Patent Application
20060244460
Battery-powered systems are often operationally constrained by battery characteristics. In this regard, the total battery capacity that may be available to a system is directly related to the rate of discharge of electrons from the battery, i.e., the current that is drawn from the battery by a corresponding load. Furthermore, when the battery exhibits a lower discharge rate, the battery retains more useable capacity. This is especially true when the battery is nearly discharged.
A direct current-direct current (DC-DC) converter refers to a device that is employed to change an input voltage, such as a voltage provided from a battery, to a different output voltage. Such DC-DC converters may be used to step-up, step-down, or invert an output voltage with respect to the input voltage. DC-DC converters are often used to manage the voltage supplied to a load in battery-powered systems. The DC-DC converter typically provides set output voltages to various system loads, and delivers a set output voltage from a varying input voltage.
While many loads, e.g., electronic components, require a tightly regulated input voltage to function properly, other loads, e.g., motors, are exceptions.
Depending on its load characteristics, a motor may function acceptably when supplied a voltage of 75% of the nominal value, even though a typical specification voltage tolerance for a motor may be ±10%. Therefore, in many cases the input voltage for a motor may be allowed to decrease during operation without adversely affecting the system's performance, i.e., the motor will continue to function properly.
Oftentimes, a DC-DC converter interfaces a power source, e.g., a battery, with a load, e.g., a motor. In this regard, a typical DC-DC converter supplies a constant output voltage to the load as long as the load current is less than a predetermined value. However, if the load attempts to draw more current than the limit value, standard design practices provide protection to the circuit;
the DC-DC converter either shuts down the converter or allows the output voltage to droop by maintaining the load current at a predetermined value.
Generally, the present disclosure provides a system and method for battery management.
A battery management system for managing current supplied by a battery to a load in accordance with an embodiment of the present disclosure comprises a circuit that detects an input current and drives the load at a substantially constant voltage if the detected input current reaches a predetermined current threshold. In addition, the circuit limits the input current to the predetermined current threshold, thereby allowing the output voltage to decrease when the input current is being limited to the threshold by the circuit.
A battery management method for managing current supplied by a battery to a load in accordance with an embodiment of the present disclosure comprises the steps of detecting a current and driving, based on the current, the load with a constant voltage if the current is below a predetermined current threshold. In addition, the method comprises the step of reducing the voltage when the input current reaches the predetermined current threshold such that the current is limited to the predetermined current threshold.
The invention can be better understood with reference to the following drawings. The elements of the drawings are not necessarily to scale relative to each other, emphasis instead being placed upon clearly illustrating the principles of the disclosure. Furthermore, like reference numerals designate corresponding parts throughout the several views.
Embodiments of the present disclosure generally pertain to systems and methods for battery management. In particular, a system in accordance with one embodiment of the present disclosure comprises a battery and a DC-DC converter that provides a constant output voltage to a load. However, if the load to which the constant voltage is being provided attempts to draw an input current to the DC-DC converter from the battery that exceeds a predetermined current threshold value, the battery management system allows the output voltage driving the load to decrease as needed to limit the input current to the predetermined current threshold value.
In this regard, if the input current to the DC-DC converter remains below a predetermined current threshold value, the battery management system provides a substantially constant output voltage to the load. However, if the input current attempts to exceed the predetermined current threshold value, i.e., the load is attempting to draw a current from the battery that exceeds the predetermined current threshold value, the battery management system of the present disclosure limits the amount of input current that is drawn from the battery and allows the output voltage provided to the load to decrease. As described hereinabove, for particular loads that have liberal input voltage tolerances, e.g., motors, such decrease is acceptable.
Thus, decreasing the output voltage to the load when the battery is significantly discharged, decreases the input current to the DC-DC converter.
Such allowable decrease in the output voltage, therefore, tends to increase the life of the battery that is supplying voltage to the DC-DC converter. This is especially so when the battery is significantly discharged.
The battery management system 10 comprises a battery 18 and a current limiting regulator circuit 14. The regulator circuit 14 connects the battery 18 to the load 12. The battery 18 applies a voltage Vin to the regulator circuit 14, and the regulator circuit 14 provides voltage Vout to the load 12. The regulator circuit 14 ensures that the output voltage Vout is substantially constant during operation, except as otherwise indicated herein.
Furthermore, the regulator circuit 14 senses a current induced in the circuit 14 by the input voltage Vin. If the input current sensed by the regulator circuit 14 is above a predetermined current threshold value based on the battery characteristics and the specifications of the load 12, then the regulator circuit 14 limits the current that is drawn at the input of the regulator circuit 14. Determination of a predetermined current threshold value is described further herein.
