GB2239359A - Circuit controlling connection of a load to a chargeable battery - Google Patents

Abstract

The circuit, which may control the connection of a refrigerator or air conditioner on a caravan to a battery B carried on a towing vehicle and charged from an alternator A1 prevents the load L being connected unless the alternator is charging the battery. With the load L disconnected, an operational amplifier A senses when the battery voltage rises above a preset value, which corresponds to charging by the alternator A1, and in response initiates a timer T which energises relay R/1 having normally open contacts R1 connecting the load to the battery. The timer T maintains relay R/1 energised but when the timer T times out relay R/1 de-energises, disconnecting the load from the battery, and unless amplifier A senses a battery voltage corresponding to a battery charging state the load remains disconnected until battery charging conditions are again sensed. An alternative sensing circuit uses a micro-computer supply voltage supervisor (A), (Fig 2), which combines a voltage standard and a comparator in one package. <IMAGE>

Description

Title: Improvements relating to control circuits for controlling application of loads to chargeable batteries" Description of Invention This invention relates to a control circuit for use in controlling a connection of a load to a chargeable battery and is intended to be applied in a case where certain of the loads or current consuming devices to be supplied from the battery represent too heavy a drain unless the battery is being concurrently charged from an associated charging source, e.g. an alternator or other rotary power driven generator.

The invention has been devised and developed primarily in relation to a specific problem which arises when low voltage high current plant is carried in motor vehicles or vehicles towed by these. Thus 12V power supply mobile vehicle refrigerators such as are supplied by Electrolux for mobile canteens, caravans etc., are generally supplied with three possible inputs of energy to operate them: 1. bottled gas; 2. mains electricity; 3. 12V D.C. supply (chargeable battery).

(1) and (2) are clearly intended for use when the vehicle is static.

The use of (3) has been limited by a 100 watt power requirement of such refrigerators. A load of 8 amps is really feasible only when the engine of the towing vehicle is running when, due to current supplied by the alternator, any discharge of the battery is prevented.

This requirement has been partly met by installing a relay activated when the ignition of the towing vehicle is switched on. This has two objections, the control really ought to be in the towed vehicle for full interchangeability as between towing vehicles, and the fact that the ignition of the towing vehicle is switched on does not necessarily mean that the engine is running and the alternator is being driven and generating.

Ideally therefore, it is considered that some device is needed in the towed vehicle which would detect when the engine driven alternator in the towing vehicle is providing a charging voltage for the battery, and then connect the supply to the refrigerator 12 volt circuit.

According to the invention, there is provided a control circuit for controlling the connection of a load to a chargeable battery in dependence upon whether the battery is undergoing charging, the control circuit comprising switch means controlled by a sensing means able to sense a battery parameter varying according to whether the battery is undergoing charging or not and serving to establish connection of the load to the battery in the former case, and sampling means for periodically removing the load from the battery to permit the sensing means to respond to said parameter without disturbance by the load itself.

The parameter sensed may be battery voltage, sensed effectively at input terminals for the load, and the sampling means may comprise a timing circuit or unit providing a timing signal initiated by response of the sensing means to a battery charging condition and terminated after a preset time interval, the switch means being responsive to initiation of the timing signal to establish connection of the load to said terminals during continuance of the timing signal.

Because the sensing of the voltage takes place with the load disconnected, the sensed parameter is not affected by voltage drop in what may be a relatively long connection between the battery and the load, and hence sensing can take place reliably at the load input terminals (at the battery side of the switch means). This is particularly important when the load is a refrigerator or comparable current consuming device carried on a towed vehicle whereas the battery is carried by the towing vehicle.

The sampling period, may be chosen to attain a satisfactory balance between minimising the discharge from the battery if the battery ceases to be charged during the timing period, and the operating life of the switch means (normally a switching relay).

