GB2228375A

GB2228375A - Battery Chargers; Battery Powered Alarm Generators

Info

Publication number: GB2228375A
Application number: GB8903692A
Authority: GB
Inventors: Adrian Robert Barnett
Original assignee: B & H Electronics Ltd
Current assignee: B & H Electronics Ltd
Priority date: 1989-02-17
Filing date: 1989-02-17
Publication date: 1990-08-22
Also published as: GB8903692D0

Abstract

A portable battery powered farm generator has a voltage generator 28 which initially applies a first stable voltage to charge the battery 22 until a sensor 34 detects that the charging current has fallen through a predetermined threshold. Generator 28 then proves a lower stable voltage for trickle charging. The battery may be a 12V, 6AH lead acid type, the first voltage being 14.4V, the threshold current 200 mA, and the trickle charge voltage 13.8 V. The alarm generator may have a siren (18), (Fig 1), activated after a delay by an infra red intruder sensor (16). <IMAGE>

Description

Alarm-Generating Device The present invention relates to alarm-generating devices.

Such devices may be designed to generate an alarm in response to the detection of an alarm condition such as the presence of an intruder, the presence of smoke or the detection of fire, for instance. It is common for a device to incorporate a variety of sensors, perhaps of different types and at various locations in order to detect alarm conditions occuring anywhere in a large building or a large part of a building. The devices and their sensors are powered from the electrical mains supply and back-up battery facilities may be provided to cater for possible disruption of or tampering with the supply.

Mains operated alarm devices cannot be used in locations such as remote buildings without mains electricity, or on boats or in caravans while laid up during the winter, for instance.

The present invention seeks to address this problem, and provides an alarm-generating device having a sensor for sensing alarm conditions, means for generating an indication that an alarm condition has been detected, and a lead acid battery for powering the device without connection to a mains electrical supply, the device further comprising a battery charger for use in re-charging the said battery when the battery charger is connected to a mains electrical supply, the battery charger comprising voltage generating means operable to generate a plurality of charging voltages for application to the battery, and further comprising current sensing means operable to cause the charging voltage to change when the charging current passes through a pre-selected threshold value.

Preferably at least one of the charging voltages exceeds the nominal working voltage of the battery. The charging voltage is preferably reduced from its initial value when the charging current passes through a threshold value. The final charging voltage supplied by the charger may be chosen to cause the battery to trickle charge.

The battery may be a 12V, 6AH lead acid battery.

For such a battery, the initial charging voltage may be in excess of 14V, such as 14.4V, the final charging voltage may be 13.8V, and the charging current may be reduced to about 200mA.

In another aspect, the invention provides a battery charger for lead acid batteries, comprising voltage generating means operable to generate a plurality of charging voltages for a battery, and further comprising current sensing means operable to cause the charging voltage to change when the charging current passes through a pre-selected threshold value.

The battery charger according to the invention may comprise any or all of the preferred features of the battery charger forming a part of the alarm-generating device referred to above.

One embodiment of an alarm-generating device according to the invention will now be described in more detail, by way of example only, and with reference to the accompanying drawings, in which: Fig. 1 is a highly schematic perspective view of the device; Fig. 2 is a schematic block diagram of a battery and battery charging circuit contained within the device of Fig. 1; Fig. 3 is a graph providing information about the operation the battery charger of Fig. 2.

The alarm-generating device 10 shown in Fig. 1 is a small portable device housed in a case 12 having a carrying handle 14. The front face of the case 12 carries a sensor 16, such as an infra red sensor. A siren 18 is also provided and a key-operated control switch 20 governs operation of the device 10.

According to the setting of the switch 20, circuitry within the device 10 may respond to the detection by the sensor 16 of a human being, to cause the siren 18 to sound, thereby warning of the presence of a potential intruder. Delay circuitry may be included to allow a user to set the alarm and retreat from the area covered by the sensor 16 without setting off the device 10, and also to allow the authorised user to approach the device 10 and switch it off by means of the switch 20, without the siren 18 sounding.

The device 10 is powered by a battery 22 (not shown in Fig. 1). This is of the lead acid type, and may be a 12V 6H unit. The battery 22 provides the small amount of current required to allow the sensor 16 to monitor for intruders, and a much larger current to sound the siren 18, when required.

It is envisaged that a lead acid battery can provide sufficient current to allow the device to be left unattended for very long periods, perhaps in excess of 1 year. This allows the device to be left unattended in a boat or caravan which has been laid up over the winter season. The device would be placed in the main cabin of the boat or within the caravan and activated, and can then be left in place to detect intruders until the boat or caravan owner returns at the beginning of the following season.

When left for such long periods, the charge on the battery will run down. Accordingly, a battery charging circuit shown in Fig. 2 is incorporated within the device 10 to allow the battery 22 to be re-charged.

The charger circuit comprises a socket 24 for connection of the device 10 to a mains electricity supply during charging, by means of an appropriate cable. The socket 24 is connected through a AC to DC converter 26 to a voltage generating circuit 28. The circuit 28 is capable of providing a stable, fixed voltage output at 30 at either of two charging voltages selected according to a selection signal received at 32 and to be described later.

The voltage output 30 from the circuit 28 is supplied to the battery 22 through a current sensing device 34.

The device 34 supplies a signal to a current amplifier 36 to generate the selection signal 32 which controls the voltage output 30.