Thus, despite the current that is demanded by the load 12, the current that is actually drawn from the battery 18 is limited by the regulator circuit 14. As a result, the voltage Vout that is provided to the load 12 by the regulator circuit 14 when the current is limited may decrease. However, as described herein, there are some loads, such as motors, for example, that have liberal input voltage tolerances. For such loads, a decrease in input voltage in order to increase battery life is acceptable.
As described herein, the regulator circuit 14 operates based upon a predetermined current threshold value. In this regard, if the input current is below the predetermined current threshold value, then the regulator circuit 14 behaves as a constant voltage source. If the input current attempts to exceed the predetermined current threshold value, then the regulator circuit 14 lowers the output voltage so that the input current is limited to the predetermined current threshold value. Therefore, in one embodiment, the circuit 14 is preferably designed around a predetermined current threshold value that is determined based upon the system amperage requirements and the particular load 12 amperage requirements.
For example, the device may be a digital camera that maintains a peak discharge rate at or below 1.5 amps for a particular battery 18, e.g., a lithium ion cell battery. Thus, if the camera requires 0.5 amps without considering the load 12, then the current that the regulator circuit 14 might allow to the load 12, i.e., the predetermined current threshold value, is 1.0 amps, i.e., the total peak discharge rate minus the total amperage required to run the camera.
The DC-DC converter circuit 22 accepts the input voltage Vin from the battery 18. The DC-DC converter circuit 22 translates the input voltage Vin into a DC output voltage Vout. The DC-DC converter circuit 22 may provide a higher output voltage Vout than the input voltage Vin, provide a lower output voltage Vout than the input voltage Vin, or provide an inverted output voltage Vout with respect to the input voltage Vin. In this regard, the circuit 22 may be a “boost converter,” a “buck converter,” or an “inverting converter,” respectively.
The DC-DC converter circuit 22 may use an energy-storage element, such as an inductor, a transformer, or a capacitor, to transfer energy from the battery 18 to the load 12 in discrete packets. Feedback circuitry employed within the circuit 22 may regulate the energy transfer to maintain a constant output voltage Vout that falls within the load limits of the load 12. A more detailed exemplary DC-DC circuit configuration having feedback circuitry is described in more detail with reference to
The current sensing circuit 20 of
As shown by
During operation, the switch 42 is opened and closed periodically. In this exemplary embodiment of the DC-DC converter circuit 22, the frequency, i.e., number of times per second, that the switch 42 is actuated is constant, and the on-time of switch 42 is modulated. The capacitor 63 exhibits a substantially constant voltage value with a small-amplitude ripple voltage caused by the switching action. When switch 42 is closed, the input voltage Vin is impressed across the inductor 44, and the diode 46 prevents the capacitor 63 from discharging to ground. Therefore, current ramps up in the inductor 44. During the period when the switch 42 is closed, the capacitor 63 supplies the load current, so the voltage across capacitor 63 drops slightly.
When the switch 42 opens again, the voltage across the inductor 44 changes such that the diode 46 is biased forward so that inductor 44 continues providing current flow and supply the load current, recharging capacitor 63 and slightly raising the voltage across capacitor 63. Additionally, the feedback circuit 40, comparator 33, ramp generator 64, and the operational amplifier 35 work in conjunction to control the output voltage Vout by modulating the time switch 42 is on during the switching period, thereby keeping the output voltage Vout at a substantially constant voltage. In this regard, the output voltage Vout is regulated.
The operational amplifier 35 comprises a non-inverting input (+) and an inverting input (−). During operation, the operational amplifier 35 operates to ensure that the voltages at both inputs, the inverting and the non-inverting, remain substantially at the same voltage. For example, if Vref produced by voltage source 62 is 1.65 Volts, then the amplifier 35 operates to ensure that the voltage at the non-inverting input (−) is 1.65 Volts, and in such an example, the feedback voltage Vfb generated by feedback divider 50 remains at 1.65 volts. Ramp generator 64 supplies a saw-tooth waveform to comparator 33, which then converts the error voltage generated by operational amplifier 35 into a duty cycle suitable for controlling switch 42. In this regard, the control circuit 41 regulates the output voltage Vout based upon the feedback voltage Vfb.