Embodiments of the invention are illustrated by way of example in the accompany drawings, wherein: FIGURE 1 shows a schematic circuit of one embodiment; FIGURE 2 shows a schematic circuit of a second embodiment.

Referring to Figure 1, a typical standard vehicle battery B is carried on a towing vehicle indicated by broken lines 10 having a generator such as an alternator Al and associated voltage regulator Vr driven from the vehicle prime mover. The battery B has a dormant voltage when fully charged but neither loaded nor undergoing charging of 12.7V (typically), and when undergoing charging at the controlled voltage of the alternator and voltage regulator combination, of from 13.6V to 14.6V (say nominally 14V).

A load L such as a refrigerator or air conditioning plant is carried on a towed vehicle, such as a caravan, indicated by broken line 11, and due to the load current which this draws, typically 8.A, it is required to prevent this being connected to the battery for a period which would cause an unacceptable discharge when the alternator is not charging the battery, e.g. because the alternator is not being driven or possibly, although driven, not delivering its proper output.

In the control circuit of Figure 1 carried on the towed vehicle sensing of the voltage on line 12 is effected by operational amplifier A used as a comparator. A Texas Instruments type TL081 would be suitable.

The output from normally closed contact of relay R/1 is taken from line 12 via line 14 to a potential divider R1 and R2, so that when the input voltage on line 12 reaches say 14V, the input voltage to the inverting input 2 of operational amplifier A rises above 7.5 volts, the potential at which the non-inverting input 3 is held by zener diode D1 connected between line 13 and earth, and the output at terminal 6 of the comparator goes from logic 1 to logic 0.

A negative impulse is thus applied to terminal 2 of the timer T (Texas Instruments type NE555) via resistance R5. This initiates a timing sequence and gives a positive output at terminal 3 to operate relay R/1 maintained until the timer "times out". During this period the load L is connected to the battery B (which is undergoing charging from alternator All). After timer T has timed out, terminal 3 no longer presents a positive output, relay R/1 returns to its unoperated condition and either: (a) the alternator is still running and the normally closed contact of R/1, having restored supply to the potential divider R1 and R2 amplifier A initiates another trigger pulse to timer T, so restarting the cycle; or (b) the generator has ceased to give its charging output, say of 14V, terminal 3 of operational amplifier A is at a higher potential than terminal 2, terminal 6 of the operational amplifier A remains at logic 1 and the timer T remains off, so that the load L remains disconnected from the battery.

The operational amplifier thus acts as a sensing means, to sense a battery parameter (voltage) dependent upon whether the battery is undergoing charging or not, relay R/1 acts as a switch means to establish connection of the battery to the load in the former case, and timer T acts as a sampling means for periodically removing the load from the battery to permit of sensing of battery voltage without disturbance by the load L.

A typical 16A single pole relay may be used for relay R/1 to allow for the use of loads greater than 8A. Such a relay typically has a mechanical life of 50 million operations and an electrical life of 100,000 operations at full load.

If one takes the switched-on time as one minute, discharging the battery at a rate of 8A for one minute would reduce the charge by 0.13 amp hours. 100,000 operations at one minute intervals would give a working contact life of 1,666 hours, representing say a life of 50,000 miles for the vehicle carrying a refrigerator.

Assuming that the useful life of the vehicle would be 100,000 miles, a realistic timing interval would be 3 minutes, but even if the timing interval were increased to 5 minutes, the maximum discharge under the worst conditions, viz when a timing period had just started, when the generator had been stopped, only 0.7 of an ampere hour, 1.6% of the total charge would be taken from the battery which would be quite acceptable.

This figure of three minutes would be generally acceptable but a reduction of the sampling period to one minute or an increase up to five minutes could safely be adopted, if conditions of operation made this desirable.

In practice, the sampling at three minute intervals gives an effectively continuous supply. An L.E.D. indicator LED1R (red) energised via R4 and showing the "on" condition for relay R/1 barely flickers during operation. A further L.E.D indicator LED2G (green) is energised via R8 indicates connection of the control circuit between the battery B of load L.

