WO2009086342A1

WO2009086342A1 - Method and system for float charging a battery

Info

Publication number: WO2009086342A1
Application number: PCT/US2008/088098
Authority: WO
Inventors: Bruce W. Mcbee
Original assignee: Cooper Technologies Co
Current assignee: Cooper Technologies Co
Priority date: 2008-01-02
Filing date: 2008-12-23
Publication date: 2009-07-09
Anticipated expiration: 2010-07-02
Also published as: US7986129B2; EP2243123A1; CA2710758A1; MX2010007365A; CA2710758C; US20090167238A1; BRPI0821463A2; EP2243123A4; TW200937700A; TWI476977B

Abstract

Float charging a battery comprises charging a battery to a reference voltage that is less than a maximum voltage of the battery. A power source provides power to a charging circuit. The circuit supplies a reference voltage. Au operational amplifier float charges the battery to the reference voltage. A comparator determines whether the battery's voltage exceeds the reference voltage. Another comparator determines whether current through the battery exceeds a reference current. A thermistor determines whether the battery's temperature exceeds a safe range. A microprocessor monitors the inputs from the comparators and the thermistor and outputs of the circuit. If the comparators or the thermistor reports a safety fault to the microprocessor, the microprocessor shuts down the charging current to the battery. If an output circuit draws too much current from the battery, the microprocessor can disconnect the battery from the output circuit.

Description

METHOD AND SYSTEM FOR FLOAT CHARGING A BATTERY

RELATED APPLICATIONS

[0001] This application is related to U.S. Patent Application No. 11/982,587, entitled "Faulted Circuit Indicator Apparatus with Transmission Line State Display and Method of Use Thereof," filed on November 2, 2007; and U.S. Patent Application No. 11/982,588, entitled "Communicating Faulted Circuit Indicator Apparatus and Method of Use Thereof," filed on November 2, 2007. The complete disclosure of each of the above-identified related applications is hereby fully incorporated herein by reference.

TECHNICAL FIELD

[0001] The present invention relates to a battery power supply and more particularly to a circuit for recharging a battery used to power remote sensors, such as faulted circuit indicators.

BACKGROUND

[0002] Faulted circuit indicators (FCIs) are well known in the field of electric power distribution systems. Generally, FCIs are electrically connected to transmission lines in a power distribution system at various locations throughout the system, often in close proximity to system loads. When a fault occurs in a transmission line, FCIs between the power source and the fault will detect that a fault has occurred. Typically, FCIs that have detected a fault then display an indication that the fault has been detected. An FCI also can include a conventional transmitter for communicating faults to another location. A technician can then identify a fault by locating the transmission line between an FCI that indicates it has detected a fault and an FCI that does not indicate a fault.

[0003] FCIs and other types of sensors may be deployed widely to monitor systems in remote locations and in harsh environmental conditions. The difficulty of powering remote sensors, such as FCIs, has long been a problem limiting the sensors' lifespan and usefulness. Generating adequate power to a remote sensor requires the use of large primary batteries, which is cost prohibitive. As a result of a lack of sufficient power to remote sensors, the sensors must power down for periods of time to conserve energy, which results in a lack of communication from the sensors during down time. [0004] Lithium primary cells, which are not rechargeable, provide one option as a power source for remote sensors. However, most transmitters for remote sensors require instantaneous power in amounts too large for lithium primary cells to provide. Additionally, non-rechargeable batteries require routine replacement at relatively short intervals.

[0005] Remote sensors could harvest power from other sources, such as solar, inductive, biological, or mechanical means. However, the inconsistent nature of these sources requires a system to capture the energy and store it for later use. Additionally, conventional sensors cannot withstand the harsh environments- — 45 to +85 degrees Celsius — in which remote sensors usually are deployed can pose nearly insurmountable challenges, especially in the small packages required by modem electronics. Obtaining a life expectancy of more than two to three years for conventional sensors is unlikely in the absence of burdensome extra circuitry and devices that cool or heat the sensor to keep the charge receptor at or close to room temperature.

[0006] Several conventional devices have been developed to try to address the deficiencies associated with conventional sensors and their power sources. However, all such conventional devices have shortcomings. The various faults of these devices result in remote sensors that are expensive, short lived, or both. For example, a conventional solution is to use devices such as supercapacitors to power a remote sensor. However, supercapacitors work well only in applications that operate in environments close to 25 degrees Celsius. Thus, supercapacitors fail to provide the necessary functionality in the extreme environments of remote sensing applications.

[0007] In addition, lithium polymer and lithium ion cells have been identified as options. However, charging these cells can be difficult because of the environmental constraints of remote sensing. Conventionally, such cells are charged to their maximum allowable voltage, which yields the maximum capacity. Storing lithium cells when charged to maximum capacity will severely limit the cells' life expectancy. Fully charging a rechargeable lithium cell greatly degrades the cell, causing premature failure and reducing shelf life to less than three to four years. Additionally, developing circuitry to monitor and charge these systems at the available low power inputs can be prohibitive.

