EP1064171A1

EP1064171A1 - Battery management device for a motor vehicle

Info

Publication number: EP1064171A1
Application number: EP99913425A
Authority: EP
Inventors: Jonathan Paul Laker
Original assignee: Smartswitch Ltd
Current assignee: Smartswitch Ltd
Priority date: 1998-03-27
Filing date: 1999-03-29
Publication date: 2001-01-03
Also published as: GB2350499A; WO1999050097A1; JP2002510190A; GB9806541D0; AU3155099A; GB0020871D0; GB2350499B

Abstract

Motor system having an engine and means for starting that engine, an electrical system powered by a battery and incorporating engine start and operate circuits, means for re-charging the battery (B) when the engine is operating and a plurality of electrical accessories in addition to the engine start and operation circuits powered from that battery.

Description

BATTERY MANAGEMENT DEVICE FOR A MOTOR VEHICLE

The present invention relates to a device, notably to one which isolates circuits in the electrical system of a motor vehicle so as to reduce the current drain on the vehicle's battery.

BACKGROUND TO THE INVENTION;

Most vehicles incorporate a battery powering a range of electrical systems, such as the ignition system, the starter motor, the lights, the in-car entertainment system and so on, connected to the battery via a wiring loom or harness, the battery being re-charged by a dynamo or alternator driven as the engine operates. The term electrical accessory will be used herein to denote an electrical circuit or appliance which is connected to the battery. The accessory may be peripheral to the operation of the vehicle, for example the in-car entertainment system, or may be essential to the operation of the vehicle, for example the lighting system. However, the term does not include the starter motor and its actuation circuit and any electrical fuel pump or spark ignition circuit, which are essential to starting the vehicle's engine. Many of the accessories require continuity of power supply, even when the vehicle's engine is at rest, for example the car security system and the engine management system. Others are usually only in use when the car is being driven, for example the head lights or the in-car entertainment system.

All such accessories drain current from the battery to varying extents and can over a period of time cause the charge held by the battery, and hence the output voltage of the battery, to drain to such low levels that the battery cannot turn the engine over to start the engine - 2 - from rest. This is particularly the case where the lights or entertainment system are left on accidentally when the user leaves the vehicle for a long time. Such draining of the battery results in the vehicle being stranded until another source of electrical power becomes available, for example the use of jump leads from another car, to provide electrical power for the starter motor to turn the engine and power the ignition system until the engine begins to run at sufficient speed to cause the dynamo or alternator to provide sufficient electrical power to maintain the running of the engine and to recharge the battery. Batteries which are discharged due to the user leaving one or more electrical accessories switched on accidentally represent a major cause of all requests for help from motorists and are a major inconvenience to the motorist. Furthermore, draining batteries repeatedly to very low voltages causes irreversible damage to the cells of the battery and reduces the useful life of the typical lead/acid accumulator used in motor vehicles.

Many proposals have been made to overcome this problem, but none has been wholly successful. For example, it has been proposed to monitor the falling voltage of the battery due to demands from one or more accessories which have been left on. After a predetermined time period and/or when the voltage drops to a given level, corresponding to the minimum level required to operate the starter motor and the ignition system, the battery is isolated from all the vehicle's electrical systems to prevent further drainage of charge from the battery.

Whilst this prevents further drainage from the battery, it isolates all circuits, including those which require to operate continuously. For example, power to the security system will be switched off when the battery is isolated and the vehicle left vulnerable to theft or other damage; - 3 - or the engine management system will be switched off and any computer memory in that system may be lost or corrupted, which removes information required for the proper servicing and maintenance of the engine . In order to avoid this, it is necessary to by pass the battery isolation system so as to maintain power to selected accessory circuits. This requires re-wiring of the vehicle ' s electrical system, which is complex and often beyond the capability of the average car mechanic and may invalidate the vehicle manufacturer's warranty. In some cases, the re-wiring, notably of the security system or engine management system, can only be carried out by the original manufacturer of the vehicle.

Furthermore, some means has to be provided to re-activate the vehicle ' s electrical system when the user desires to start the engine. This can be done by actuating a manual over-ride switch which re-connects the battery to the remainder of the vehicle's electrical system. Alternatively, reconnection of the battery can be achieved in response to actuation of the ignition switch or some other electrical circuit responsive to the presence of the user in the car, for example the actuation of one or more of the door courtesy light switches as the user enters the car. Whilst this overcomes the need for the user to determine that the battery isolation switch has operated and needs to be de-activated, it will require additional circuitry and additional wiring to the vehicle ' s electrical system.