Diode D3 protects the timer unit T against damage which could arise from induced voltage upon de-energisation of relay R/1, and diode D4 and resistor R6 provide a discharge path for any current so induced.

A protective regulator P in line 13 prevents any induced voltage above a value of say 15 volts being applied to any circuit connected to it as the supply voltage to the operational amplifier and the timer should not exceed 18V.

The sensing means, using operational amplifier A, is able reliably to detect and respond to a rise of voltage on lines 12, 13, to a value of 14 volts which will occur when charging takes place, but will not respond to a dormant battery voltage of 12.7V (for a fully charged battery).

The length of the connection from the supply, i.e. positive line 12, and earth line (not shown) on the towing vehicle to the corresponding terminals of load L may be considerable and the voltage drop in these lines would be such that depending upon the state of operation the refrigerator, e.g. heating element (for an absorption type) that sensing by amplifier A of the voltage parameter would be unreliable. However, since sensing is carried out at intervals determined by the timer T with concurrent disconnection of the load, the voltage parameter is not disturbed by the load itself.

In the circuit of Figure 2 functionally corresponding components are designated by like references and the preceding description applies.

Sensing of the voltage on line 12 is done by a micro-computer supply voltage supervisor which combines a voltage standard and a comparator in one package and is marked A on the circuit diagram.

The voltage supervisor used is TL7715 (Texas Instruments). With this device, when the supply voltage is below a typical value of 13.5 volts, one of the output terminals 6 is at logic 1 and the other 5 not used) is at logic 0. When the supply voltage goes above 13.5 volts, by a value of say 0.2 volts, the outputs change over in logic level so initiating the timer T and operation of relay R/1 as before.

The use of the integrated circuit TL7715 (Texas Instruments) not only reduces the number of components used compared with the circuit of Figure 1 but as it operates to closer tolerances than the zener diode of Figure 1, the circuit of Figure 2 is preferable for bulk production.

Appropriate values for other components are shown on both circuit diagrams.

Claims (8)

CLAIMS:

1. A control circuit for controlling the connection of a load to a chargeable battery in dependence upon whether the battery is undergoing charging, the control circuit comprising switch means controlled by a sensing means able to sense a battery parameter varying according to whether the battery is undergoing charging or not and serving to establish connection of the load to the battery in the former case, and sampling means for periodically removing the load from the battery to permit the sensing means to respond to said parameter without disturbance by the load itself.

2. A control circuit according to Claim 1 wherein the parameter is battery voltage sensed effectively at input terminals for the load, and the sampling means comprises a timing circuit or unit providing a timing signal initiated by response of the sensing means to a battery charging condition and terminated after a preset time interval, the switch means being responsive to initiation of the timing signal to establish connection of the load to said terminals during continuance of the timing signal.

3. A combination of the control circuit according to Claim 1 or Claim 2 with a load in the form of a refrigerator, air cooling means or other current consuming device which would draw current from the battery at a level such that premature discharge would occur without concurrent charging of the battery.

4. The combination according to Claim 3 wherein the current consuming device is carried on a caravan or other trailer vehicle, the battery is carried by a towing vehicle having a prime mover for driving the vehicle and also for driving an electrical generator forming the source of charging current.

5. The matter claimed in any one of Claims 1 to 4 wherein the battery has a nominal 12 volt rating and the sensing means is responsive to a voltage of 13.6 to 14.6 volts or thereabouts at the load terminals to bring about connection of the load, and the sample means provides sampling intervals of between about one minute and five minutes.

6. A control circuit substantially as hereinbefore described with reference to and as illustrated in Figure 1 of the accompanying drawings.

7. A control circuit substantially as hereinbefore described with reference to and as illustrated in Figure 2 of the accompanying drawings.

8. A control circuit including any novel feature or novel combination of features disclosed herein and/or shown in the accompanying drawings.