AU8906082A - Measurement of a state of a liquid - Google Patents

Description

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MEASUREMENT OF A STATE OF A LIQUID

Background of the Invention

This invention relates to measurement of a state of a liquid. The invention has particular application although not exclusive application in the art of measuring a specific gravity state of a liquid such as the electrolyte in a wet cell lead acid battery. Description of Prior Art

It is well known that the condition of a wet cell lead acid battery is determined by the specific gravity of the electrolyte. It is also known that the speed of propagation of sound through a liσuid varies in accordance with the specific gravity of the liquid. The specific gravity measurement means of this invention -z - operates by detecting the time taken for sound to travel through the liquid and then using that information to determine the specific gravity. Statements of the Invention 5 Therefore, in accordance with a first broad aspect of the present invention there may be provided apparatus for measuring a state of a liquid which can be determined by the time taken for sound waves to travel .through the liquid comprising transducer means for ^* 10 generating sound waves and for receiving such generated sound waves after the sound waves have propagated a known distance within the liσuid, said transducer means being connectable with electric circuit means which will cause a sound wave signal to be propagated from said

15 transducer means within said liquid and will process a received signal by said transducer means to ascertain the state of said liquid by measuring the time taken of the sound wave signal to travel between the known distance. Preferably the transducer means comprises a sound wave

20 generating transducer and a spaced sound receiving transducer, the positioning of the sound wave generating transducer and the sound wave receiving transducer being such that when inserted into a liquid the sound waves generated by the sound wave generating transducer can

25 pass to said sound wave receiving transducer by passing through said liquid, electric lead means extending from each of said transducers for respective connection to signal supplying means for causing said sound wave signal transmitting transducer to emit sound waves, and to allow

30 electrical signals generated in said sound wave receiving transducer representative of sound waves detected by said sound wave receiving transducer to be fed to detection circuitry to be processed to ascertain the state of said liσuid dependent on the time taken for the sound to

35 travel between the transducers.

It is particularly preferred that when the apparatus is measuring a specific gravity state of a liquid that a temperature sensing means be incorporated with apparatus so as to take into account the variation in the speed of propagation of sound waves within the liquid consequent on any change of temperature of that liquid.

By using the apparatus of the present 5 invention in a liquid which has a known specific gravity it is possible to obtain a measure of the temperature of the liquid by measuring the time taken for sound waves to travel through the liquid. Brief Description of Drawings 0 In order that the invention can be more clearly ascertained a preferred construction for use in wet cell lead acid battery specific gravity state measurement will now be described with reference to the accompanying drawings wherein:- 5 Figures 1, 2- and 3 show respective perspective view, vertical cross sectional view and horizontal cross sectional view of a typical probe incorporating a sound wave generating transducer and a sound wave receiving transducer; 0 Figure 4 is a perspective view of an assembly of the transmitting transducer and the receiving transducer;

Figure 5 shows a block circuit diagram of a preferred control apparatus used to give an indication of 5 the specific gravity and/or to control the charging and/or discharging of a battery;

Figure 6 is a detailed circuit diagram of a circuit used for supplying signals to the sound wave generating transducer; and Figures 7 and 8 are collectively a detailed circuit diagram of a circuit used for receiving and _t processing signals from the receiving transducer. Description of Preferred Embodiments

Referring firstly to Figures 1, 2 and 3 there is shown a probe handle 1 made of a plastics material inert to wet cell lead acid battery electrolyte. The probe handle 1 is generally circular in cross section and ^• substantially hollow. The probe handle 1 receives a transducer-housing 3^* which is snap fittable within the hollow interior of the handle 1. The means for permitting snap engagement has not been shown in the drawings but may ^"simply comprise raisures on the surface of the transducer housing 3 and corresponding recesses on the inside of the hollow handle 1. Other means for permitting snap engagement of the transducer housing 3 to the handle 1 are possible and are within the scope of this invention. - The handle 1 has a generally square cross sectional shaped interior and the portion of the transducer housing 3 which is receivable in the handle 1 is correspondingly squarely shaped. The transducer housing 3 has a forward rimmed head 5 which positively locates the transducer housing 3 at the end of the handle 1. The transducer housing 3 is hollow with a square cross sectional shaped interior corresponding generally to the shape of that in the handle 1. Opposed sides of the transducer housing 3 have a transducer receiving aperture 7 therein. Grooves 9 are provided in the outside walls of the transducer housing 3 to enable electrical leads to pass to the transducers which are fitted in the apertures 7. The electrical leads pass through the handle 1 by passing through passageways 11 in a portion 15 which bridges across the walls of the handle 1 and divides the handle 1 into a forward end 17 and a rear or holding end 19. The leads which pass through the passageways 11 pass through the rear or holding end 19 and through an opening 21 in a plug member 23 which closes the rear or holding end 19 of the handle 1. Fluid passageways 25 are provided in the forward end 17 of the handle 1 to allow liquid such as electrolyte to fully enter the forward end 17 and completely fill the space between the opposed transducers which are mounted in the apertures 7.