In order to reduce the above problems, it has been proposed, for example in US Patents Nos 5,615,076 and 5,764,469, to use the falling voltage of the battery as it is discharged to actuate a latch switch mechanism to isolate the electrical accessories from the battery when the voltage falls to a value below which it would not be - 4 - possible to re-start the vehicle's engine. The electrical accessories are re-connected to the battery by reversing the latch switch when the voltage applied by the battery rises to a level corresponding to the running of the engine and charging of the battery by the dynamo or alternator. Such a proposal overcomes the need to provide a manually operated switch or other signal to re-connect the electrical accessories once the engine is re-started and the battery voltage is raised to its normal operating level. However, the device described in such proposals has as its major objective the protection of the accessories against electrical spikes caused by the starting of the vehicle's engine. This requires that the electrical accessories be isolated from the battery during such engine start up, necessitating additional circuitry and complexity. Furthermore, in order to maintain power to the accessories during engine start up, a second battery is required which has to be switched in to the accessory circuit during engine start up, further adding to the cost and complexity of the proposals. Moreover, such proposals introduce the voltage monitoring circuit and the latched isolation switch between the battery and the accessories it serves so that power to all the accessories is switched off when the battery voltage falls below the selected level. Where it is necessary to maintain power continuously to an accessory, for example the vehicle security system, it is necessary to by pass the voltage monitoring circuit and latch switch, further adding to the complexity of the re-wiring required in using such proposals.

We have found that many of the problems associated with the prior proposals are due to the fact that a single voltage monitoring and battery isolation mechanism serves a number of electrical accessories. This enables a single device to be used, thus reducing costs, and simplifies its - 5 - installation by requiring merely the insertion of a single device in the main cable leading from the battery to the vehicle's electrical system. Such a device requires the use of components which can carry a comparatively large current in view of the total electrical load which is to be handled by the device and such components are usually robust. However, as pointed out above, it is necessary to by pass the device to enable power to be supplied to accessories or electrical circuits, such as the starter motor and ignition circuits, which require continuity of power supply.

We have found that if only those circuits which are likely to draw power from the battery inadvertently or unnecessarily when the vehicle's engine is at rest are each provided with an individual means for isolating that circuit from the battery, a simple and effective system can be provided which minimises the need for additional wiring and allows the vehicle user total flexibility in selecting which electrical accessories are to be isolated from the battery. Moreover, since each isolating device is required to carry only a small electrical load and not the total load of all the accessories to be isolated, the components required in such devices can be small and the isolation device can be compact. It is thus possible to incorporate the isolation device within another device commonly used in the accessory circuit, for example in the conventional relay switch used to switch the accessory on or off at the command of the vehicle's user. In a particularly preferred embodiment, the isolation device is incorporated within the housing of the conventional relay switch so that the invention can be applied to the existing electrical system of a vehicle merely by interchanging a conventional relay switch serving the desired accessory with a modified switch incorporating the isolation device. Such interchange can be effected by an - 6 - unskilled person and requires no re-wiring of the vehicle's electrical system. Since the interchange is effected individually for each accessory which it is desired to isolate, no by passing of a central unit is required to maintain continuity of power to other circuits or accessories. Furthermore, since failure of one isolation device will not affect accessories other than the one its serves, no general isolation of all accessories will occur if a component of the isolation device fails, as is the case with prior proposals.

The invention thus provides a simple and effective means by which an unskilled user can selectively protect the battery from drainage by individual accessories using a single interchange component. Moreover, the shape and pin configuration of relay switches have been harmonised to a large extent and are the subject of industry wide standards, for example the DIN standard for relay switches in the automotive field. This enables a single design of modified relay switch to be used in a wide range of vehicles and to serve a wide range of accessories . Where necessary, the isolation device can readily be modified to respond to different voltages so that some non-essential accessories, for example the in-car entertainment system, can be isolated from the battery at higher voltages, whilst more essential accessories, for example lighting circuits, can be retained connected to the battery until a lower voltage is reached.

SUMMARY OF THE INVENTION;

Accordingly, the present invention provides a motor system, notably in a vehicle, having an engine and means for starting that engine, an electrical system powered by a battery and incorporating engine start and operate circuits, means for re-charging the battery when the - 7 - engine is operating and a plurality of electrical accessories in addition to the engine start and operation circuits powered from that battery; characterised in that : a. At least a selected one of the accessories is provided with an individual voltage responsive mechanism by which that accessory can be isolated from the battery individually from other accessories, so as to reduce the drain of charge from the battery by that accessory; and in that b. That voltage responsive mechanism comprises: i. A first monitoring mechanism for monitoring the voltage applied by the battery to that accessory so as to detect when the applied voltage falls below a predetermined first, low value at which the battery still retains sufficient charge to enable the engine of the motor system to be started; and ii. A first isolation mechanism which is actuated in response to the detection of said first, low voltage so as electrically to isolate said accessory from the battery when said voltage falls to said first low value; and iii A mechanism, which need not be separate from said first monitoring mechanism, for monitoring the voltage applied by said battery to the monitoring mechanism and to detect when the applied voltage rises to a predetermined second, high value corresponding to that achieved when the battery is being re-charged by the operation of the engine; and iv. A mechanism, which need not be separate from said first isolation mechanism, which is actuated in response to the detection of said second, high voltage so as to re-connect said - 8 - accessory to the battery when the voltage rises to said second, high value.