[0008] As discussed previously, lithium cells ordinarily are charged to the maximum allowable voltage. By charging to the maximum voltage, users of laptop computers, for example, can extend the maximum period of continuous use. However, this method of charging results in successive periods of charging to the maximum voltage capacity followed by periods of partially discharging the lithium cell. Such patterns of use severely degrade the life of a battery. Accordingly, traditional methods of charging lithium cells cannot provide the consistency and longevity required in remote sensing applications.

[0009] Accordingly, a need exists in the art for a power source for remote sensors that addresses the deficiencies of conventional sensors and associated power sources. For example, a need exists in the art for extended battery life in remote sensing operations. A further need exists for a circuit for float charging a battery to a predetermined voltage that will extend the life of the battery. A need also exists for float charging a battery as a way to provide sufficient and consistent power to remote sensors and to prevent down time resulting in loss of communication.

SUMMARY

[0010] The proposed solution is to float charge a rechargeable battery at its optimum storage voltage. The present invention involves using a limited current to charge a battery to a specified target voltage below the battery's maximum allowable voltage. This float charging method increases a shelf life and longevity of the battery.

[0011] In one aspect, a power source provides power to a charging circuit. The circuit supplies a reference voltage. An operational amplifier float charges a battery to the reference voltage. A resistor limits the maximum charging current to the battery, and a diode prevents reverse current that would discharge the battery. The circuit performs several safety checks. A comparator determines whether the battery's voltage exceeds the reference voltage. Similarly, another comparator determines whether current through the battery exceeds a reference current. Also, a thermistor determines whether the battery's temperature exceeds a safe range. A microprocessor monitors the inputs from the comparators and the thermistor and outputs of the circuit. If the comparators or the thermistor reports a safety fault to the microprocessor, the microprocessor can operate a switch off to shut down the charging current to the battery. If an output circuit draws too much current from the battery, the microprocessor also can operate a second switch to disconnect the battery from the output circuit. [0012] In an alternative aspect, a linear regulator float charges the battery to the reference voltage. In addition to the safety features listed above, the microprocessor can directly shut off the linear regulator to prevent overcharging or other dangerous conditions. When operating with high voltages, the charging circuit can accommodate multiple capacitors for filtering purposes.

[0013] Additional aspects, features, and advantages of the invention will become apparent to those having ordinary skill in the art upon consideration of the following detailed description of illustrated embodiments exemplifying the best mode of carrying out the invention as presently perceived.

BRIEF DESCRIPTION OF DRAWINGS

[0014] Fig. 1 is a functional block diagram illustration of a method for float charging a battery according to an exemplary embodiment of the invention.

[0015] Fig. 2 is a functional block diagram illustration of a method for charging a battery using an operational amplifier according to an exemplary embodiment of the invention.

[0016] Fig. 3 is a schematic illustration of circuit for float charging a battery using an operational amplifier according to an exemplary embodiment of the invention.

[0017] Fig. 4 is a schematic illustration of a circuit for float charging a battery using a linear regulator according to an exemplary embodiment of the invention.

DETAILED DESCRIPTION

[0018] It is not an accepted practice in the art to float charge lithium cell batteries. In fact, manufacturers of lithium cells generally recommend charging the cells to the maximum allowable voltage to extend a potential period of continuous use. However, by charging to the maximum allowable voltage a user will drastically reduce the life expectancy of a lithium cell. Thus, the manufacturers' recommendations foreclose the use of lithium cells in remote sensing applications.

[0019] According to the present invention, a user can lengthen the life of a lithium cell considerably by float charging the cell to a specified target voltage below the maximum capacity voltage. "Float charging" means charging a battery at the same rate it is discharging. The float charging circuit charges the battery essentially at the same rate the battery discharges to an output circuit or dissipates during inactive periods, thus maintaining the battery's voltage at the specified target voltage. This method of charging can increase the life of the battery to a decade or more, making the lithium cell suitable for remote sensing applications. Additionally, this method of float charging demonstrates inherent safety qualities by reducing the risk of overheating or explosion of the lithium cell associated with overcharging. The present invention can accommodate additional safety features as well, including over-voltage, over-current, and temperature protection.

[0020] Figure 1 illustrates a method 100 for float charging a battery according to an exemplary embodiment of the invention. Figure 1 will be described with reference to Figure 3, which illustrates a circuit 300 for float charging a battery Vl using an operational amplifier OPl according to an exemplary embodiment of the invention. In step 105, a power source 302 provides power to the circuit 300. The circuit may use any suitable power source, including a solar cell, a fuel cell, a battery, a current transformer, or another suitable source. In one exemplary embodiment, the float charging circuit 300 can draw power from the current transformer (not shown) of a faulted circuit indicator (not shown).