A cut-out portion 27 is provided in the top and bottom of the transducer housing 3 so as to leave the passageways 25 open for the passage of electrolyte through the forward end 17 of the handle 1- The transducer assembly is shown in Figure 4. It can be seen that there are provided two identical transducer elements 50 and 51. One of the transducer elements is, in use, connected to receive electrical 5 signals so as to transmit sound waves therefrom. The other of the transducer elements is, in use, connected to receive those transmitted sound waves. Typical transducers are those known as MURATA, B.F.B., 455kHz intermediate frequency filter crystals. 0 These transducers comprise flat piezo-electric elements which fit neatly within the apertures 7. The piezo-electric elements are coated with a suitable corrosion inhibiting material which is inert relative to the electrolyte e.g., that manufactured by Monsanto and 5 known as modified BISPHENO resin. Suitable transducers could be manufactured from a sheet of piezo-electric material and by bonding electrodes onto the respective faces by a suitable electrically conductive epoxy resin. Thus, when a voltage is applied across the electrodes the 0 transducer can be made to vibrate in accordance with any electrical signals across the electrodes or alternatively, if the transducer is vibrated by sound waves then it will generate an electrical voltage across the electrodes, proportional to the displacement of the 5 transducer. The transducers are operated at the mechanical resonant frequency of the transducers in order to maximize the accoustic intensities and thereby enhance efficiency-of the assembly. In the present case this is 455kHz. In order to inhibit sound vibrations from 0 passing through the transducer housing 3 to the sound wave receiving transducer, each of the respective ^• transducers is sound insulated from the housing 3 by a P.T.F.E. (Teflon, Trade Mark) tape annulus 55 which extends around the transducer assembly. The transducers together with the sound insulation tape annulus 55 are force fitted in the respective apertures 7.

If desired a further aperture 7 (not shown) can be provided in the top or bottom wall of the transducer housing 3^{" "}and contain a temperature sensing transducer element. A further groove 9 (also not shown) can be provided in the transducer housing 3 similar to that as shown for the apertures 7, whereby to carry the leads for that temperature sensing .transducer. Similar passageway means 11 may be provided to that shown for carrying these leads through the bridging portion 15 and out through the opening 21 in the plug member 23 which closes the handle 1.

Referring now to Figure 5 there is shown an electrical block circuit diagram of electronic control apparatus connected with the transducers in the probe 3 shown in Figures 1 to 4. In Figure 5 the probe 3 has been designated by the letter T and comprises the sound transmitting transducer 50 and a sound receiving transducer 51 and the handle 1. A temperature sensing transducer 52 has also been shown. Each of the respective transducers 50, 51 and 52 have two leads extending therefrom. The sound transmitting transducer 50 is fed from a voltage controlled oscillator VCO, the output of which is passed through a suitable amplifier Al before passing to the sound transmitting transducer 50. The amplifier Al amplifies the output of the voltage controlled oscillator VCO at a suitable level to drive the transducer 50. The transducers 50 and 51, are adjusted in the respective apertures so that the spacing therebetween results in a zero, phase shift of the received signal for a calibrated specific gravity. It should be realized that the received signal may be one or several half cycles later. Further, because the transducers are operated at the mechanical resonant frequency, the mechanical output signal will be in phase with the driving signal. Thus should there be a change in the specific gravity then the received signal will arrive sooner or later and will be correspondingly out of phase with that of the transmitted signal. In order to detect the phase difference the output of the sound receiving transducer 51 is fed to an amplifier A2 which, in turn, supplies an amplified signal to a phase sensitive detector PSD. The phase sensitive detector PSD also receives a signal directly from the output of the voltage controlled oscillator VCO. Thus, the phase sensitive detector PSD has two signals at its inputs, one of which is a reference signal and the other of which is received from the output of the transducer 51 and is phase dependent.