As stated above, the term accessory is used herein to denote an electrical circuit or appliance which is not essential to the starting and running of the engine. The term thus excludes the starter motor and the circuit for actuating the starter motor and any spark ignition and electrical fuel pumps and the circuits for actuating them. The term also includes the circuits connecting those appliances to the battery. In a particularly preferred embodiment of the invention, the monitoring and isolation mechanisms are incorporated in the circuit connecting the appliance to the battery, notably in a switch for connecting or disconnecting the power supply from the battery to the appliance. It is especially preferred to incorporate the voltage monitoring mechanisms in a relay switch in the circuit, the monitoring mechanisms operating the relay switch to disconnect the appliance from the battery when the voltage detected by the monitoring mechanism drops to or below the predetermined first, low value and to re-connect the appliance to the battery when the voltage detected by the monitoring mechanism rises to or above the predetermined second, high value.

The present invention also provides a switch mechanism adapted to be incorporated in the electrical circuit of a single accessory in a motor system of the invention, characterised in that the switch is adapted individually to isolate that accessory from the battery, which switch incorporates a voltage responsive mechanism which comprises : i. A first monitoring mechanism for monitoring the voltage applied by the battery to that accessory so as to detect when the applied voltage falls below a predetermined first, low - 9 - value at which the battery still retains sufficient charge to enable the engine of the motor system to be started; and ii. A first isolation mechanism which is actuated in response to the detection of said first, low voltage so as electrically to isolate said accessory from the battery when said voltage falls to said first low value; and iii A mechanism, which need not be separate from said first monitoring mechanism, for monitoring the voltage applied by said battery to the monitoring mechanism and to detect when the applied voltage rises to a predetermined second, high value corresponding to that achieved when the battery is being re-charged by the operation of the engine; and iv. A mechanism, which need not be separate from said first isolation mechanism, which is actuated in response to the detection of said second, high voltage so as to re-connect said accessory to the battery when the voltage rises to said second, high value.

Preferably, the switch mechanism is a relay switch of the type which is commonly used in a car electrical system to respond to a switch operated by the user of the car, for example the windscreen wiper actuation switch on the steering column, and which serve to connect or disconnect the accessory to or from the battery. Such relay switches operate at low current loadings and incorporate or actuate a switch capable of handling high current loadings to connect the battery to the load circuit of the accessory. Such relay switches are usually plug in units having protruding pin, spade or other connectors which engage corresponding connectors present in a switch box between the battery and the accessory, typically of the type and - 10 - configuration as specified in the relevant DIN specification. Typically, such relay switches comprise a plastic or similar housing containing the operative components mounted on a base plate carrying the spade or other connectors by which the switch is connected to the electrical circuit and which serve to locate and support the housing and base plate as a unitary member at the desired position in a switch box carried by the vehicle structure.

The invention also provides a method for reducing the drainage of charge from a battery of a motor system due to one or more accessories inadvertently drawing current from the battery, which method comprises incorporating a separate switch mechanism of the invention in each of a selected one or more of the accessories or the electrical circuits serving it, so as to isolate that accessory from the battery once the voltage applied to that accessory falls to or below a predetermined first, low value and to re-connect that accessory to the battery once the voltage applied to that accessory rises to a predetermined second, high value .

The present invention can be applied to a wide range of types of motor system powered by a wide range of types of engine which are operated non-continuously and which require to be re-started by the operation of a starter mechanism. Thus, the invention can be applied to spark ignition engines, where it is necessary to provide both a starter motor and its operating circuit and a spark ignition circuit, both powered from the battery. However, the invention can be applied to compression ignition engines which require no spark ignition circuit .

Furthermore, the fuel may be fed to the engine by a mechanical pump or by an electrical pump which is also powered by the battery. Such starter motor, ignition and - 11 - fuel pump circuits are essential to the starting and operation of the engine and are not circuits ^' to which the present invention is applied. The engine can be a static engine, for example to power an emergency generator, or can be carried by a vehicle such as a ship, plane, railway locomotive, automobile or other road or land vehicle.

For convenience, the invention will be described hereinafter in terms of a spark ignition engine in a car.

The invention can be applied to motor systems in which the electrical system operates over a wide range of voltages. However, the battery will typically be a nominal 6, 12 or 24 volt battery.