Notably, the DC-DC converter circuit 22 is an exemplary implementation known in the art. Other circuitry implementations of the DC-DC converter circuit 22 now known or future-developed are possible in other embodiments. Furthermore, as described herein, the DC-DC converter circuit 22 may be implemented in such a manner as to increase the output voltage Vout, decrease the output voltage Vout, or invert the output voltage Vout with respect to the input voltage Vin. The exemplary DC-DC converter circuit 22 increases the input voltage Vin and regulates the output voltage Vout to a substantially constant output voltage Vout.
The current sensing circuit 20 is electrically connected to the input voltage Vin and the DC-DC converter circuit 22. Generally, the current sensing circuit 20 detects the input current of the regulator circuit 14 from the battery 18. If the input current remains below a predetermined current threshold value, then the DC-DC converter circuit 22 boosts the input voltage Vin, converts the input voltage Vin into a substantially constant output voltage Vout, and provides such substantially constant output voltage Vout to the load 12, as described hereinabove. As noted herein, the load 12 may be a motor, for example. In this regard, the control circuit 41 regulates the output voltage Vout based upon the feedback voltage Vfb generated by feedback divider 50.
However, if the current exceeds the predetermined current threshold value, then the current sensing circuit 20 provides a control signal to the control circuit 41, and the control circuit 41 regulates the output voltage Vout based upon the control signal provided by the current sensing circuit 20 as opposed to the regulator circuit's internal feedback voltage Vfb.
In this regard, the current sensing circuit 20 comprises a current-controlled voltage amplifier 30, a resistance/capacitance filter (R/C filter) 31, a voltage-controlled voltage amplifier 32, and a diode 34. Generally, each of these components works in conjunction to detect the input current and limit the input current to a predetermined current threshold value.
During operation, Vin is impressed across the current-controlled voltage amplifier 30. The current-controlled voltage amplifier 30 measures the current induced in the circuit 20 by regulator circuit 14, provided by battery 18, and translates the measured current into a voltage having a gain specified by a particular circuit element. For example, if the current-controlled voltage amplifier 30 had a constant gain of 1, and the current through the wire is 1 amp, then there will be 1 volt at the output of the amplifier 30.
The current-controlled voltage amplifier 30 can be effectuated in numerous ways known to those skilled in the art. Such a current-controlled voltage amplifier can comprise a plurality of electronic components that work in conjunction to detect the input current, translate the current to a voltage and apply a gain to the voltage. For example, the current-controlled amplifier 30 might comprise a “sense resistor,” which refers to an electronic component comprising a resistor placed in a current path to allow the current to be measured. The voltage across the sense resistor is proportional to the current that is being measured and an amplifier produces a voltage or current that drives the measurement. Additionally, a difference amplifier might be used to measure the current induced in the circuit 20 by the Vin provided by the battery 18. In this regard, the amplifier 30 generally senses the current through the amplifier 30 and translates the current into a voltage which has a gain value dependent upon a gain constant implemented in the amplifier 30.
The inductor 44 receives the voltage output of the current controlled voltage amplifier 30, and the current through the inductor appears as a DC component representing the average input current required to supply the load summed with a triangular wave due to the switching action of switch 42.
The current sensing circuit 20 of
The voltage-controlled voltage amplifier 32 receives the averaged current from the R/C filter 31. The voltage-controlled voltage amplifier 32 scales the current so that the voltage output at an input current equal to the predetermined current threshold value is equal to the reference voltage Vref of the voltage source 62 plus one forward-biased diode voltage drop accommodating the drop across diode 34. The averaged current output from the R/C filter 31 is provided to the voltage-controlled voltage amplifier 32, and the voltage-controlled voltage amplifier 32 takes the gain as a function of the current and translates and/or scales the current so that at the desired current limit, e.g., 0.5 amps, the voltage output at the cathode of diode 34 is equal to Vref.
In this regard, because of diode 34, if the input current is higher than the predetermined current threshold value, the current sensing circuit 20 will provide a voltage at Vfb higher than Vref. Such voltage Vfb provided by the current sensing circuit 20 overrides the voltage feedback from Vout Thus, the current sensing circuit 20 regulates the circuit 14 by lowering the output voltage Vout to maintain Vfb substantially equal to Vref. In this regard, the current sensing circuit 20 effectively lowers the output voltage Vout and limits the input current as desired. The current sensing circuit 20 holds feedback voltage Vfb at the reference voltage Vref either by choosing the output voltage of the voltage-controlled voltage amplifier 32, which is scaled such that the Vfb voltage at an input current equal to the predetermined current threshold value is held at Vref, or the voltage feedback from the feedback divider 50, whichever is higher. If the input current attempts to increase above a predetermined current threshold value of 0.5 amps, e.g., the diode 34 appears as a closed circuit. In this regard, the diode 34 closes the loop and causes the loop to regulate to the current instead of the circuit 14 being regulated by the feedback voltage Vfb supplied by feedback divider 50.
