BATTERY TEMPERATURE COMPENSATING DEVICE
FOR BATTERY RECHARGING SYSTEMS
BACKGROUND TO THE INVENTION
Battery manufacturers, in the past few years, have developed sealed, lead-acid maintenance-free batteries as a power source, or prime movers, for the tele-communications industry, telephone switching systemsand the like, and in the computer industry for uninterrupted power supplies; they have found their way into automotive battery applications, and also as a future electric source in electric-motored automobiles.
It is recommended, by such manufacturers of lead-acid maintenance-free batteries that the optimum operating temperature for the longest life of the battery be maintained at 25 C. The battery life is considerably shortened (and in some cases to less than half of the expected life), if this temperature is not maintained during the charging cycle, or trickling charge cycle, for the batteries.
It is known that the internal resistance of such batteries decreases as the battery temperature rises either as a result of extra current flowing through the battery, or because of the ambient (room) temperature increases. The decreased resistance through the battery allows higher current flows leading to the generation by the battery of more heat. If left unchecked, such condition results in thermal runaway.
These are major problems with batteries which are also used on power back-up systems, as aforesaid noted.
Conversely, overcharging or over-temperature rising will allow such sealed batteries to "gas", and under extreme conditions to explode or bulge or blow out the seal on the battery thus, effectively ending the battery's life. The loss of the integrity of the battery generally causes substantial ancillary damage to equipment and personnel.
ENG, in U.S.A. Patent No. 4,847,547 issued n 11 " .
-- 21213~8 July, 1989 discloses a battery charger with a Vbe temperature compensation circuit which is integral to the battery charger system itself. It compensates for ambient temperature variations, but not for battery temperature variations.
Similarly, WORTMAN, in U.S.A Patent No. 4,663,580 issued on 5 May, 1987 discloses a sealed lead-acid battery float charge and power supply provided with a pre-determined non-linear temperature co-efficient located within the power supply that provides a pre-determined nonlinear temperature co-efficient by using a linear temperature co-efficient element (a forward biased silicon diode) and a nonlinear temperature co-efficient element tthermistor) in combination.
COOPER et al., in U.S.A. Patent No. 4,667,143 issued on 19 May, 1987 discloses a battery charger having temperature compensated charge rates. The circuitry disclosed is one of a battery charger providing a control circuit for switching the type of regulation in response to the current, voltage and temperature signals, but the temperature signal is one of ambient temperature and not of battery temperature. It is only the battery temperature which affects the internal impedance of the battery during the recharging cycle.
It is an object of the invention therefore to adjust, in response to the battery temperature, the recharging voltage to a battery.
It is a further object of the invention to regulate the voltage on recharge to within 1~ throughout the recharging temperature range.
It is a further object of the invention to provide a battery-temperature-compensating device which is able to be retro-fitted to existing battery chargers, hence avoiding the need for expensive replacement of battery charging devices.
THE INVENTION
The invention achieves the foregoing by the use of a temperature-sensing element, preferably an integrated circuit, physically mounted on the battery to sense the temperature of the battery. The same is connected by appropriate electrical leads to a monitoring circuit which proportionately regulates, in response to the temperature of the battery, the voltage applied by a battery charger electrically connected to the battery, whereupon regulation follows along a linear graph, over an operating range of -40 C to +60 C. The preferred linear voltage compensation is between the temperatures ranging from 0C to 50C at voltages of -2.5 to -4.5 mV/ C/cell (60-108mV/ C/string); the slope of the linear compensation can be changed depending upon the type of batteries being recharged and the total voltage of the battery pack string.
It can be observed from the foregoing that a realistic summertime day-to-night temperature change of about 15 C will result in the equivalent of a 900-1620mV change in the reference voltage (sometimes called, herein, the "float voltage") that should be effectively applied (negatively) to the battery since it's temperature rises during recharging; and, as is disclosed herein, this temperature compensation is achieved even with existing battery charging units namely rectifiers (R) connected to existing battery recharging systems.
The invention achieves, in response to the actual ambient or current temperature of the battery, during its recharging cycle, a dynamic changing of the recharging voltage applied to the battery terminals by changing the recharging reference voltage or "float voltage", through the range of -1.5 volts to +1.5 volts, as a deviation voltage around the preferred or recommended recharging voltage, for recharging purposes, (the null point) at any given temperature, which generally is the actual recharge voltage, per cell; of the , battery, at the battery reference temperature, 25C.
The invention will now be described by way of example and with reference to the accompanying drawings in which:
Figures lA and lB are block and circuit diagrams of the preferred embodiment.
Figure 2 is a typical voltage vs. temperature graph of "compensating" or "float voltages" for a nominal 48V
battery string having 24 operating lead-acid cells.
Figure 3 is a circuit diagram, illustrating the outboard compensating system, according to the invention, interconnected between a battery bank (BB), for recharging a plurality of serially connected batteries (BAT), and a battery charger (R) for recharging the same.
