, a flashing LED alarm lamp is latched on in the unit 7, indicating a failure on the last load test applied and an alarm circuit is activated.

Referring now to the meters (not shown) associated with module 3, it is desirable to provide a high resolution. To achieve this the unwanted part of the meter range, for example 0 to 1.5 V for a lead acid cell and 0 to 0.5 Vfor a nickel cadmium cell, is removed and the full scale deflection of the meter operates over the voltage range most appropriate to the particular application and user need.

To achieve this aim it is possible to use certain types of precision voltage reference diodes to develop a fixed voltage drop across these diodes which is essentially independent of the current passing through them. This voltage tends to be in the range of 1.2 to 1.4 volts and as such has limited applications. However, as shown in Figure 2 by placing a suitable voltage multiplying amplifier 10 before these diodes 11, almost any effective voltage drop may be achieved simply by providing the appropriate gain factor.

In such a circuit V out = (V in x B) - V ref.

Many electronic instruments and circuits require to "lose" a fixed portion of an input voltage. In particular, applications incorporating moving pointer panel meters benefit by the facility to "lose" unnecessary portions of the scale where in practical use the pointer will never travel. For example, a voltmeter monitoring the voltage of a 48 volt battery system may vary in use between 36 and 60 volts, hence the zero to 60 volt meter often used has poor resolution. By incorporating a suitably configured 30 volt offset, the meter may display 30 to 60 volts resulting in a substantial improvement in resolution and a more meaningful meter reading.

A practical circuit is shown in Figure 3 and illustrates how a portion of the meter is "lost".

The circuit displays a battery voltage in "volts per cell" (average voltage of each cell) and in this case there is no need to display the portion below 1.5 V.

The gain of the amplifier is set to provide sufficient gain to "match" the 1.5 volts to be removed to the 2.24 volts (i.e. two voltage reference diodes in series) of the voltage reference.

The amplifier gain is set by the combination of the 10K and the 15K resistors and is nominally 1.6133 times.

The inclusion of the 2K7 resistor in parallel with the output enables the rise in current, for input voltages near to the displayed minimum, to be increased ensuring maximum stability of the reference voltage with varying current.

Claims (6)

1. A system for controlling the charging of batteries, including control means for sensing the high or low charge of the batteries and means for altering the charge of the batteries in response to said sensing by the control means.

2. A system according to Claim 1, wherein the batteries form part of a stand-by electrical system.

3. A system according to Claim 1 or 2, wherein the state of charge of the batteries is indicated on a meter.

4. A system according to Claim 3, wherein the meter is a voltmeter which measures voltage of the batteries only over a desired range above zero voltage.

5. A system for controlling the charging of batteries constructed and arranged substantially as herein described with reference to the accompanying drawings.

6. A voltmeter which measures voltage only over a desired range above zero voltage.