The accessories to which the present invention can be applied can be selected from a wide range, for example the internal or external lighting circuits of the car, the windscreen wipers, the radio or other entertainment system, a heater circuit and so on. The invention can be applied to protect the battery against excessive drainage where an accessory is left on accidentally by the user of the car, for example the lighting system or the entertainment system; or where an accessory or its circuit develops a fault, for example due to breakdown of an electrical component or insulation, which causes an electrical short to occur.

For convenience, the invention will be described hereinafter in terms of a 12 volt car system for a spark ignition engine having the head lamp circuit provided with a voltage responsive mechanism of the invention to protect the battery against excessive drainage due to the head lights being left on by the user of the car. - 12 -

The voltage responsive mechanism of the invention comprises two voltage monitoring mechanisms which operate the switch or other device for isolating or re-connecting the accessory from the battery. Typically, the switch is a conventional relay switch which comprises a leaf spring carrying a terminal contact which is moved against the bias of the spring by one or more coils so that the contact bears against one or more other stationary contact (s) to complete the circuit allowing electrical current to flow to the accessory served by the switch. When the coil (s) are energised, for example by the user operating the headlamp on/off switch on the vehicle dashboard, the bias of the spring is overcome by the electromagnetic field generated by the electric current and the contact between the contacts is made. If desired, the electromagnetic forces can move the leaf spring between a central rest, or fail safe, position at which the contact carried by the leaf spring makes no contact with the fixed contacts and the accessory is isolated from the battery, and positions to either side of the rest position. At one of these positions the leaf spring contacts a first fixed contact to connect the accessory to the battery. At the other position, the leaf spring is positively retained away from the first fixed contact or it contacts a second fixed contact to connect a secondary circuit to the battery, for example the dipped beam as opposed to the full beam of the headlamp bulb. Such a form of the switch is denoted as a reed type relay switch. Alternatively, the relay switch can be a solid state switch, for example a latched gate switch or equivalent circuit in which the output from the voltage monitoring circuits described below latch the gate in the open or closed position to connect or disconnect the accessory which the switch serves. Other forms of switch, for example a linearly acting single or double action solenoid switch, may be used if desired. - 13 -

For convenience, the invention will be described hereinafter in terms of the use of a reed type relay switch.

The relay switch may itself directly connect or disconnect the accessory to the battery. Alternatively, the relay switch may act upon another, heavier duty, switch (denoted as the primary switch) to effect the connection and disconnection of the electrical load of the accessory where that load is great, for example 20 amps or more. For convenience, the invention will be described in terms of a relay switch which acts directly to connect or disconnect the accessory to the battery.

Such switches, their design, construction and operation are of conventional form. Typically, they comprise a housing for the electrical components and a base board carrying the components . The base board carries the appropriate protruding pins or spade connectors which serve to connect the electrical components to the input and output circuits or wires and also to support and locate the switch within a connection box which may itself carry a plurality of switches each serving a separate accessory circuit. Such switches and connection boxes are conventionally used to control the operation of the accessories in cars.

The voltage monitoring mechanisms may take a number of forms and are preferably solid state devices which are cheap, compact and rugged. Thus, the mechanism will typically comprise a reference substantially constant voltage source, for example a Zenner diode network. This reference voltage is typically from 10 to 35% of the voltage applied by the battery when the engine is operating normally and the battery is being charged by the - 14 - dynamo or alternator. The reference voltage is also selected so that it is below, typically from^'l to 3 volts below, the lowest voltage expected to be applied by the battery in normal operation of the vehicle. For example, during operation of the starter motor the voltage applied by the battery may drop to as low as 5 to 6 volts. For a nominal 12 volt battery, the reference voltage will be in the range 3 to 5 volts, preferably about 4 volts.

The reference voltage provides the reference level against which the voltage applied to the electrical system by the battery is monitored. The applied voltage can be monitored by a transistor circuit, operational amplifier or other suitable solid state device which switches its output when the voltage applied to one of its input connectors falls below or rises above a given level. Such devices can be of conventional design and construction and many are available commercially and may be used in their commercially available forms without modification. Preferably, the voltage monitoring is done by a dual comparitor device. It is also preferred that the low level voltage be monitored by one channel of such a device and the high level voltage monitored by the other channel, so that each channel controls a latched gate switch device to achieve positive change from the connected to disconnected position of the relay switch. Many of such latched gate devices are held positively in the open or closed position until the signal holding it in that position is over-ridden by a signal reversing the latching. Such latched gate switches are commercially available and may be used as such in the voltage monitoring device of the invention.

The latched gate device can act directly to connect or disconnect the current fed via the relay switch from the battery to the accessory. However, it may be preferred to - 15 - feed the output from the latched gate device to the input of the coils controlling the position of the leaf spring in a reed relay switch so that the solid state devices are not required to carry a large current .

