GB2247366A - A two state constant current battery charging system - Google Patents

Abstract

An automatically operating battery charging system has a storage battery 18 connected in a feedback loop that includes two switched constant current sources I1 and I2. These produce a charging current and a trickle current sufficient to maintain the battery at a predetermined float voltage when charging is complete. The switching is controlled by the output of a comparator 11. During the initial charging stage the charging current is continuous. During the final charging stage an asymmetric time delay network 13 and comparator hysteresis cause a train of charging current pulses to be delivered to the battery. As charging progresses the frequency of the current pulses decreases. The pulse train terminates when the float voyage produces a comparator input V1 sufficient to maintain the system in the trickle current state. The system possesses all the advantages of voltage sensing between the pulses, and temperature compensation 12 may be reliably applied to the final float voltage. Progress of charging may be indicated by visual indicators L1, L2. <IMAGE>

Description

SPECIFICATION A Two State Constant Current Battery Charging System This invention relates to an automatically operating battery charging system for applications that require a battery in standby service to be recharged rapidly and efficiently after a discharge.

When the recharge time of a battery is critical then constant current charging is recommended. Also when a battery is required to be maintained in a float or standby regime then a constant current may be used. If this is applied continuously then it is known as a trickle current. For both vented or sealed cells of various types the charging is most efficient at about the 20 hour rate or faster, and this is generally at least one order of magnitude greater than the maximum permissible trickle current. To prevent overcharging and damage to the cells the use of a two state charger is therefore indicated.

The problem of reliably determining the end point of the charge has been tackled by various methods with partial success. Of these methods, pulse current charging with battery voltage sensing between the pulses has proved most effective, where a relatively simple charging system is required. In some systems the battery voltage is sensed in combination with other parameters such as temperature or time of fall to a pre-set value after the end of the current pulse.

The advantages of voltage sensing between the pulses are that internal resistance variations and battery interconnections do not influence the voltage discriminator and it is easier to compensate for variations with temperature.

Pulse current systems that operate automatically usually utilize a two level voltage discriminator otherwise known as a comparator with hysteresis. A reference voltage and the battery voltage are fed to the comparator the output of which determines the continuation or termination of the train of charging pulses. When the battery voltage reaches the upoer switching level the pulse train is terminated and trickle charging commences.

The battery voltage falls after the charging pulses cease and the width of the hysteresis band is required to be sufficient to prevent the circuit from oscillating between the charging and standby states. If, after discharge, the battery voltage has fallen to the lower switching level then the charging pulses recommence.

The current pulses are of fixed duration and the width of the hysteresis band is typically from 15t to 25t of the battery voltage. For example from 2.lv to 2.6v per cell for a lead-acid accumulator.

A number of devices have been developed that possess some or all of the features mentioned above, for example those disclosed in US Patent 3,659,181 and UK Patent 2 047 025.

However with any device that uses voltage sensing, at a level appreciably above the cell e.m.f., the effects of age and temperature may prevent the charge termination voltage from being attained. In this circumstance overcharging and damage to the battery may result. Systems have been developed that overcome this disadvantage by using various types of timer that are activated when the cell reaches the gassing voltage. To be effective in automatic standby use the circuitry needs to be relatively complex since the recent discharge history of the battery must be taken into account.

The system described herein is a pulse current charger that does not use complex timing or memory circuits. The current pulses are not of fixed duration. They are modified by the state of charge and the comparator hysteresis. Voltage sensing is used but the upper switching level is close to the final float voltage so that there is almost no possibility of overcharging.

The charger has been developed specifically for use with sealed lead-acid cylindrical cells but the method may be applied almost as effectively to sealed Ni/Cd sintered cells.

The charger comprises a two state constant current source or two separate constant current sources that may be switched alternately to connect with the battery on charge. The switching being achieved by electronic, optoelectronic, electromechanical or any other automatic means.

The aforementioned current sources deliver a charging current and a trickle current sufficient to maintain the battery at a predetermined float voltage. The switching is controlled by the output of a voltage comparator with hysteresis.

The signals input to the comparator are a fixed voltage to the reference input and a fraction of the battery voltage to the variable input. The width of the comparator hysteresis in volts is less than the change in the signal applied to the variable input when the current switches between the two states.

The comparator, current sources, battery and battery voltage sampling network are connected in a closed feedback loop. Also included in the feedback loop is an active or passive phase lag or time delay network. The time delay introduced is longer than the time taken for the battery voltage to change from the float to the charging regime after the current has switched from the trickle to charging state. Whether it be active or passive the phase lag or time delay network is asymmetric. The phase lag or time delay being significant for positive going voltage changes only. Therefore when the current source switches from trickle current to charging current the signal transmission around the feedback loop is delayed significantly. However the delay introduced is negligible as the battery voltage falls when the charging current is switched off.

The feedback loop is so arranged that when the signal applied to the comparator variable input exceeds the upper switching or threshold level then the charger will eventually switch to the float regime. Likewise with the variable input signal below the lower threshold level the current is switched to the charging state.