The temperature sensor 52 is connected with a temperature sensing circuit T.S.C. which, in turn, applies a temperature signal to a meter or monitor M which is summed in the meter or monitor M with the output signal of the phase sensitive detector P.S.D. to give a temperature corrected indication at the meter or monitor. As the velocity of the propagation of sound as measured is compensated to be substantially independent of temperature within the range to be tolerated. Thus, as the specific gravity may be functionally related to the speed of propagation of sound in the liquid then the indication on the meter or monitor^' M can be calibrated to indicate the specific gravity of a particular liquid, in this case the electrolyte of lead acid batteries.

If desired the meter or monitoring device M may be a digital read out meter or other suitable meter to provide a display in a required format. It should be noted that the resin coating disclosed, herein, modified.BISPHENOL resin by Monsanto, results in desired operation of the transducers for a limited time. In this connection we have found that we need to replace the transducers every so often because the resin, when substantially hardened, results in a shifting of the mechanical resonant frequency of the transducers. Thus, we have used the transducers within a few days of being coated with the resin and we have found that they must then be replaced about one week later. Figures 6, 7 and 8 show detailed circuit diagrams of one particular embodiment which operates in accordance with the teachings of the block circuit diagram of Figure 5. - S

In Figure 6 there is shown the circuitry which is connected with the transducer 50. In this circuit, a square wave signal is applied across the transducer 50 and a series connected inductance LTl. The circuit comprising the inductance LTl and the transducer 50 are such that at mechanical resonance of the transducer 50, an electrical sine wave appears across the transducer 50 and this optimizes accoustic energy output at the resonant frequency. In the circuitry ICl is a dual voltage controlled oscillator type 74LS124. The circuitry is such that IC2, IC3 and IC4 together with transistors 2N3643 and 2N3644 ensure that the transducer 50 oscillates at mechanical resonance of 455kKz with maximized power output. As is known, at the centre frequency of a phase locked loop servo system employing an (exclusive OR) digital phase comparator, (IC2 pins 1, 2, 3), the reference phase (pin 2 IC2) will be 90 degrees out of phase with the input phase (in this case, the phase of the current through the transducer 50 onto pin 1 of IC2) . The function of the dual JK flip flops, IC3, and the inverter IC2 (pins 4, 5, 6), is to ensure:-

1. A desired 90 degree phase shift at the centre frequency between the two phase locked loop (PLL) signals;

2. A square wave drive wave form is applied from the output of transistors 2N3643 and 2N3644 to maximize the effective transducer 50 drive amplitude; and 3. To ensure the PLL always attempts to lock the transducer 50 at maximum power. (Without IC3 flip flop pins 1, 3, 14, 12 and 13 it will be possible to attempt to lock the PLL with 90 degree phase shift between the output of transistors 2N3643 and 2N3644 and its current, i.e., no net power input to the transducer 50. This would be a random turn on or disturbance occurrence depending upon the initial state of a simple dividing by 2 squaring flip flop. An output signal B is provided -from IC3 and is the PLL reference - - input to pin 2 of IC2 and is 90 degrees* out of phase with the amplified driving wave form to the transducer 50 from transistors 2N3643 and 2N3644. Signal B is also applied to the receiver circuit as will be described later. The^" phase comparator for the PLL comprises pins 1, 2 and 3 of IC2. The transmitter 50 current wave form is "infinitely clipped" to measure the phase information only at pin 1. This is compared, with the reference phase supplied to pin 2. Rl and Cl form the PLL filter and the values are adjusted to achieve satisfactory locking range and pull in range. RTl is an adjustable resistance which allows the PLL centre frequency to be set at the -resonance frequency of 455kHz. LTl is an inductor used to allow resonance of the transducer 50 to be made to compensate for its passive shunt capacitance. It is adjusted so that the transducer 50 resonates at its desired frequency - 455kHz. IC4 is a high speed comparator used to obtain phase information (the current zero crossings) of the currents supplied to the transducer 50. IC4 is type LM360N by National. R2 is a current sensing resistor. Its value is in the order of 33ohms, the exact value being determined so as not to lower the "Q" of the tuned circuit formed by LTl and transducer 50. The transistor pair comprising types 2N3643 and 2N3644 form a power amplifier to drive the transducer 50. R3 which is typically 680ohms allows IC3 pin 9 to reach a high positive output level. C2 in conjunction with Rl7 allows a capacitive coupling to ensure that the output wave form from transistors 2N3643 and 2N3644 is within the linear operating range of the complimentary emitter followers which constitute the. power amplifier.

