GB2073435A

GB2073435A - Standby equipment

Info

Publication number: GB2073435A
Application number: GB8108962A
Authority: GB
Original assignee: Cardiac Recorders Ltd
Current assignee: Cardiac Recorders Ltd
Priority date: 1980-03-21
Filing date: 1981-03-23
Publication date: 1981-10-14
Also published as: DE3111116A1; IT8120650A1; IT1167723B; FR2478893A1; IT8120650A0

Abstract

The invention relates to battery operated standby devices and in particular to an electrical power source for such devices. The invention finds important applications with defibrillators (10) and other medical appliances where it is essential that the state of charge of the battery (40) is known in case the device is ever called into use in an emergency. The invention proposes the use of a current integrating device (48) for integrating the total current taken from the battery (40) so as to provide an indication of the usable charge that is left. The integrating device (48) may conveniently be a mercury microcoulometer. <IMAGE>

Description

SPECIFICATION Standby equipment The present invention relates to battery operated standby devices and in particular defibrillators and other medical appliances.

Standby devices, by their very nature, may stand unused for long periods. This leads to the difficulty that batteries used in such devices inevitably lose power, so that the eventual user does not know whether the stored power is adequate. A further difficulty is that unless the battery is replaced or recharged after every use, the user will not know whether it retains enough power for further use.

In may cases these problems are not of over-riding importance, but in the case of medical standby equipment, which if used at all will be used in an emergency and mustfunction immediately and reliably, such problems are very serious.

Long shelf life can be achieved by the use of reserve cells, but they are not suitable for equipment to be used in an emergency, owing to the time taken to activate such cells.

Secondary cells can be used, on permanent trickle charge, but this can be expensive and is not always feasible.

The problem is particularly acute in the case of equipment such as defibrillators which, when used, deliver a substantial quantity of electricity, so that the battery state after use must be known in order to determine whether the equipment can safely be reused.

Batteries with very long shelf lives, in particular lithium/organic electrolyte batteries, are now available, and in standby medical equipment it is often desirable that such batteries be used instead of rechargeable batteries, but such use is only possible if the charge state at any time is readily ascertainable.

It is accordingly a first object of the present invention to enable the charge state of a battery of a standby device to be readily ascertained.

in accordance with a first aspect of the present invention there is provided an electrical power source for a standby device comprising a primary battery and a device for providing an indication of the time integral of the current taken from said electrical power source in order to provide an indication of the usable charge remaining in said electrical power source.

Preferably, in addition to the time integral of the current taken from the electrical power source when the standby device is in use, the integrating device is further connected to integrate a current less than that taken by the standby device in order additionally to take into account loss of charge as a result of ageing of the electrical power source.

The means for integrating the current taken from the electrical power source may for example be a microcoulometer arranged to give a reading dependent upon the time integral of the current flowing therethrough. Alternatively, there may be used as an integrating device a counter connected to a clock energized to run at a rate proportional to the current being integrated, the rate being high when current is drawn by the standby device and the rate being lower when accounting for natural ageing of the power source.

A mercury microcoulometer consists of a glass capillary tube containing two columns of mercury separated by a small gap containing an electrolyte.

Passage of direct curent between the mercury columns transfers mercury across the gap thereby giving the impression of the gap moving along the tube at a rate proportional to the current flow. The position of the gap is therefore a function of the current4ime integral and is read from a scale on or beside the tube.

The current made to flow through the mirocoulometer may either be the current through the equipment or it may be drawn in parallel through a calibrating resistor, or through a regulating circuit containing, for example, a Zener diode. The current drain of the coulometer monitoring the charged state of the primary cell need only be very small.

It has been found that such a device is particularly suitable for monitoring the charge state of long life primary cells in defibrillators.

The invention will now be described further, by way of example, with reference to the accompanying drawing which is a circuit diagram, partly in block form, of a defibrillator having a battery cassette incorporating a charge state monitoring device.

In the accompanying drawing, the part constituting a defibrillator is contained within a first box generally designated 10. A battery cassette is generally designated 12 and is arranged within a second box. In practice, the battery cassette slots into the housing of the defibrilltor and the necessary connections are established automatically on inserting the battery cassette into the housing of the defibrillator.

The defibrillator has two elctrodes 14 and 16 to be applied to a patient in order to cause a substantial current pulse to be applied to the patient's heart. The pulse is obtained by rapidly discharging a charge stored in a capacitor 20 through a discharge circuit including the patient connected between the electrodes 14 and 16 and an inductor 18.