It is particularly preferred to incorporate the voltage responsive mechanism, for example the low and high voltage monitoring devices and the latched gate device, in a single IC chip which can be incorporated on the battery input pin of the relay switch and feeds its output to the coil or coils of the relay switch to control the position of the leaf spring to make or break the circuit between the battery and the accessory.

As indicated above, such voltage responsive devices can readily be incorporated into a conventional reed relay switch and will not affect the function of the pin connectors of the switch. The switch incorporating the voltage devices can thus be used directly in place of a conventional relay switch, thus simplifying the use of the invention either by original equipment manufacturers or by the user of the car as a retrofit device.

The invention can be applied across a wide range of accessories operating over a wide range of current requirements by suitable selection of the various electrical components in the device. For example, some accessories may be peripheral to the operation of the car and can be disconnected from the battery after a comparatively small drop in battery voltage, so that the low voltage for such an accessory may be selected to be only 0.5 to 1 volt below that normal operating voltage of the battery so as to minimise drain from the battery. However, it is preferred to select the voltage responsive device so that it isolates the battery from the accessory which the device serves at close to the minimum voltage at - 16 - which the engine can readily be started by the partially discharged battery so that the accessory remains in operation for as long as possible. Typically, a nominal 6, 12 or 24 volt battery which is being charged by the alternator or dynamo as the engine is running will deliver about 7.8, 13.8 or 25.8 volts; and the minimum voltage required to operate the starter motor and the spark ignition/fuel pump circuits will be about 6, 12 or 24 volts, depending upon the type of engine, battery and starter motor. The optimum first, low voltage value at which the device of the invention should operate can readily be determined by simple trial and error tests and will typically be about 6.1 volts for a nominal 6 volt battery, 12.1 volts for a nominal 12 volt battery and 24.1 volts for a nominal 24 volt battery. The low voltage monitoring device can be specifically designed and manufactured to operate at that low voltage. Alternatively, the device can be designed to operate over a range of values and a balancing/resistor circuit can be applied to the device to achieve operation at the desired low voltage value. Such resistor/balancing circuits can be of conventional design and construction as is known in the art .

Similarly, the device monitoring the second, high voltage is set to operate at just below, eg. from 0.1 to 1 volts below, the voltage achieved when the engine is running. The second, high voltage value will thus typically be about 7.5, 13.5 and 25.5 volts for nominal 6, 12 and 24 volt batteries respectively. By selecting a value which is close to the normal operating voltage of the battery, the device does not respond to transient surges in the battery voltage as other accessories are disconnected from battery. Such high value voltage monitoring mechanisms may also have a resistor/balancing circuit if desired. - 17 -

The voltage responsive devices of the invention may incorporate other features to enhance the utility and/or extend the range of applications of the device. Thus, the device may incorporate a time delay mechanism so that the battery is not connected or disconnected from the load of the accessory for a period, say from 10 to 120 seconds after the relevant low or high voltage value has been detected. This will reduce the risk of premature connection or disconnection of the battery in response to a transient rise or fall in the voltage applied by the battery due to operation of some other component in the car's electrical system. Alternatively, the voltage responsive device may monitor the applied voltage intermittently, for example at intervals of from 0.1 to 120 seconds, and respond to the average of the voltages detected over the previous say 2 to 10 readings, thus smoothing the response of the device to fluctuating applied voltages. Such functions can readily be incorporated into the IC chip containing the other functions of the device. The device may incorporate means for trimming or adjusting one or more of the components of the low and/or high voltage monitoring mechanism so that a single design of device can be adjusted by the manufacturer or user to accommodate a range of applied voltages and/or high or low voltages at which the device is to actuate the connection or disconnection of the battery from the accessory.

DESCRIPTION OF THE DRAWINGS

The invention will now be described by way of illustration only with respect to a preferred embodiment as shown in the accompanying drawings, in which Figure 1 is a diagrammatic block diagram of the electrical system of a car having a relay switch incorporating a voltage responsive device of the invention to operate the - 18 - connection or disconnection of an accessory from the battery; Figure 2 is a block flow diagram of 'the logic of the device of the invention; Figure 3 is a diagrammatic part sectioned view of a power relay switch incorporating a voltage responsive circuit according to the invention; Figure 4 is a diagrammatic circuit diagram of the components of a form of the device of Figure 3 acting on the coil of a power relay switch; Figure 5 is a schematic logic chart for the sequence of operations of the device of Figure 4; and Figure 6 is a diagrammatic representation of a connection box of the car of Figure 1 incorporating a plurality of the relay switches of Figure 3.

