GB2072966A - Battery discharge warning system - Google Patents

Abstract

A battery discharge warning system particularly suitable for use in vehicles to warn of an electrical load on the system when the charging current is not operative, comprises a first set of inputs (1-6) for monitoring the condition of load circuits; a second input (7) for monitoring the condition of the ignition circuit, and a latch circuit (G2, I1, D3) arranged to latch to an "ON" condition only when both inputs are "ON", so as to give rise to a warning actuating signal which is arranged to be inhibited as long as there is an "ON" signal at the second input (7), the latch circuit being arranged to unlatch only when an "OFF" signal appears at the first input. <IMAGE>

Description

SPECIFICATION Battery protection system This invention relates to a warning device for electrical systems including a secondary battery and a charging circuit, in which a warning is required if an electrical load is left connected after the charging circuit has been disconnected. It has particular application in automobile electrical systems, in which it acts to provide a warning if lights (for example) are inadvertently left on after the ignition has been switched off.

According to a first aspect of the invention there is provided a warning device of the type described, comprising at least one first input for monitoring the condition of a load circuit or circuits; a second input for monitoring the condition of a charging circuit (for example an ignition circuit of an automobile engine); a latching circuit adapted to latch to an 'ON' condition only when 'ON' signals are present at both said inputs, so as to produce a warningactuating signal, and to unlatch only when an 'OFF' signal appears at said first input; and an inhibit circuit which acts to inhibit said warningactuating signal in the presence of an 'ON' signal at said second input. The warning-actuating signal is preferabiy used to drive an audible warning device.

Preferably, the latching circuit comprises at least one AND (or NAND) gate having first and second inputs connected to the respective inputs at the device and whose output is fed back to its second input in such a way as to provide a latching function.

According to a further aspect of the invention, there is provided a load-circuit current-monitoring device particularly adapted for use with a warning device of the type described, and comprising a ferromagnetic member of material having low magnetic retentivity and high permeability, adapted to be positioned around a currentcarrying conductor of the load circuit, and having an air-gap in which there is positioned a transducer adapted to produce an electrical signal in a detection circuit in response to a change of magnetic flux in the gap beyond a predetermined level corresponding to a predetermined level of current in the conductor. Preferably the transducer is a reed-switch. The magnetic circuit may be biassed to a predetermined flux level by means of a permanent magnet positioned across the gap.

The use of such a device to monitor the flow of current out of the battery (for example) renders the installation of the device much simpler than is the case if separate connections need to be made to different parts of the electrical circuit of an automobile, for example.

When the warning device of the invention is used in an automobile, for example, in conjunction with a current detection device, the said second input preferably incorporates a delay circuit which is arranged to delay the removal of the inhibiting signal by a specified period after the charging circuit is deactivated, to allow time for the charging current present in the battery circuit to die away.

Some embodiments of the invention will now be described by way of example with reference to the accompanying drawings, in which: Figure 1 is a circuit diagram of a first type of warning device in accordance with the invention; Figure 2 is a schematic diagram of a load current-monitoring system; Figure 3 is a diagrammatic perspective view of a first type of load-current monitoring device; Figure 4 is a vertical cross-section through another type of load-current monitoring device; and Figure 5 is a circuit diagram of a second type of warning device in accordance with the invention.

Referring to figure 1 , which shows a "negative earth" system, the positive sides of all electric loads to be supervised are connected to the individual inputs of multiple (6 in the example) input logic OR gate G,. Output from G, is connected to the input of a NAND gate G2 whose other input connects via a diode D, to an input 7 from a point in the automotive installation which remains energised when the engine is running and de-energised when the engine is stopped. When an electric load is on and the engine is running, both inputs to G2 are at "high" and the output from G2 becomes "low". This results in the output of inverter 11, connected to the output of G2 becoming "high". This output is returned via diode D3 to the input of G2. "High" from input 7 is also applied through diode D2 to the input of inverter 12.

The resultant "low" from the output of the inverter applied via the resistance Rg to the base of the npn transistorTR1 maintains it in a cut off state. When the engine is switched off input 7 assumes a "low" state. The logic gate G2 remains latched with its output at "low" because one of its inputs is kept "high" due to an electric load remaining on and keepint output from gate G, "high", whilst the other input to G2 is kept "high" by the feedback.

from inverter 11. Since the pull up action of D2 has now ceased, "low" from G2 is applied via resistor Rs to the input of inverter 12. The resultant "high" from its output turns transistor TR1 on and the alarm signal is turned on.

When the load or loads resulting in "high" from gate G1 are switched off output of G2 becomes "high", the alarm signal ceases and the latching action via D3 is cancelled. If it is desired to re-apply the load deliberately, e.g. switching on the parking lights, this can be done without the resultant alarm because the input via diodes D1 and D3 remains at "low" and output from G2 will remain "high", keeping TR1 cut off.

It might prove inconvenient to make the individual connections from various loads to the inputs of gate G,. It is possible to replace the voltage signals from such loads by a single signal obtained by monitoring battery current resulting from any such load or loads being switched on.

A simple method of current monitoring is to measure voltage drop across a low value resistor R3 inserted in the common supply lead from the battery as in figure 2, or even measure the voltage drop along the lead itself. The two measuring points are connected to a difference amplifier A.

When the output from the amplifier is of a sufficient value signifying a threshold value of current passing through Rs, the Schmitt trigger circuit ST operates applying a logic "high" state to the input of G2 in figure 1 connected previousiy to the output of G1.

If current detection signal is employed it is necessary to employ a delay of "high" state at the input 7, to maintain this signal until the engine comes to rest and the reverse, charging current ceases, enabling the true load current to establish itself. This delay may consist of a capacitor C, charged from input 7 and a Schmitt trigger circuit ST2, as in figure 2.