Figure 4 is a top plan view, of the sensor housing and in phantom, showing the sensor (S) and connecting cable, in a side elevational view of the sensor adhesively attached to the side of a battery being in the battery bank string of figure 3.
Referring to figure 2 there is plotted, the preferred reference of said voltages (V) depicted, according to battery temperature, in centigrade, as against voltage, that is the voltage deviating around the meridian which for lead acid batteries of 24 cells is a nominal 48 volts. Where other voltage strings are used, or other different lead-acid batteries of different manufacturers are used, or when batteries of a different type are used, for instance, nickel cadmium battery, the slope of the line passing through the null point (NP) needs to be changed, and this is accomodated by the switches in switch (S1), in figure lB, switching the resistances (RN3) and (RN4) in or out of the circuit to be described with reference to figure lB.
Referring to figure 1, it is a composite block float diagram and detail circuit diagram.
The circuit in its present form was designed for use with voltage regulated rectifiers and 24 cell battery strings. The required co-efficient of compensation is about -72mV/C and is constant with varying temperature. The prototype has a measured co-efficient of 0.71.49mV/C . The compensator is spliced into the reference leads and takes its power from the battery and its output drives the reference input of the rectifier. The compensator will develop a voltage between input and output terminals, increasing proportionally with temperature.
Referring to figure 1, JP2 is a connector connected to a battery bank (BB), see figure 3, at one end, and through a fuse element to Block A, an oscillator and output buffer, for generating a square wave of the voltage on JP2. Block A connects to voltage multiplier and regulator, Block B, their combined output feed into summing amplifier Block D which is controlled, in part, by reference voltage generator (C) having a reference means, REF.1. From the summing amplifier and the reference voltage generator, some voltage is fed to a slope-selection circuit, Block F. Block F is designed to accommodate various types of batteries requiring different slopes for the purposes of charging, and is provided with various selections of switches, (S1) through (S4) in Block F. Slopes of -60, -72, -84, -96, or, -108 millivolts per centigrade degree can be obtained for the purposes of feedback, as will be described in detail hereafter, see figure lB. The output from the summing amplifier, Block D, is passed through a voltage amplifier and output buffer (E), for connection to reference leads (JP3), which are eventually connected to the charger (R,100). The differential in voltage at JP3 and JP2, following the chart of figure 2, maintains the voltage for charging, when a battery charger 100 is connected in accordance with figure 3 to the battery bank (BB), and to the circuit of figures lA and lB, and maintains the voltage across the terminals of the battery bank (BB), so as to follow along the linear slope of the graph of figure 2 as the battery temperature changes. The connections (JP3) connect directly to the charger (R,100), as seen in figure 3 and provide a feedback of a voltage so that the output from the charger (B/O+-) can change as it is applied to the positive and negative terminal of the battery bank (BB).
Referring to the oscillator and output buffer Block A, the negative terminal of a battery string is connected to plug (JP2) and fed by a fuse (F1), through resistor R4, into the collector of transistor Q3, which, with Q1, acts as switching elements of an astable multivibrator, also consisting of capacitors C1 and C2 interconnected between base and collectors of these transistors. The resistors Rl, R2, R4 and R5 are appropriately connected to either base or to collectors so as to establish the multivibrating circuit while the emitters of both transistors are interconnected in the normal fashion. The frequency of oscillation is determined largely by the RC constant of (Cl,R5), (C2,R1) and is about, preferably, 8 kHz with this configuration. Transistors Q2 and Q4 form a complementary push-pull buffer, driven by Q1 of the multivibrator aforesaid described, since the bases of Q2 and Q4 are interconnected, as are the emitters. The collector of Q4 is connected to the emitters of the multivibrator Q3 and to the collector resistor R4.
Thus, the -50 volt battery input, from connector JP2, is chopped into a squarewave between 0 volts and -50 volts with a frequency of about 8kHz.
Referring to the voltage multiplier and regulators, Block B, there are two voltage multipliers and both are driven by the oscillator and Q4-Q2 buffer of Block A.
The first voltage doubler is an inverting doubler consisting of capacitors C4, C6 and serially connected diodes D5 and D6. The capacitor C4 has its other lead connected to the output of the oscillator and buffer Block A, as noted. The output of this inverting doubler consists of +50 volts and is passed through resistor R3 to supply current to a reversed bias Zener diode D2, fixing the reference voltage output thereat, because of filter capacitor C7 which is parallel to the Zener, at +15 volts. This now acts as the positive part of the power supply for the operational amplifiers of figure 1.
Within Block B is a second voltage multiplier that in fact is a doubler and has its output voltage referenced at twice the battery voltage or approximately -100 volts DC. It consists of capacitor components C3 and C5 and series diodes D4 and Dl, wherein the capacitor C3 is connected between the junction of the two diodes to the oscillator and output buffer Block A and to the common lead of capacitor C4.