As noted herein, Vout is the output voltage of the regulator circuit 14 and Vin is the input voltage produced by the battery 18. Vfb is the feedback voltage of the control circuit 41 as indicated in
The battery 18 cycles between 4.2 volts and 1.8 volts, as indicated by the plot Vin. As the battery cycles, the input voltage Vin drops, and the circuit 14 compensates for the cycling and tends to maintain the output voltage Vout at or around 5 Volts, as indicated by the plot Vout. There is a slight drop in the output voltage Vout during the transition as indicated. Note that the temporary slight drop in Vout during the Vin transition is due to the output response of the DC-DC converter circuit 22 for a line transient, i.e., Vin is changing dynamically. Further note that other voltage ranges are possible in other examples.
To better illustrate the foregoing, assume a 0.5 amp predetermined current threshold value. Further, assume the load 12 draws a current of 250 miliamps and Vout is 5.0V, for example. 250 miliamps at the output translates to an input current of approximately 350 miliamps, which is less than the 0.5 amp predetermined current threshold value. As Vin drops from 4.2 Volts to 1.8 Volts, the load 12 attempts to draw a greater input current from the battery 18, which is consistent with a dc-dc converter characteristic generally due to the need to supply a constant output power demanded by the load regardless of input voltage. In this regard, as the input voltage Vin drops, and the current voltage Vcur increases, i.e., the load 12 attempts to draw greater current from the battery 18.
Thus, at 1.8 Volts, the exemplary 250 miliamp load 12 needs approximately 881 miliamp input current, which translates to the approximate 0.8 Volts of the voltage Vcur representing the input current when the input voltage Vin is at 1.8 Volts. Therefore, in order to retain the output voltage Vout at 5 Volts at 250 miliamps, the input current drawn from the battery 18 to retain these output characteristics is approximately 881 miliamps. The output voltage Vout, as indicated, is regulated at substantially 5 Volts. Whether the input voltage is 4.2 Volts or 1.8 Volts, the regulator circuit 14 draws the needed current represented by voltage Vcur from the battery 18, i.e., 350 miliamps at 4.2 Volts or 881 miliamps at 1.8 Volts, to whatever value is needed to ensure that the output voltage Vout is regulated at substantially 5 Volts.
The battery 18 cycles between 4.2 volts and 1.8 volts, as indicated by the plot Vin. As the battery cycles, the input voltage Vin drops, however, unlike the regulator circuit 14 not employing current limiting, the output voltage Vout tends to migrate downward and remain at a level substantially below the constant output voltage Vout maintained when current limiting is not employed, as illustrated in
To better illustrate the foregoing, assume a 0.5 amp predetermined current threshold value. Further, assume the load 12 draws a current of 250 miliamps, for example. 250 miliamps translates to an input current of approximately 350 miliamps, which is less than the 0.5 amp predetermined current threshold value. As Vin drops from 4.2 Volts to 1.8 Volts, the load 12 attempts to draw a greater input current from the battery 18, which is consistent with a dc-dc converter characteristic generally as described hereinabove. In this regard, the input voltage Vin drops, and the current voltage Vcur increases, i.e., the load attempts to draw greater current from the battery 18.
However, instead of allowing the load 12 to draw 881 miliamps in order to adjust for the decrease in input voltage Vin, the current limiting circuit limits the current drawn when the input voltage decreases to 1.8 Volts to the 0.5 amp predetermined current threshold value. Thus, when the current sensing circuit 20 is operating, at an input voltage of 1.8 Volts, the load 12 receives less than 5 Volts. Such is illustrated in
Not unlike the behavior of the regulator circuit 14 with reference to
The regulator circuit 14 (
However, if the input current exceeds the predetermined current threshold value in step 82, then the current sensing circuit 20 provides an input control current to the control circuit 41, thereby limiting the current drawn from the battery 18 in step 84. Therefore, the current drawn from the battery 18 is limited to the predetermined current threshold value, and the output voltage Vout is maintained by the feedback circuit 40 via the input control current provided by the current sensing circuit 20. As described herein, when the current sensing circuit 20 limits the current that can be drawn from the battery 18, the output voltage Vout is not maintained at substantially 5 Volts. Instead, as in the example provided, while the input voltage Vin is at the 1.8 Volts and the input current drawn from the batter is limited to the 0.5 amps, the output voltage decreases from the substantially constant 5 Volts, as is illustrated with reference to the graph in