Referring now to Figure 7 there is shown the phase sensitive detector and the meter or monitor M. In this circuit the receiving transducer 51 is connected in parallel with a tuning coil LT2. LT2 is adjustable to optimize the output from the transducer 51 at the resonant frequency. IC5 type LM310C is a voltage follower with a high input impedance and wide bandwidth. This integrated circuit isolates the transducer 51 from - to- the relatively low input impedance of comparator IC6 thereby maintaining a high "Q". IC6 type LM360N is a comparator, similar to IC4, which derives the phase information (zero crossings) of the received signal from the transducer 51. Resistor RT3 is used to adjust offsets within IC5 and IC6 to maintain a square wave output at pin 6 of IC6. IC2 type 74S86 is the phase comparator between the transmitted signal from the "B" •output of the transmitter circuit of Figure 6 and the received signal from the transducer 51 which is

"infinitely clipped" on pin 10 of IC2. The output on pin 8 of IC2 is smoothed to form an essentially D.C- signal functionally related to the propagating mediums velocity of sound between the transducers 50 and 51. If the accoustic separation of the transducers 50 and 51 is an integer multiple of the transmitted frequencies half wavelength, then the functional relationship will be ideally linear. R16 and C4 form a smoothing filter which is buffered by a voltage follower formed by IC8 type LM324 pins 5, 6 and 7. R16 is made relatively large in comparison with the output impedance of IC2, pin 8, for optimum accuracy. In circuit part M of Figure 7 the output signal from the phase sensitive detector is summed with a temperature correcting signal (from the circuit of Figure 8) and used to drive a meter m. Suitable zero adjusting circuitry is provided for the meter m. In this connection RT5 adjusts the D.C. offset to compensate for non exactly, integer half wave separation between the transducers 50 and 51 for the calibration..point zero of a chosen electrolyte. Adjustment of RT5 effectively allows "Zeroing" of the meter m of a chosen liquid at a reference temperature and a calibration specific gravity. IC8 pins 8, 9 and 10 buffers the "zero" set trim of RT5. R12 sets the gain, or degree of temperature dependence compensation received from the temperature sensing circuit of Figure 8. R12 is adjusted so that anticipated variations can be just covered with variation of RT4. R13 is adjusted to cover likely manufacturing tolerances of the transducers 50 51 and the s acin a tm - -

be within the range of RT5. R14 basically sets the "span" of the full scale range of meter m. IC8 pins 14, 13 and 12 provide in the simplest configuration, a metered output provision via meter m. The output at pin ^• 14 of IC8 is a linear function of the velocity of sound through the liquid. The meter m can be calibrated directly in specific gravity units or alternatively when temperature is to be measured instead of specific gravity then it may have a scale thereon calibrated in the required temperature degrees when the liquid has a known specific gravity or other known state.

Referring now to the circuitry shown in Figure 8 there is shown a temperature sensing circuit T.S.C. which applies a correcting signal to the monitor M. - In this circuit D.c-c. is a constant current diode (2-5mA) to provide the temperature sensor 52 which is a temperature dependent diode with constant current. DTS is the temperature sensing transducer 52. It is known that a silicon diodes forward voltage drop is very nearly linearly temperature related over a range of 0 degrees Celsius to 70 degrees Celsius at approximately -1 to -2mV /degrees Celsius. RT2, R4 and R6 form a variable D-C- voltage divider to set a potential at pin 3, IC7 equal to the potential at DTS at a reference temperature e.g. 25 degrees Celsius. IC7 (pins 2, 3, 4) buffer the set potential of RT2, R4 and R6 divider. IC7 pins 5, 6 and 7 buffer the temperature sensing transducer 52 potential. IC7 pins 8, 9 and 10 form a difference amplifier to increase the temperature dependence of the output at pin 8 of IC7 to approximately -lOmV/degrees Celsius. In this circuit the voltage at pin 8 of IC7 is adjusted to O V at a particular reference temperature e.g. 25 degrees Celsius by adjustment of RT2. R7, R7 ^» , R8 and R8 * are resistors which are chosen with the aim of ensuring IC7, pins 8, 9 and 10 form a sufficient accurate difference amplifier (generally R7=R7 ^■ + R8=R8 ' + or - 1 %) . IC7 pins 14, 13 and 12 inverts the polarity of the output at pin 8 of IC7 to provide an opposite sense temperature dependence, e.g. +10mV/degrees Celsius. RT4 allows - /£ -