[0021] In step 110, a reference voltage VSl is supplied to the circuit 300. The reference voltage VSl provides a target float charge voltage for the battery Vl . The reference voltage can be calibrated according to the float charge voltage that will maximize the life of the battery Vl. The appropriate float charge voltage can generally be determined from a manufacturer's recommended voltage for storage of the battery Vl. In an exemplary embodiment, the battery Vl can be any rechargeable lithium-ion cell, lithium phosphate cell, or lithium polymer cell, or any other suitable type of battery. In an exemplary embodiment, the float charging circuit 300 will charge the battery Vl to between forty percent and seventy percent of the battery Vl 's maximum capacity. In another exemplary embodiment, the float charging circuit 300 will charge the battery Vl to between fifty percent and fifty-five percent of the battery Vl 's maximum capacity. For example, float charge for a lithium ion or lithium polymer cell can be between 3.6 and 3.7 volts for a battery having a maximum voltage of 4.2 volts, and float charge for a lithium phosphate cell can be 3.4 volts for a battery having a maximum voltage of 4.2 volts. In one exemplary embodiment, maximum capacity can be measured in ampere hours. [0022] In stqp 115, through the use of operational amplifier OPl, the circuit 300 charges the battery Vl to the desired float charge voltage. Step 115 will be described in further detail hereinafter with reference to Figure 2. Figure 2 illustrates a method 115 describing the function of operational amplifier OPl as a charging mechanism according to an exemplary embodiment of the invention. Alternative embodiments may use a linear regulator or other suitable device to charge the battery to the desired float charge voltage.

[0023] While the circuit 300 charges the battery, it also conducts several safety checks, as illustrated in steps 120-145 of Figure 1. As shown in Figure 1, the circuit 300 performs the safety checks sequentially. In alternative embodiments, the safety checks may be performed simultaneously.

[0024] In step 120, a comparator CPl compares the battery Vl voltage to a maximum voltage (Vmax) for the battery Vl. Resistors R5 and R6 function to set the appropriate maximum voltage for the over- voltage function of comparator CPl . A person having ordinary skill in the art will be able to choose the resistance levels of resistors R5 and R6 appropriately to set the desired trip point for over-voltage protection. In an exemplary embodiment, the maximum voltage can be calibrated to a manufacturer's maximum suggested voltage for the battery Vl.

[0025] Then, in step 125, the comparator CPl determines whether the battery Vl voltage is greater or less than the maximum voltage. If the battery Vl voltage is greater than the maximum voltage, the comparator CPl outputs the information to a microprocessor 306 and the method 100 branches to step 150. In step 150, the microprocessor 306 switches off Field Effect Transistor (FET) Tl. When switched off, FET Tl prevents the circuit from charging the battery Vl .

[0026] Referring back to step 125, if comparator CPl determines that battery Vl voltage is less than or equal to the maximum voltage, the circuit 300 continues to charge the battery Vl to the desired float charge voltage as determined by the reference voltage VSl.

[0027] As a second safety check, a comparator CP2 compares a current I- across the battery Vl to a maximum current 1+ in step 130. Resistors R9 and R10 function to set the appropriate reference current 1+ for the over-current function of comparator CP2. A person having ordinary skill in the art will be able to choose the resistance levels of resistors R9 and R10 appropriately to set the desired trip point for over-current protection. In an exemplary embodiment, the maximum current 1+ can be calibrated to a manufacturer's maximum suggested current for the battery Vl.

[0028] Then, in step 135, the comparator CP2 determines whether the current I- across the battery Vl exceeds the maximum current I+. If the battery Vl current I- exceeds maximum current I+, comparator CP2 outputs the information to the microprocessor 306 and the method 100 branches to step 150. In step 150, the microprocessor 306 switches off FET Tl, as discussed previously.

[0029] Referring back to step 150, if the battery Vl current I- is less than or equal to the maximum current I+, the circuit 300 continues to charge the battery Vl to the desired float charge voltage as determined by the reference voltage Vl .

[0030] As a third safety check, the microprocessor 306 compares in step 140 a temperature of the battery Vl, as measured by a temperature sensor NTC, to a predetermined temperature, which can comprise a specified temperature or a temperature range. In one exemplary embodiment, the temperature sensor NTC comprises a thermistor. In alternative embodiments, the microprocessor 306 can directly gauge the battery Vl 's temperature via its own temperature gauge or another suitable device may be used. In an exemplary embodiment, the predetermined temperature can be calibrated to a manufacturer's suggested safe temperature for the battery Vl.