The final float voltage of the battery produces an input signal that exceeds the lower threshold level of the comparator.

General and particular embodiments of the invention are now described with reference to the accompanying drawings, in which: Figure 1 is a schematic block diagram showing the interconnection of the subsystems comprising the overall system.

Figure 2a is a time trace of the signal to the comparator variable input during the final stages of charging.

Figure 2b is a graph of the battery voltage sample for the same time interval.

Figure 3 is a circuit diagram of a simple implementation of the charging system in accordance with the invention.

Referring to Figure 1 with the battery 18 in a discharged condition the signal V1 input to the comparator 11 is below the lower threshold level.

The comparator output controls the two state switch 15 which is caused to connect in charging current source I1. The switch 15 may be configured to simultaneously connect in visual indicator L1, although this is not essential for the operation of the circuit. The battery voltage rises slowly, as is shown by section A of the graph of Figure 2b, and the comparator input V1 rises until it attains the upper threshold level U2 in Figure 2a.

When V1 just exceeds level U2 the comparator changes state and switch 15 is caused to disconnect I1 and connect in trickle current source I2.

Referring again to Figure 1, the battery voltage sampling network 14 has output signal VB. VB then falls rapidly and due to the asymmetric nature of the phase lag or time delay network 13 the comparator input V1 follows closely. When V1 is just less than the lower threshold level U1 in Figure 2a the comparator changes state and causes switch 15 to connect in charging current I1 and disconnect I2.

The sample battery voltage VB then rises rapidly but the phase lag or time delay network 13 causes V1 to rise more slowly than VB or at a later time than VB. The time delays produced by 13 and the other propagation delays or time constants in the feedback loop thus cause a pulse of charging current to be delivered to the battery.

V1 eventually rises to the upper threshold level U2 and the comparator changes state again. The cycle as described above is thus repeated causing a train of charging pulses. This is shown in section B of the graphs of Figure 2a and Figure 2b.

During the final stage of charging the system thus behaves in a manner similar to that of a relaxation oscillator. With each successive charging current pulse the mean battery voltage increases slightly and the battery voltage relaxes more slowly after the pulse. The frequency of the charging pulses therefore decreases as shown in Figure 2b. The pulse train terminates when the float voltage maintained by the trickle current T2 is sufficient to keep V7 at or above the lower threshold level U1. (As shown at points E,E1 in Figures 2a and 2b.) The duration of the charging pulses is not fixed but varies slightly with the state of charge of the battery.

It is principally governed by the phase lag or time delay in the feedback loop and the width of the hysteresis band.

Since the width of the hysteresis band is relatively narrow (usually about 1t of the cell e.m.f.) temperature variations will not prevent the upper threshold level being reached. However it is desirable to compensate for the variation in temperature of the float voltage. This is achieved by means of an active or passive temperature compensation network 12 as shown in Figure 1. In alternative embodiments of the invention the temperature compensation network may be applied at any position in the feedback loop between the battery 18 and the comparator 11. In another alternative embodiment temperature compensation may be applied to the reference voltage network 10 in Figure 1. In the latter case the temperature variation of the reference signal VR in Figure 1 is caused to be identical with the temperature variation of V1 when the battery has reached its final float voltage.

In accordance with the invention and referring to Figure 1 the time delay or phase lag network 13 may be connected at any point in the feedback loop.

Also in accordance with the invention the battery voltage sampling network 14 may be connected at any point between the battery 18 and the comparator 11 in Figure 1.

The subsystems 10, 11, 12, 13, 14 shown in Figure 1 may be active or passive or combined together in such a way that their functions are performed by other networks in accordance with the operation of the overall system substantially as described herein. The power supply for the system OV, VS as shown in Figure 1 is required to be smoothed direct-voltage such as may be obtained from a conventional mains transformer and rectifier unit with a smoothing capacitor. It is not required to be regulated by electronic or other means, provided that any supply voltage variation is not such as to impair the operation of the subsystem networks or the current sources Il and 12.

A visual indication of the progress of the charging process may be obtained by arranging for the switch 15 of Figure 1 to connect in simultaneously I2 and visual indicator L2. During the first charging stage Ll is active and L2 is inactive. During the final changing stage Ll and L2 are active alternately. As the charging progresses and the time interval between charging pulses increases so L2 is active for longer periods, until eventually L2 is active continuously. This indicates that the battery is being maintained at or above the predetermined float voltage by the trickle current.

In a different embodiment of the invention means may be included in the system to determine when the time interval between the charging pulses exceeds a certain pre-defined value and to signal the completion of charging at this point. The signalling may be by visual, audible, electronic or any other means.

A simple implementation of the charging system in accordance with the invention is now described with reference to Figure 3. The circuit configurations are of standard types and their operation will not be described in detail.

Mains transformer T, bridge rectifier G and smoothing capacitor Cl produce a suitable direct voltage OV, VS.