adjustment and thereby compensation of the measured temperature dependence of the specific gravity measurement apparatus. Linear compensation is adjustable over positive and negative coefficients which is satisfactory for most applications. (Non-linear compensation may be accorded by making IC8, pins 1, 2, 3 some form of non-linear voltage transfer device) . IC8 pins 1, 2 and 3 buffer the adjustable temperature •compensation from RT4. In the embodiment described in Figures 1 to 3 it must be appreciated that it is important that the dimensioning and/or spacing apart of the electrodes be substantially constant independent of any expected temperature changes in the transducer housing 3 as otherwise any change in the spacing apart of the transducers would be detectable as a change in phase of the transmitted and received signals. This, in turn, would effect the readings of the specific gravity and/or temperature but it is minimized by appropriate adjustment of the temperature sensing control T.S.C.

The temperature measurement could also be performed by measuring the change in resonant frequency of the presumably matched or at least approximately so, transmitting and receiving transducers. If the shift in resonant frequency of the receiving means is used for the measurement of temperature- of the liquid the circuitry would necessarily be more complex than in the embodiment shown in Figure 4, but it is considered that the final circuit could be produced by a suitable electronics addressee.

The output of the phase sensitive detector P.S.D. to the monitor M may be used to control a charger for charging wet cell lead acid batteries where the probe T is inserted in the electrolyte of the battery and where the output to the meter or monitor M is fed^' to a gate of a MOSFET power transistor which is in series with the battery and which controls the charging rate of the battery. A suitable MOSFET power transistor is BUZ 10 by ^*73-

Siemens. Other forms of control devices may be substituted for the MOSFET transistor and be within the scope of the invention.

In a further modification the device may be used to control the discharging of a lead acid wet cell battery as the output of the phase sensitive detector P.S.D. to the monitor M may be fed to the gate of a MOSFET power transistor used to control the current taken from the battery. In this case the probe T would be inserted in the electrolyte of the battery such that when the specific gravity changes to an undesirable value then the output of the battery can be suitably reduced.

In a further modification of both the charging and/or discharging of wet cell lead acid batteries, a ^• temperature sensing means may be provided in accordance with the teachings of Australian Patent Application No. 41099.78 in the name of N.T. Bowman et al whereby to additionally control the charging and/or discharging of the wet cell lead acid battery in accordance with the temperature of the electrolyte therein as well as in accordance with the specific gravity. The subject matter of that application is imported herein.

In a further modification of the embodiment described herein, the probe may be positioned a "low" electrolyte level of a wet cell lead acid battery so that when the electrolyte reaches that level the transducers 50 and 51 will give an abnormal measure of the time taken for sound to travel between the. transducers. Appropriate electric comparator circuitry may then be included which can have a reference signal applied thereto representative of such time and when the measured time corresponds, it can provide an output which will either give an alarm or prevent current being supplied to or taken from the battery. In a further modification the scale of the meter or the display of the monitor M may be arranged to display a temperature state of a liquid with known specific gravity by appropriate calibration thereof. In a further modification the scale of the meter or the display of the monitor may be calibrated to give an indication of the energy remaining state of a wet cell lead acid battery by measuring the specific gravity thereof in the manner disclosed herein. It is well known that the energy remaining in a wet cell lead acid battery is related to the specific gravity of the electrolyte. Accordingly the meter or monitor M can be suitably calibrated in energy units. It should also be appreciated that instead of using a sound wave generating transducer and a sound wave receiving transducer, a single sound generating and receiving transducer could be used. In this embodiment the transducer would be operated to generate sound waves and then to receive the sound waves which propagate therefrom and return over a known distance. In this way a measure of the time taken for the sound wave to travel a known distance can be made. Such an embodiment is within the scope of the broad inventive concept herein. These and other modifications may be made to the invention as would be apparent to persons skilled in the art and all such modifications are deemed to be within the scope thereof the nature of which is to be determined from the foregoing description.