Another method of current detection, which may be employed, for instance to avoid making too many connections to the existing wiring, is to detect magnetic field resulting from the current flowing in the lead connecting the battery to various loads. For this purpose the lead is passed through a ferromagnetic core illustrated in figure 3. The core contains an air-gap into which a magnetic detector is inserted. This can be in form of a Hall effect device or a magneto-resistor. The electric output from the Hall device is applied to an amplifier the output of which is connected, via a Schmitt trigger, in place of G, in figure 1. The magneto resistor type of device is connected into a bridge network of resistors. The output from the bridge is connected to an amplifier which in turn is again, via a Schmitt trigger circuit, connected in place of the output from G, in figure 1.

A third, simpler arrangement for magnetic detection of current is shown in figure 4. It contains a ferromagnetic core 1 of material with low magnetic retentivity and high permeability.

The core is provided with an air-gap 2 and the current carrying cable 3 is threaded through the core aperture. A small hole is drilled through the centre of the pole pieces on both sides of the airgap and at right angles to it. A small, sensitive magnetic reed-switch 4 is inserted into the hole, with the contact gap approximately in the centre of the air-gap of the core. A small biasing magnet 5 is applied across the air-gap. Current flowing in conductor 3 induces magnetic flux in the core with a polarity aiding the flux from magnet 5 so as to force the flux through the ferromagnetic contacts of the reed-switch. The resultant closure of the contacts produces the signal, verifying the flow of current in lead 3.

This form of current detector is employed in the circuit shown in figure 5 which is suitable for both positive and negative earth systems. The function of the circuit is as follows: Input 1 is used when the vehicle wiring is "negative earth" and is connected to a part of vehicle wiring which is energised when the engine is running is connected via diode D, to a delay circuit consisting of resistors R3, R4, capacitor C, and NAND gates G2 to G4 connected in a Schmitt trigger circuit. This circuit produces a signal "high" at the output from G3 when the engine is switched on and for some 3-4 secs. after switching the engine off.

If the wiring of the vehicle is of "positive earth" type, the "engine on" signal is applied to Input 2 and the inverter circuit consisting of D2, R1, G, and D3 produces a positive output required by the remaining circuitry.

If a load is connected, resulting in a significant value of current, the electromagnetic detector shown in figure 4 produces a "high" signal vra reed-switch S1 in figure 5. This signal and the output from G3 are applied to the input of G5 with resultant "low" from its output. "High" from G3 is also applied via D5 to the inputs of inverter connected gate G7. Its output is then at "low" holding the transistor TR1 in the cut off state via resistor R8.

If the engine is switched off but the load current remains on, the output from G3 reverts to "low" after the prescribed delay period. However G6 latched via the inverter G6 and diode D6 remains with its output at "low". The inhibiting action, when either diode D5 or D7 is conducting, is removed and the multivibrator consisting of gates G, and G8 and C2 and R7 commences to oscillate and an intermittent audible signal is produced by the buzzer B switched by TR1. If the load is switched off, S1 opens and circuit G5, G6, D6 unlatches. If the load is now re-applied G5 remains with its output at "high" and D7 inhibits multivibrator G7, G8 and no warning signal is produced.

Claims (14)

1. A discharge device for an electrical system of the kind including a secondary battery and a charging circuit, comprising at least one first input for monitoring the condition of a load circuit or circuits; a second input for monitoring the condition of a charging circuit; a latching circuit adapted to latch to an "ON" condition only when "ON" signals are preset at both said inputs, so as to produce a warning-device actuating signal, and to unlatch only when an "OFF" signal appears at said first input; and an inhibit circuit which acts to inhibit the operation of said warning device in the presence of an "ON" signal at said second input.

2. A warning device according to claim 1 in which the latching circuit comprises at least one AND (or NAND) gate having first and second inputs connected to the respective inputs of the device and whose output is fed back to its second input in such a way as to provide a latching function.

3. A warning device according to claim 1 or claim 2 further comprising an audible warning device driven by the said warning-device actuating signal.

4. A warning device according to any preceding claim further comprising a delay circuit connected to the said second input and arranged to delay the removal of the inhibiting signal by a predetermined period after deactivation of the charging circuit, to allow time for the charging current in the battery circuit to die away.

5. A discharge warning device according to any preceding claim including sensor means adapted to detect the current in the charging circuit without a physical connection to the said circuit.

6. A current-monitoring device comprising a ferromagnetic member of material having low magnetic retentivity and high permeability, adapted to be positioned around a current carrying conductor, and having an air-gap in which there is positioned a transducer adapted to produce an electrical signal in a detection circuit in response to a change of magnetic flux in the gap beyond a predetermined level.

7. A current monitoring device according to claim 6 in which the magnetic circuit is biased to a predetermined flux level by means of a permanent magnet positioned across the gap.

8. A load circuit current monitoring device according to claim 6 or claim 7 in which the transducer is a reed switch.

9. A current monitoring device according to claim 6 or claim 7 in which the transducer comprises a Hall-effect device.

10. A current monitoring device according to claim 6 or claim 7 in which the transducer comprises a magneto-resistor.

1 A warning device according to claim 5 in which the sensor comprises a current-monitoring device according to claim 6 or claim 7 or any one of claims 8 to 10.

12. An automobile having a battery discharge warning device according to any of claims 1 to 5 our claim 11.

13. A battery discharge warning device substantially as herein described with reference to Figure 1 or Figure 5 of the accompanying drawings.

14. A load current monitoring device substantially as herein described with reference to Figure 3 or Figure 4 of the accompanying drawings.