DESCRIPTION OF THE PREFERRED EMBODIMENT:

As shown in Figure 1, a car electrical system comprises a wiring harness H connecting the various electrical components to a battery B. Each of the accessories A is individually operated by a first accessory control switch SWl, usually on the dashboard, for example the heater or air conditioning on/off or control switches, or on a stalk on the steering column, for example the side and headlight selector switches, the direction indicators or the windscreen wiper control switches . These control switches operate a power relay switch PWl in a switch box which connects the battery to the accessory. In this way the operating switches SWl etc on the dashboard and steering column need carry only a small current load, with the main operating current for the accessory passing through the heavy duty contacts and wiring of the power relay switch PWl.

As shown in Figure 3, the relay switch PWl typically comprises a pair of spaced apart fixed contacts 1, 2 which are wired into the battery to accessory circuit via the spade contacts la and 2a carried by the base plate 3 of - 19 - the relay housing. As shown in Figure 5, the spade contacts engage corresponding sockets in the base of the switch box connected to the battery and to the accessory by the wiring harness and also serve to locate and support the relay switch in the switch box. One of the fixed contacts 1 is provided with a leaf spring arm 4 which extends towards the other fixed contact 2 and is normally biased away from the other fixed contact by the action of the spring of arm 4. A coil 5 is located adjacent the leaf spring 4 and is energised by passing a current through it from the control switch SWl operated by the user of the car. This applies a magnetic field to the leaf spring 4 which overcomes the bias of the spring and moves the contacts 6 carried at the free end of the spring 4 into contact with the other fixed contact 2, as shown, to complete the circuit between the battery and the accessory and actuate the accessory. When the control switch is reversed or released by the user, the current no longer passes through the coil , the leaf spring biases away from the other fixed contact and the circuit between the battery and the accessory is broken. Such a design of relay switch is conventional and its form and design in the motor industry are governed by strict standards, for example a DIN standard. Since the current applied to the coil by the control switch is low, the operation of the coil draws very little current from the battery and it is the fixed contacts and the leaf spring which are required to carry large current loads. The electrical components of the relay switch are housed within a housing 10 which is a snap fit onto base 3 to provide a robust sealed unit.

As shown in Figure 1, the voltage responsive device 20 of the invention is located on the electrical circuit of the coil 5 acting on the leaf spring 4 of the relay switch PWl. The device comprises a Zenner diode network 21 which provides a fixed reference voltage of 4 volts to the dual - 20 - comparitor circuit 22. One channel of the comparitor circuit 22 controls a latched gate switch of other solid state switch 23 which acts to open the switch as the voltage applied by the battery B falls below the level at which it would not be possible to start the car engine, about 12.1 volts. The other channel of the dual comparitor circuit 22 operates to reverse the latched switch 23 once the voltage applied by the battery rises above 13.5 volts, which corresponds to near the level when the engine is running and the battery is being recharged by the alternator or dynamo (not shown) . When the latched switch 23 is held in the open position, current cannot pass through the coil 5 of the relay switch PWl and the circuit between the battery B and the accessory A is cut . When the latched switch 23 is held in the closed position, current can pass through the coil 5 and bring the contact 6 carried by the leaf spring 4 and the other fixed contact 2 together to make the circuit between the battery B and the accessory A.

Figure 2 illustrates the logic process which the voltage responsive mechanism follows in monitoring the voltage applied by the battery and the operation of the switch PWl.

As shown in Figure 3, the components of the voltage responsive device of the invention can be compact and fitted onto a small circuit board or a single IC chip since they do not have to carry a large current load. This circuit board or chip can readily be accommodated within the housing of the relay switch so that the modified switch presents an external size and shape which is substantially identical to a conventional relay switch and in which the spade connector pins retain the orientation and function of the spade connector pins of a conventional relay switch. The modified relay switch can - 21 - thus be readily inserted in place of a conventional relay switch in a conventional switch box by an unskilled user without the need to modify the electrical system or wiring harness of the car. The user merely identifies which accessory circuit is the one which needs to be isolated, either to protect it against battery discharge or to be disconnected as the one which is most likely to cause battery discharge, for example by being left on inadvertently. The user may identify a circuit which is susceptible to failure, for example due to incipient break down of a component thereof, and protect the battery against premature discharge by such failure by inserting a modified relay switch of the invention in that circuit .

The invention has been described above in terms of a coil operated relay switch. However, the connection of the battery input and the accessory output circuits may also be achieved by an entirely solid state device.

Figure 4 illustrates a specific form of the voltage monitoring and switch devices used in the device of Figure 3 integrated into a relay switch 40. The device 40 receives electrical power from a car battery 42 along a negative earth line 44 and via an accessory switch SWl along power rail 46. The device 40 has three connectors 47, 48, 49 corresponding to the conventional connectors to a double poll single throw relay switch R A1 that is incorporated as the output of device 40.

