Abstract:
An apparatus and method for detecting the presence of a battery in a battery charging circuit is provided. An inductor is placed in series with a charging circuit and the battery connection leads. A switching device and a capacitive device are connected in parallel to one another and in series with a gated device to form an indicator circuit. The indicator circuit is connected in parallel with the battery connection terminals. A series of pulses is applied to the gated device, allowing current to flow into the capacitive device during each pulse and energize the switching device when a battery is present. When no battery is present, the inductor prevents sufficient current from the charging circuit from energizing the capacitive device and the switching device.

Description:
FIELD OF THE INVENTION 
     The present invention is related to battery chargers. More specifically, the present invention is related to a battery charger that is capable of detecting the presence or absence of a battery connected across the battery charging terminals. 
     BACKGROUND OF THE INVENTION 
     Diesel firepump controllers and other systems that contain battery charging circuits often require a means to determine if a battery is connected to the charging circuit. Prior art systems typically monitor the output current of the charging circuit to verify the presence of a battery. Unfortunately, simply monitoring the output current of the charging circuit is not sufficient under all circumstances to verify connection of the battery across the charging terminals. 
     By way of example, additional circuit loading on the battery charger can mask the loss of low levels of float charge current, and prevent the detection of a disconnected battery. Also, external charging sources, such as engine-driven alternators, can temporarily raise the battery terminal voltage above float charge levels which will terminate the charging circuit&#39;s float charge current. Simple current monitoring schemes will falsely report battery disconnection in this case. 
     An example of a prior art battery detector is disclosed in U.S. Pat. No. 5,821,730 to Drapkin. The battery detection circuit uses a sensing resistor, in combination with a transformer, which has a dual secondary winding to sense current to the battery. When no load is present, there is little or no current through the sensing resistor, and a diode connected to the secondary winding is reverse biased. When a battery is present, current through the sensing resistor causes the diode to be forward biased, which in turn causes a step up in voltage at a node which turns on a transistor, indicating that a battery is present. Battery detecting techniques such as the battery detector disclosed in this patent fail to detect a false load which allows current to flow through the sensing resistor, resulting in a false battery detection. 
     Another example of a prior art battery detector is disclosed in U.S. Pat. No. 5,825,100 to Kim. This battery detector is an example of an electro-mechanical battery detector. When a battery is present, conduction surfaces are caused to contact connection points. The closed circuit between the connection points indicates that a battery is present. Unfortunately, this battery detection circuit will not detect whether the object inserted is actually a battery or not. Any physical object which causes the conduction surfaces to contact the connection points will be detected as a battery. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an object of the present invention to correctly detect the connection or disconnection of a battery from a charging circuit. A battery detector according to an embodiment of the present invention accurately detects the presence of a battery, even when external charging sources such as an engine alternator temporarily stop the charging circuit&#39;s current. A battery detector according to an embodiment of the present invention also accurately detects a disconnected battery, even in the presence of an unexpected circuit load that may draw current from the charging circuit. 
     These and other objects of the present invention are achieved by connecting a relatively low value inductor in series between the battery and the charging circuit. The series inductor provides a low resistance path for battery charging current when the battery is present. A relay and a capacitor are connected in parallel to each other and in series with a switching transistor. The relay, capacitor, and transistor combination is connected in parallel across the battery connector terminals. The transistor is activated periodically by a series of pulses. When a battery is present, current from the battery flows through the low resistance path to the capacitor, and energizes the capacitor and the relay when the transistor is turned on (during each pulse). If a battery is not present, the series inductor provides a high impedance path from the charging circuit to the capacitor and relay, preventing current from charging the capacitor, and in turn causing the relay to de-energize. A device according to the present invention works correctly, even in the presence of additional loading on the charging circuit or elevated battery voltage. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other objects, advantages and novel features of the invention will be more readily appreciated from the following detailed description and in conjunction with the accompanying drawings in which: 
     FIG. 1 is a circuit diagram illustrating an embodiment of the present invention; 
     FIG. 2 is a circuit diagram illustrating an embodiment of the present invention showing a battery present; 
     FIG. 3 is a timing diagram illustrating an embodiment of the present invention when a battery is present; 
     FIG. 4 is a circuit diagram illustrating an embodiment of the present invention showing a battery absent; 
     FIG. 5 is a timing diagram illustrating an embodiment of the present invention when a battery is absent; 
     FIG. 6 is a schematic diagram of a 12 volt or 24 volt negative ground battery detector circuit showing component connections and values; and 
     FIG. 7 is a schematic diagram of a dual 12 volt negative or positive ground battery detector circuit showing component connections and values. 
     Throughout the drawing figures, the same reference numerals will be understood to refer to the same parts and components. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A circuit  100  according to an embodiment of the present invention is illustrated in FIG. 1. A charging circuit  102  is connected in series to an inductor  104  and a set of battery terminals  106  for connecting a battery  108 . A current limiting resistor  110  can be included in series with the battery  108 . An indicator circuit  111  is connected in parallel across the battery terminals  106 . The indicator circuit is comprised of a relay  112  connected in parallel to a capacitor  114 . Indicator circuit  111  is further comprised of relay  112  and capacitor  114  both connected in series to a switching transistor  116 . Those of ordinary skill in the art will recognize that any suitable switching device can be used in place of the NPN transistor shown in FIG.  1 . The NPN transistor is shown merely for illustrative purposes. For instance, an FET can be used, or a simple mechanical switch can also be used. Series inductor  104  is placed in between the charging circuit  102  and the indicator circuit  111 . 