While the battery 22 is being charged, the output voltage 30 remains at the higher of the two possible voltages until the charging current has dropped below a threshold value, at which point the signal 32 causes the voltage output to drop to the lower value. The functioning of the circuits will now be described in more detail with reference to Fig. 3, which is a plot of the charging current (vertical axis) applied to the battery 22 during charging, against time (horizontal axis).

Figures given in relation to the graph relate to the use of a 12V 6AH hour battery.

When charging commences, with the battery 22 being significantly undercharged, the battery will present relatively low resistance to charging current which will accordingly be relatively high. The circuit 28 will generate the higher of the two output voltages, which is 14.4V in this example and is significantly greater than the nominal voltage of the battery. Thus, the charge on the battery quickly increases. At 14.4V, the initial charging current may be in the region of 1.5A. This will slowly reduce as the battery becomes more highly charged.

Eventually, after seven hours or more, the charging current will drop to and through a threshold value such as 200mA, indicated at 36. The time at which this occurs is indicated by a vertical line 38 in Fig. 3. At that time, the circuits 34 and 36 provide an input 32 which changes the output 30 of the circuit 28 from its higher value, 14.4V, to a lower value of 13.8V.

The threshold current 36 is chosen so that the change over time 38 occurs when the battery is almost wholly re-charged, such as when the battery has more than 90 Ó of a full charge. However, the threshold current is sufficiently high as to ensure that damage to the battery cannot occur by excessive charging current being driven into the battery by the high charging voltage.

The charging voltage after the time 38 is chosen to be low enough to substantially only trickle charge the battery, that is to do very little more than to compensate for natural losses of charge occuring within the battery. Consequently, the charging current drops off very substantially and very quickly when the change occurs at 38 and the current thereafter will remain at a very low level, such as ImA or less. The charge on the battery will then rise very slowly towards full charge, but the charging current will be sufficiently small that the battery can be left connected to the charger indefinitely without damage.

After 8 hours or more, the battery will be fully charged. In this way, a new battery capable of powering the device 10 for 12 months can be returned to a state of charge which allows the device to operate unattended for a further period of 12 months. By contrast, a conventional charging technique might return the battery to only a 75 full charge, thereby significantly reducing the length of time for which the device can be left unattended, without re-charging. After a second charging by conventional techniques, the state of the battery may have reduced to only 508 Ó of the charge on a new battery.

Variations and modifications to the apparatus described above can be made without departing from the spirit and scope of the present invention. In particular, other choices of charging voltage, charging current thresholds and battery size can be made according to the nature and the intended use of the device.

Whilst endeavouring in the foregoing specification to draw attention to those features of the invention believed to be of particular importance it should be understood that the Applicant claims protection in respect of any patentable feature or combination of features hereinbefore referred to and/or shown in the drawings whether or not particular emphasis has been placed thereon.

Claims

1. An alarm-generating device having a sensor for sensing alarm conditions, means for generating an indication that an alarm condition has been detected, and a lead acid battery for powering the device without connection to a mains electrical supply, the device further comprising a battery charger for use in re-charging the said battery when the battery charger is connected to a mains electrical supply, the battery charger comprising voltage generating means operable to generate a plurality of charging voltages for application to the battery, and further comprising current sensing means operable to cause the charging voltage to change when the charging current passes through a pre-selected threshold value.

2. A device according to claim 1, wherein at least one of the charging voltages exceeds the nominal working voltage of the battery.

3. A device according to claim 1 or 2, wherein the charging voltage is reduced from its initial value when the charging current passes through a threshold value.

4. A device according to any preceding claim, wherein the final charging voltage supplied by the charger causes the battery to trickle charge.

5. A device according to any preceding claim, wherein the battery is a 12V, 6AH lead acid battery.

6. A device according to claim 5, wherein the initial charging voltage is in excess of 14V.,

7. A device according to claim 6, wherein the initial charging voltage is 14.4V.

8. A device according to claim 5, 6 or 7, wherein the final charging voltage is 13.8V.

9. A device according to any of claims 5 to 8, wherein the charging current is reduced to 200 mA or less when the charging voltage is changed.

10. A battery charger for lead acid batteries, comprising voltage generating means operable to generate a plurality of charging voltages for a battery, and further comprising current sensing means operable to cause the charging voltage to change when the charging current passes through a pre-selected threshold value.

11. A battery charger according to claim 10, wherein at least one of the charging voltages exceeds the nominal working voltage of the battery.

12. A battery charger according to claim 10 or 11, wherein the charging voltage is reduced from its initial value when the charging current passes through a threshold value.

13. A battery charger according to claim 10, 11 or 12, wherein the final charging voltage supplied by the charger causes the battery to trickle charge.

14. A battery charger according to any of claims 10 to 13, operable to charge a 12V, 6AH lead acid battery.

15. A device according to claim 14, wherein the initial charging voltage is in excess of 14V.

16. A battery charger according to claim 15, wherein the initial charging voltage is 14.4V.

17. A battery charger according to any of claims 14 to 16, wherein the final charging voltage is 13.8V.

18. A battery charger according to any of claims 14 to 17, wherein the charging current is reduced to 200mA or less when the charging voltage is changed.

19. An alarm-generating device substantially as described above with reference to the accompanying drawings.

20. A battery charger substantially as described above, with reference to the accompanying drawings.

21. Any novel subject matter or combination including novel subject matter disclosed, whether or not within the scope of or relating to the same invention as any of the preceding claims.