The device 40 contains a switching circuit 50 which receives as an input the voltages on power rail 46 and earth line 44 and provides a switched output on line 52 which, together with earth line 44, is connected to control relay R A1. - 22 -

The switching circuit 50 has an integrated circuit IC1 with two comparators 53, 54 with outputs 55, 56 wired via input resistors 57, 58 to a solid state switch 60 that provides the output 52. Each comparator 53, 54 receives on its positive input 61, 62 a reference voltage of 4.1 volts from a Zenner diode ZD1 that is wired to earth line 44 and, via resistor 64, to power rail 46. The negative inputs 65, 66 of each comparator 53, 54 are each connected to a reference voltage on variable resistors VR1, VR2. The variable resistors VR1, VR2 each span the earth rail 46 and earth line 44, and are set so that the comparator negative inputs 65, 66 each receive a set fraction of the total voltage between power rail 46 and earth line 44.

The operation of the switching circuit 50 will now be described also with reference to Figure 5, which is a status diagram for solid state switch 60 in the case where variable resistor VR1 is set so that comparator 53 switches at 13.5 volts, and where variable resistor VR2 is set so that comparator 54 switches at 12.1 volts. These voltages represent, respectively, the second, higher voltage value at which the devices operates to re-connect the accessory load 70 to the battery, which is slightly below the expected normal operating voltage of battery 42 ; and the first, low voltage value at which the device operates to disconnect the accessory load 70 from the battery 42, and is slightly above the minimum battery voltage required to re-start the engine when the battery is partially discharged. In step 1 of Figure 5, the battery voltage is in the normal region of greater than

13.5 volts. In this case, the comparator negative inputs 65, 66 each receive a voltage greater than the reference voltage from Zenner diode ZD1 on positive inputs 61, 62. Therefore, comparator outputs 55, 56 are each low, and so therefore are inputs IP1, IP2 of the solid state switch 60. The solid state switch 60 includes discrete or - 23 - integrated sequential logic (not shown) which, when both inputs IP1, IP2 are low, generates a high output on line 52. Relay R A1 is then activated to switch power supplied to output line 47 through to output line 48 in order to power an accessory unit 70 associated with accessory switch SWl .

In step 2, the battery voltage has dropped below the normal expected operating voltage of 13.5 volts, but is still above the limit of 12.1 volts associated with a drained battery. In this voltage region, the variable resistors VR1, VR2 are set such that only comparator 53 will have switched from a low to a high output owning to the drop in voltages on negative input lines 65, 66. The solid state switch input IP1 is then high, pulled up by pull up resistor 67 wired to comparator output 55. At this point, the sequential logic in solid state switch 60 still generates a high output, so that the accessory load 70 is still connected to the battery.

If the voltage continues to drop, then step 3 is reached with the battery voltage at or below 12.1 volts, the minimum value at which the engine can be re-started. At this point, the output from the other comparator 54 has also switched owing to the output from variable resistor VR2 dropping to below the reference voltage from Zenner diode ZD1. The solid state switch input IP2 is then also high, pulled up by pull up resistor 68 wired to comparator output 56. The sequential logic in solid state switch 60 then switches the output line 52 low, in order to deactivate relay RL1, and so disconnect the accessory 70 from the battery.

If the voltage then rises above the lower limit of 12.1 volts, but still remains below the normal expected operating voltage of 13.1 volts, as shown in step 4, then - 24 - the sequential logic in solid state switch 60 provides some hysteresis so that the output on line 52 continues low even though IP2 will have switched from high to low. The accessory 70 therefore remains disconnected from the battery.

Once the voltage has reached the normal operating voltage of 13.5 volts, then both inputs IP1, IP2 to solid state switch 60 will have switched low, with the result that the output line 52 goes high. The switching circuit 50 therefore provides a measure of hysteresis so that the accessory 70 is not rapidly connected and disconnected at the transition state. This could happen if, for example, the accessory 70 was drawing sufficient current in order to make the voltage from the battery 42 drop below a set single limit point, only for the voltage from the battery 42 to rise above this set single limit point once accessory 70 had been disconnected.