Claims (16)

1. Apparatus for measuring a state of a liquid which can be determined by the time taken for sound waves to travel through the liquid comprising transducer means for generating sound waves and for receiving such generated sound waves - after the sound waves have propagated a known distance within the liquid, said transducer means being connectable with electric circuit means which will cause a sound wave signal to be propagated from said transducer means within said liquid and will process a received signal by said transducer means to ascertain the state of said liquid by measuring the time taken of the sound wave signal to travel between the known distance.

2. Apparatus as claimed in Claim 1 wherein said transducer means comprises a sound wave generating transducer. nd a spaced sound wave receiving transducer, the positioning of the sound wave generating transducer and the sound wave receiving transducer being such that when inserted into a liquid the sound waves generated by the sound wave generating transducer can pass to said sound wave receiving transducer by passing through said liquid, electric lead means extending from each of said transducers for respective- connection to signal supplying means for causing said sound wave signal transmitting transducer _to emit sound waves, and to allow electrical signals generated in said sound wave receiving transducer representative of sound waves detected by said sound wave receiving transducer to be fed to detection circuitry to be processed to ascertain the state of said liquid by measuring the time taken for said sound waves to travel between the transducers.

3. Apparatus as claimed in Claim 2 including said signal supplying means and said detection circuitry.

4. Apparatus as claimed in Claim 3 wherein said signal supplying means contains driving circuits adjusted, in use, to cause the sound wave generating transducer to oscillate at its mechanical resonant frequency.

5. Apparatus as claimed in Claim 4 wherein the spacing of said sound wave generating transducer and said sound wave receiving transducer is an integer number of half wavelengths for a calibration state of said liquid.

6. Apparatus as claimed in any one Claims 3 to 5 including a temperature sensing transducer which, in use, measures the temperature of the liquid, temperature sensing circuitry connected with said temperature sensing transducer and said detection circuitry to provide temperature corrected signals of the time taϊcen for the signals to travel between the transducers to, in turn, provide temperature corrected state measurements.

7. Apparatus as claimed in any one of Claims 3 to 6 wherein said detection circuit includes phase comparator circuitry which in use, measures the phase difference of a transmitted signal to a received signal for ascertaining the time taken for said sound waves to travel between the transducers.

8. Apparatus as claimed in Claim 7 wherein said signal supplying means includes a square wave signal generating means, and an inductance connected in series with said sound wave generating transducer so that, in use, the output sound waves will be of a sinusoidal nature.

9. Apparatus as claimed in any one of Claims 3 to 8 wherein said detection circuit includes a πcr.itor calibrated to indicate a specific gravity state of said liquid.

10. Apparatus as claimed in any one of the Claims 3 to 9 including a battery charging circuit including a series connected power transistor, said battery charging circuit being controllable by said power transistor to control charging of a battery in accordance with the specific gravity thereof, said transducers being, in use, situated in the electrolyte thereof.

11. Apparatus as claimed in any one of Claims 3 to 9 including a battery current discharging circuit including a series connected power transistor, said apparatus being connectable- with a lead acid battery whereby to measure the specific gravity of the ■electrolyte thereof and wherein said current discharging circuit is controllable by said power transistor in • accordance with the specific gravity of said electrolyte whereby to control the current which can be drawn from said lead- acid battery.

12. Apparatus as claimed in any one of Claims 1, ^• 2 or 3 and connected in an elongate body whereby to provide a specific gravity measuring probe.

13. Apparatus as claimed in Claim 1 or Claim 2 fitted, in a lead acid storage battery so that said sound wave receiving transducer and said sound wave generating transducer are in the electrolyte and wherein said electric lead means extend from said battery.

14. Apparatus as claimed. in Claim 12 and positioned at a "low" battery electrolyte level position.

15. Apparatus as claimed in any one of Claims 3 to 5 wherein said detection circuit includes a monitor calibrated to measure a temperature state of said liquid where the specific gravity of said liquid is known.

16. Apparatus as claimed in any one of Claims 3 to 8 wherein said detection circuit includes a monitor calibrated to provide an indication of the energy remaining state of a lead acid cell battery.

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