The capacitor 20 has each of its terminals connected to a respective one of two ganged switches 22 and 24 which connect the capacitor either to a charging device or to the discharge circuit. To charge the capacitor 20, power is derived from the battery cassette 12 through a closed ON/OFF switch 26 by a charge control circuit 28. When the capacitor 20 is to be charged a switch 30 is closed to apply a voltage to a DC to DC convertor 32 which steps up the voltage significantly and charges the capacitor 20. The DC to DC convertor may for example incorporate a chopper circuit or a series invertorto convert the DC voltage to an AC signal followed by a step up transformer and a rectifier. When a charge is to be applied to the patient, a fire button 34 actuated which is connected to a discharge conrol circuit 36.

The latter changes over the position of the ganged switches 22 and 24 to allow the capacitor 20 to discharge into the patient.

It is to be noted that the defibrillator is in itself a known device and is given only as an example of a load which is used infrequently, requires a substantial amount of current and whose inmmediate operation is very important. It is therefore necessary to ensure that the battery within the battery cassette 12 is always capable of supplying the required current should the defibrillator be called into use.

The battery cassette 12 incorporates a primary cell 40 the positive terminal of which is connected to the ON/OFF switch 26 and the negative terminal of which is connected by way of a resistor 42 of low value for example 0.06 ohms. to the common earth line 44 of the defibrillator. A series combination of a high value resistor 46 and a microcoulometer 48 is connected across the battery 40. The value of the resistor 46 may typically be 1,000 Mohms. The junction between the resistor 46 and the microcoulometer is connected by way of a resistor 50 and a switch 52 to the ground line 44. The resistor 50 may typically have a value of 1,000 ohms, and the switch 52 is ganged with the ON/OFF switch 26.

When no current is taken by the defibrillator both the switches 26 and 52 are open and a small current flows through the microcoulometer 48 by way of the large value resistor 46. When however current is taken by the defibrillator a current of several amps flows through the resistor 42 whereupon the voltage developed across the resistor causes a current to flow through the resistor 50 and the microcoulometer 48, the latter current being substantially greater than that flowing through the resistor 46. As a result, a small current proportional to the ageing rate of the battery 40 is integrated by the microcoulometer 48 whilst the defibrillator is not in use but a current porportional to the current taken by the defibrillator is integrated by the microcoulometer while the defibrillator is in use.Consequently, the sum of the two currents integrated provides a true representation of the loss of charge from the battery and from this it is possible to determine the quantity of charge remaining.

Though the invention has so far been described by reference to a microcoulometer, it will be appreciated that this is only one of several integrating devices capable of being used. For example, the integrator may be constituted as a counter connected to a source of pulses of which the frequency is proportional to the amount of charge lost by the power source be it as a result of load current or as a result of ageing.

While in the preferred embodiment ageing is automatically taken into consideration, it is alternatively possible to integrate only the current taken by the load and account for ageing by reference to charts. The charts may conveniently be expressed by curves placed adjacent the microcoulometerseale.

An advantage of a mercury microcoulometer is that on reversing the polarity the gap movement direction is reversed consequently the microcoulometer is reversable requiring only to be zeroed and reversed upon replacement of the primary battery.

Claims

1. An electrical power source for a standby device comprising a primary battery and a device for providing indication of the time integral of the currenttaken from said electrical power source in order to provide an indication of the usable charge remaining in said electrical power source.

2. An electrical power source as claimed in Claim 1, wherein the device for indicating the time integral of the current taken from said electrical power source is a mercury microcoulometer.

3. An electrical power source as claimed in Clainp 2, wherein the microcoulometer is connected by way of a first resistor across the power source, the said first resistor being dimensioned such that the current flowing through the microcoulometer is proportional to the loss of charge by the primary battery as a result of ageing and wherein a second resistor is connected in series between the primary battery and the standby device to conduct the operating current of the standby device, the voltage developed across the said second resistor being applied by way of a Third resistor to the microcoulometer, whereby the microcoulometer provides an indication representative of the total loss of charge by the primary battery.

4. An electrical power source as claimed in Claim 1, wherein the device for providing an indication of the time integral of the current taken from said electrical power source comprises a counter and means for applying to the counter a series of pulses the repetition frequency of which is proportional to the rate of loss of charge by the primary battery.

5. In a defibrillator comprising electrodes to be applied to a patient, a charge storage means for storing a charge to be applied to the patient by way of the said electrodes, charging means for charging the charge storage device and a discharge control means for controlling the discharge of the storage means through the patient, the improvement comprising an electrical power source connected to the charging means and including a primary battery and means for integrating the current applied by the primary battery to the charging means to provide an indication of the usable charge remaining in the primary battery.