Claims

- 25 -CLAIMS :

1. A motor system having an engine and means for starting that engine, an electrical system powered by a battery and incorporating engine start and operate circuits, means for re-charging the battery when the engine is operating and a plurality of electrical accessories in addition to the engine start and operation circuits powered from that battery; characterised in that: a. At least a selected one of the accessories is provided with an individual voltage responsive mechanism by which that accessory can be isolated from the battery individually from other accessories, so as to reduce the drain of charge from the battery by that accessory; and in that b. That voltage responsive mechanism comprises: i. A first monitoring mechanism for monitoring the voltage applied by the battery to that accessory so as to detect when the applied voltage falls below a predetermined first, low value at which the battery still retains sufficient charge to enable the engine of the motor system to be started; and ii. A first isolation mechanism which is actuated in response to the detection of said first, low voltage so as electrically to isolate said accessory from the battery when said voltage falls to said first low value; and iii A mechanism, which need not be separate from said first monitoring mechanism, for monitoring the voltage applied by said battery to the monitoring mechanism and to detect when the applied voltage rises to a predetermined second, high value corresponding to that - 26 - achieved when the battery is being re-charged by the operation of the engine; and iv. A mechanism, which need not be separate from said first isolation mechanism, which is actuated in response to the detection of said second, high voltage so as to re-connect said accessory to the battery when the voltage rises to said second, high value.

2. A motor system as claimed in claim 1, characterised in that the voltage responsive mechanism is incorporated in the electrical circuit connecting the accessory to the battery in a switch for connecting or disconnecting the power supply from the battery to the accessory.

3. A motor system as claimed in claim 2, characterised in that the voltage responsive mechanism is incorporated in a relay switch in the electrical circuit, the monitoring mechanisms operating the relay switch to disconnect the appliance from the battery when the voltage detected by the monitoring mechanism drops to or below the predetermined first, low value and to re-connect the appliance to the battery when the voltage detected by the monitoring mechanism rises to or above the predetermined second, high value.

4. A motor system as claimed in claim 3 characterised in that the relay switch actuates a second switch which connects or disconnects the current load to the accessory.

5. A switch mechanism adapted to be incorporated in the electrical circuit of a single accessory in a motor system as claimed in claim 1, characterised in that the switch is adapted individually to isolate that accessory from the battery, which switch incorporates a voltage responsive mechanism which comprises: - 27 - i . A first monitoring mechanism for monitoring the voltage applied by the battery to that accessory so as to detect when the applied voltage falls below a predetermined first, low value at which the battery still retains sufficient charge to enable the engine of the motor system to be started; and ii. A first isolation mechanism which is actuated in response to the detection of said first, low voltage so as electrically to isolate said accessory from the battery when said voltage falls to said first low value; and iii A mechanism, which need not be separate from said first monitoring mechanism, for monitoring the voltage applied by said battery to the monitoring mechanism and to detect when the applied voltage rises to a predetermined second, high value corresponding to that achieved when the battery is being re-charged by the operation of the engine; and iv. A mechanism, which need not be separate from said first isolation mechanism, which is actuated in response to the detection of said second, high voltage so as to re-connect said accessory to the battery when the voltage rises to said second, high value.

6. A switch as claimed in claim 5, characterised in that the switch is a relay switch of the type in which the switch mechanism is carried by a base plate within a protective housing, the base plate having extending pin, spade or other electrical connectors whereby the switch can be connected to the circuit is serves .

7. A switch as claimed in claim 6, characterised in that the relay switch incorporates a moveable contact carried - 28 - by a flexible support arm for movement between an operative position in which the moveable contact makes contact with a fixed contact to complete an electrical circuit under the influence of an electric coil and the voltage monitoring mechanism controls the flow of electric current to the coil.

8. A method for reducing the current drain on a battery in a motor system having an engine and means for starting that engine, an electrical system powered by a battery and incorporating engine start and operate circuits, means for re-charging the battery when the engine is operating and a plurality of electrical accessories in addition to the engine start and operation circuits powered from that battery, characterised in that a separate switch mechanism as claimed in claim 5 is incorporated in each of a selected one or more of the accessories or the electrical circuits serving it as to isolate that accessory from the battery once the voltage applied to that accessory falls to or below the predetermined first, low value and to reconnect that accessory to the battery once the voltage applied to that accessory rises to the predetermined second, high value.

9. A method as claimed in claim 8, characterised in that the switch mechanism is incorporated into only those selected individual circuits between the battery and the accessory from whose draining effect it is desired to protect the battery.

10. A motor system, switch or method as claimed in any one of the preceding claims, characterised in that first and second voltage monitoring mechanisms of the voltage responsive mechanism comprise a dual comparitor circuit having a constant reference voltage applied thereto and which operate a primary switch to apply or disconnect an - 29 - actuating current either directly to the accessory which the voltage responsive mechanism serves or indirectly via an intermediate switch which actuates the primary switch.

11. A motor system, switch or method as claimed in any one of the preceding claims, characterised in that the first, low voltage value is substantially 6.1, 12.1 or 24.1 volts, and said second, high voltage value is substantially 7.5, 13.5 or 25.5 volts for a nominal 6, 12 or 24 volt battery respectively.

12. A motor system, switch or method substantially as hereinbefore described with respect to and as shown in any one of the accompanying drawings .