Zener diodes ZDl, ZD2, ZD3 and resistors R1 and R2 produce a temperature independent reference voltage VR.

The amplifier Al with open collector output constitutes the voltage comparator section. R3 and R4 provide regenerative feedback and govern the width of the hysteresis band.

Resistors R5 to R10, transistors Trl, Tr2 and Tr3 and light emitting diodes Ll and L2 comprise the two state constant current source. In this case L2 is part of an opto-isolator OP. With the output transistor of Al conducting Trl conducts and delivers charging current I1 to the battery 18 via diode D1.

When Al changes state Tr2 and Tr3 conduct, with Trl cut off, trickle current I2 is then delivered to the battery via D1.

Ril, R12 and RV1 form the battery voltage sampling network. Variable resistor RV1 enables VB to be adjusted so that the comparator switches at the correct float voltage of the battery.

R13, Rl4, R15, D2 and C2 form a simple- phase lag network the output of which is connected to A2.

A2 is configured as a non-inverting amplifier with a gain of approximately unity. This provides temperature compensation. The feedback components, resistors R16 and R17 and thermistor RT, can be selected to produce a temperature dependent gain that almost exactly cancels out the temperature variation of VB. Thus the comparator variable input Vl is independent of temperature.

The section of the circuit above the line Xy constitutes a simple means for signalling when the time interval between charging pulses exceeds a specified period. The block M represents a negative-edge triggered retriggerable monostable. C3, R18, D3 and D4 form a differentiating network to produce a negative-going voltage pulse when transistor Tr3 switches on. This occurs at the end of each charging pulse. The output Q goes high thus keeping NOR-gate output S low and therefore Tr4 and indicator L21 off.

If the interval between pulses exceeds the monostable period both inputs to the NOR-gate become low and S goes high thus causing Tr4 to activate L21.

In this circumstance L21 begins to flash on with each charging pulse, eventually remaining on continually. In a more compact implementation this section of the circuit may be omitted and OP replaced by a light emitting diode.

Indication that the charge is complete may be given by any means of monitoring the state of the current sources and a particular means is not essential for the operation of the system in accordance with the invention.

In Figure 3 the charging current is set by R8 and the trickle current by R9. The latter must not exceed the maximum continuous overcharge current.

For cylindrical lead-acid cells this is usually stated to be about C/400.

For Ni/Cd cells with higher self discharge rates the trickle current may be up to about C/100.

When the ambient temperature is very low it may be necessary to prevent the system switching to the charging state. This may be achieved by a variety of means. By way of example applied to the circuit of Figure 3, resistor Rl may be connected as the collector load of a transistor similar to the output stage of Al. The transistor can be caused to turn on at a critical temperature thus shorting out ZDl. This drives the system into the trickle charge state but prevents the indicator L21 from operating.

Claims (6)

1An automatically operating battery charging system comprising a two state constant current source or two separate constant current sources that may be switched alternately to connect with the battery on charge, the switching being achieved by electronic, optoelectronic, electromechanical or any other automatic means: said current sources producing a charging current and a trickle current sufficient to maintain the battery at a predetermined float voltage at the end of the charging process: the switching of said current sources being controlled by the output of a voltage comparator with hysteresis: the signals input to said comparator being a fixed voltage to the reference input and a fraction of the battery voltage to the variable input, said battery voltage fraction being the output of a battery voltage sampling network:: the width of the comparator hysteresis in volts being less than the change in the signal applied to the comparator variable input when the current source is caused to switch in either direction between the two states and the charging process is not complete: said comparator, current sources, battery and battery voltage sampling network being connected in a closed feedback loop:: also included in said feedback loop being an active or passive phase lag or time delay network, said delay network being characterized in that; a) the time delay or time constant introduced is longer than the time taken for the battery voltage to change from the float to the charging regime after the current has switched to the charging state, b) the response of the phase lag or time delay network is asymmetric so that the phase lag or time delay produced is significant when the current source switches from trickle current to charging current and negligible when the charging current is switched off: the predetermined float voltage of the battery being such as to produce an input to said comparator that, by the action of said feedback loop, maintains said current switch in the trickle current state.

2 A battery charging system according to claim 1 wherein said feedback loop is so arranged that when the signal applied to the comparator variable input exceeds the upper threshold level then the system eventually switches to the float regime and when said signal is below the lower threshold level the current is switched to the charging state.

3 A battery charging system according to claim 1 or claim 2 wherein means are provided to compensate for the variation with temperature of the predetermined float voltage.

4 A battery charging system according to claim 3 wherein said temperature compensation means comprises either i) an active or passive network inserted in the feedback loop so as to modify the signal to the comparator variable input or ii) an active or passive network applied to or incorporated in the reference voltage network.

5 A battery charging system according to any one of the preceding claims wherein means are provided to determine the time interval between the charging current pulses and to signal the end of the charging process when said time interval exceeds a pre-defined value.

6 A battery charging system substantially as described herein with reference to Figures 1-3 of the accompanying drawings.