The cathode of the diode D4 is connected through series resistor R16 to the cathode of Zener diode D3 whose cathode is connected in series to Zener diode D2 and is in parallel to a filter capacitor C8 whereby a common reference voltage of -15 volts is developed along the anode side of Zener D3 to become the negative part of the power supply from the remainder of circuitry of figure 1. A doubling of voltage takes place at the cathode side of diode Dl so that the output voltage thereat is -100 volts. The 100 volts is used to supply power to the output amplifier stage Block E.
Block C is a reference voltage generator. It consists of a stable reference voltage element REF.l having an output of 2.5 volts to operational amplifier U3. The other input of the operational amplifier U3 is connected through a potentiometer R23 in series, at one end, to resistor R21 and ground; the other end of the potentiometer, through resistor R22 is connected to the - output of the operational amplifier U3. The potentiometer R23 is used to adjust the voltage of the output, at pin 6 of the operational amplifier U3 and to "pad out" any errors caused by component tolerances used. Preferably, the output voltage of the operational amplifier U3 is nominally at +2.98 volts.
With a probe temperature at 25 C which is connected to .
- 8 2 12133g point J1, the output of the operational amplifier U3, is adjusted by R23 so that there is no voltage difference between input and output terminals JP2 and JP3 of figure 1.
Referring to figure 3, a temperature sensing element, S, such as a LM335, being an integrated circuit, is mounted on the physical surface of a battery BAT, such that the sensing element S is not subjected to any air currents circulating about the battery BAT. The sensing element S has an output linearity proportional to temperature with a slope of 10 millivolts per degree Celsius, which for the lead-acid batteries at 25 C, the output is approximately 2.98 volts. The sensing element is driven from the negative power supply; -15 volts, of the voltage multiplier regulator Block B so that the output thereof is always negative with respect to the circuit ground of the compensator of figure 1. The output of the reference voltage generator Block C feeds into summing amplifier, Block D, which has an operational amplifier U2 and a bank, RN1 is provided for a plurality, 7 in number, of resistive inputs in order to provide various slopes, for various battery manufacturers' published specifications of slope. The summing amplifier has three weighted inputs, one of which is the 2.98 volt reference from U3, serially connected through input resistor R6 to provide a forward gain of 1.2 for U2.
The output from the temperature-sensing element, or temperature sensor (S), which is -2.98 volts at 25 C is connected to the outer input through resistor R7 with a gain of 1.2. The negative terminal of the battery is connected to the third input, from JP2 is connected to pin 8 of resistor network RN1 for a gain 1/6. A gain of 1.2 for the reference REF1 and the temperature sensor (S) will cause battery voltage to vary -72 millivolts per C and will be a normal rest position.
However, some batteries, because of their manufacture, may require different slopes. To allow for slope - 9 21213~8 variation reference offset to C (figure 2), a network of resistors comprising any or all of the elements RN3 and RN4, which is the slope selection circuit, Block F, can be connected in parallel with resistor R6 and also with resistor R7 respectively, using the various switches located in switch S1 so that various slopes in the following ranges can be achieved; -60, -72, -84, -96, and -108 millivolts per C. The feedback resistor for the operational amplifier U2 is in fact, parallel resistors 1 through 6 of RN1 which is connected to the output terminal 6 of the operational amplifier U2.
Because the resistance and also the temperature co-efficient of the resistive elements are close, a fairly accurate and stable gain for this operational amplifier U2 can be selected for battery strings of selected voltage ranges, i.e., 36 volts, 24 volts, or 12 volts by selecting the resistors of RN1 in or out of the circuit.
Referring to voltage amplifier and output buffer, Block E, the same consists of an operational amplifier U1, and transistor Q5, operating as a Class "A"
amplifier, has the base of Q5 connected through resistor R13 to the output of operational amplifier U1, providing temperature-compensated voltage to the reference terminals of the battery charger connected to JP3. This circuit has a gain of 6, which is set by using a single resistive network RN2. U2 is a low offset type and thus, the trimming of potentiometer R17, connected between terminals 1, 8 and 7 of the operational amplifier Ul, is not normally needed.
=~A
lo 2121338 EXAMPLE COMPONENT LIST (Figures lA, lB, 3, 4) Cl 3 nF Ql MPSA92 R14 750 n, C2 3 nF Q2 MJE350 R15 20K n C3 22 ~F Q3 MPSA92 R16 2.4K n, C4 10 ~F Q4 MJE340 R17 20K n C5 22 ~F Q5 MJE340 R20 1.87K n C6 10 ~F Rl 390K n R21 10K n C7 10 ~F R2 2.2K n R22 1.87K n C8 10 ~F R3 2.4K n R23 lK n C9 100~F* R4 10k n RNl 120K n C10 10~F* R5 390K n RN2 120K Q
Cll 1 nF R6 20K n RN3 100K n Dl lN4004 R7 20K n RN4 100k n D2 lN4744A R8 12K n Ul 0P07 D3 lN4744A Rll 120 n U2 0P07 D4 lN4004 R12 3.3K n U3 0P07 D5 lN4004 R13 2K n REFl MC1403 D6 lN4004 Sl Grayhill Switch Fl lA VAC
25 * (or not used)