     FIG. 2 illustrates the operation of the circuit  100  when a battery  108  is connected across the battery terminals  106 . A series of pulses are delivered to the base terminal of the transistor  116  (not shown), causing the transistor to conduct. The pulses may be generated and applied to the transistor in any conventional method familiar to those of skill in the art. During each pulse, the transistor conducts, and allows current from the battery  108  to flow into the capacitor  114 , generating a voltage (V batt ) which energizes the relay  112 . While a battery  108  is connected across the battery terminals  106 , the inductor  104  provides a low impedance DC path for current from the charging circuit  102  to flow into the battery  108  and capacitor  114 . Thus, introduction of the inductor  104  between the charging circuit  102  and the battery  108  does not limit the charge provided to the battery  108  by the charging circuit  102  under normal operating conditions. 
     FIG. 3 is a timing diagram further illustrating the operation of the circuit  100  as illustrated in FIG. 2. V b  represents pulses which are applied to the base terminal of the transistor  116  (not shown). V cap  represents the voltage across the capacitor  114 . As shown, the value of C is chosen such that V cap  remains high in between pulses. This ensures that the relay  112  remains energized continuously while a battery  108  is connected to the battery terminals  106 . Thus, when the relay  112  is in an energized state, the circuit indicates that a battery is connected to the charging circuit. 
     FIG. 4 illustrates the operation of the circuit  100  when a battery  108  is not connected across the battery terminals  106 . Pulses continue to be applied to the base terminal of the transistor  116  (not shown), causing the transistor  116  to conduct during each pulse. Each time the transistor begins to conduct, a voltage is induced across inductor  104 , which is approximately equal to the voltage across the charging circuit  102 . A value of inductance of the inductor  104  is selected such that very little current flows into the capacitor  114  from the charging circuit  102  during the short pulses applied to the transistor. Between pulses, the transistor does not conduct, and current does not flow into capacitor  114 . 
     FIG. 5 is a timing diagram further illustrating the operation of the circuit  100  as shown in FIG. 4. V b  illustrates the pulses applied to the base terminal of the transistor  116  (not shown). Each time a pulse is applied to the transistor  116 , the transistor  116  begins to conduct, inducing a voltage across the series inductor  104 . Due to the inductor, current is not able to flow immediately into the capacitor  114 . The value of the inductor and capacitor are chosen such that when the battery is not present, the voltage across the capacitor never reaches the value required to energize the relay  112 . The voltage induced across the inductor (V L ) during the pulses is roughly equivalent to the voltage across the charging circuit. The voltage across the capacitor (V cap ) may increase slightly during the pulses, but not enough to energize the relay  112 . Thus, when the relay  112  is not energized, the circuit  100  indicates that a battery is not connected to the charging circuit. 
     FIG. 6 shows an embodiment of the invention, including some of the components and connections that can be used in a circuit for charging 12V or 24V batteries. As shown, the circuit  100  includes an IC  118  which is connected to a network  120  of resistors, capacitors and diodes to generate pulses that are delivered to an FET  122 . A 7.5V Zener diode  124 , a capcitor  126 , and a current limiting resistor  128  are used to deliver 7.5V supply voltage to pin # 1  of the IC  118 . Pin # 8  of the IC  118  is connected to ground. The value of current limiting resistor  128  can be 750 ohms for a 12V battery charging circuit, or 2.7 k ohms for a 24V circuit. The IC  118  is a standard 4049 IC containing six buffer/invertors. As will be understood by those of skill in the art, the connection of the IC  118  to the network  120  provides an oscillator which delivers pulses to FET  122 . By way of example, resistor  130  and  132  can have a value of 47 k ohms each. Resistors  134  and  136  can have a value of 20 k ohms in a 12V circuit, and 10 k ohms in a 24V circuit. Capacitor  138  can have a value of 0.01 Farads, while capacitor  140  can have a value of 0.0012 Farads. 
     The embodiment of the present invention illustrated in FIG. 6 is further provided with a current limiting resistor  142  and LED  144  which combine to visually indicate when a battery is detected. The embodiment is further provided with an energy dissipating resistor  146  and diode  148  to dissipate energy stored in inductor  104  in the event that battery  108  is suddenly disconnected. Resistor  150  and capacitor  152  form a snubber circuit that protects the FET  122  as will be understood by those of skill in the art. A normally open contact  154  and a normally closed contact  156  can be provided as output from the battery detection circuit. 
     FIG. 7 illustrates a dual battery charging circuit according to the present invention that is adapted to charge either positive or negative terminal ground batteries. The embodiment of FIG. 7 is essentially two circuits as described in FIG.  6 . FIG. 7 illustrates how the buffer/intertors  158  of IC  118  are connected to the network  120  of resistors, capacitors and diodes. The embodiment of FIG. 7 also includes several jumpers  160  which provide a means for reversing the polarity of various circuit elements to accommodate positive or negative ground batteries, as will be understood by those of skill in the art. 
     It will also be understood by one of skill in the art that relay  112  and capacitor  114  can be replaced by a battery detecting resistor (not shown). In such an embodiment, when a battery is connected across the battery connection terminals  106 , current flows from the battery  108  through the battery detecting resistor when the transistor  116  conducts, resulting in a potential drop across the battery detecting resistor. Conversely, when a battery is not connected across the battery connection terminals  106 , current is prevented from flowing through the battery detecting resistor when transistor  116  conducts during the short pulses delivered to the transistor base terminal due to the inductor  104 . Thus, no potential drop would occur across the battery detecting resistor when a battery was not present. In this embodiment, the presence or absence of a battery  108  connected across battery connection terminals  106  is detected by sensing the potential drop across the battery detecting resistor. 
     Although only a few exemplary embodiments of this invention have been described in detail above, those skilled in the art will readily appreciate that many modifications are possible in the exemplary embodiments without materially departing from the novel teaching and advantages of this invention. Accordingly, all such modifications are intended to be included in the scope of this invention as defined in the following claims.