[0031] Then, in step 145, the microprocessor 306 determines whether the battery Vl temperature is greater or less than the predetermined temperature. If the battery Vl's temperature exceeds the predetermined temperature, the method 100 branches to step 150, in which the microprocessor 306 switches off FET Tl, as discussed previously.

[0032] Referring back to step 145, if the battery Vl temperature is less than or equal to the predetermined temperature, then the method 100 branches back to step 147 in which the microprocessor ensures that the switch FET Tl is closed. The method 100 then returns to step 115 to continue charging the battery Vl to the desired float charge voltage. Thus, if all three safety checks (steps 125, 135, and 145) are passed, then the circuit 300 will continue to charge the battery Vl. If the microprocessor previously opened the switch FET Tl due to a failure of a safety check, the switch FET Tl will be closed in step 147 after all three safety checks are passed. [0033] Referring back to step 150, the method 100 returns from step 150 to step 120 to continue performing the safety checks. Thus, if a previous condition that triggered discontinuing charging the battery Vl returns to a normal state, then the method 100 can continue to monitor the safety checks until all safety checks are passed. At that point, the method 100 branches to step 147 as discussed earlier to continue charging the battery V 1.

[0034] Because the circuit 300 float charges the battery Vl, charging will be performed continuously in the absence of a safety shut down. The method 100 of float charging exhibits inherent safety features. By float charging the battery Vl to a predetermined float charge voltage below the maximum voltage of the battery Vl, the method 100 reduces the risk of overheating or explosion associated with batteries, and lithium cells in particular. In addition, the exemplary embodiment of Figure 3 includes at least one further safety feature not illustrated in Figure 1. The microprocessor 306 can determine if a load is drawing too much current from an output 308 of the battery Vl. If the output 308 draws too much current, microprocessor 308 will switch off FET T2, allowing the float charging circuit to continue float charging without outputting current to the load.

[0035] Figure 2 illustrates a float charging mechanism using an operational amplifier according to an exemplary embodiment of the invention. Figure 2 will be described with reference to Figure 3.

[0036] In step 205, the float charging circuit 300 supplies the reference voltage VSl to the + pin of the operational amplifier OPl (hereinafter V+) and the battery Vl provides its voltage to the - pin of the operational amplifier OPl (hereinafter V-). In step 207, the operational amplifier OPl attempts to match the voltage V- to the voltage V+ and charges the battery Vl appropriately as described in steps 210-225.

[0037] In step 210, the operational amplifier OPl determines whether the voltage applied to the - pin (V-) matches the voltage applied to the + pin (V+). If the voltage at the + pin exceeds the voltage at the - pin, the method 115 branches to step 215 in which a current Il flows from the operational amplifier OPl to the battery Vl, thus charging the battery Vl. Resistor Rl functions to limit the amount of current Il flowing from the operational amplifier OPl. A person having ordinary skill in the art will understand how to choose the appropriate resistance level of Rl to limit current Il to the desired level. [0038] Referring back to step 210, as the voltages at the + and - pins of operational amplifier OPl approach equivalent, the method 115 branches to step 220 in which the operational amplifier OPl controls the current Il such that current Il approaches zero to prevent charging the battery Vl beyond the reference voltage VSl. When the voltage at the + pin equals the voltage at the - pin, the current Il equals zero, and the charging of the battery Vl ceases.

[0039] Referring back to step 210, if the voltage at the - pin exceeds the voltage at the + pin, diode Dl prevents backfeed current from the battery Vl, Il equals zero, and charging of the battery Vl ceases.

[0040] The method 115 is performed continuously. Thus, from each of steps 215, 220, and 225, the method 115 returns to step 210 to provide the proper current Il for charging the battery Vl. As shown in Figure 2, the method 115 also proceeds to step 120 (Figure 1) to allow performance of the safety checks described previously.

[0041] Figure 4 illustrates a circuit 400 for float charging a battery BTl using a linear regulator U5 according to an exemplary embodiment of the invention. The circuit 400 can be used as the circuit that charges the battery as referenced in step 115 of Figure 1, in which case the battery BTl is the battery Vl.

[0042] The circuit 400 of Figure 4 operates in much the same way as the circuit 300 of Figure 3 to float charge a battery BTl or Vl, respectively. According to the embodiment of Figure 4, a linear regulator U5 provides a constant voltage to battery BTl. A power source (not shown) provides a voltage Vin to the #8 pin of the linear regulator U5 through a resistor Rl 7 that limits the charge current. The linear regulator U5 provides the constant voltage to the battery BTl via resistor Rl 2 from the linear regulator U5's #1 pin. The battery Vl voltage is provided to the linear regulator's #2 pin, and the linear regulator U5 controls the voltage to the battery Vl in response.

[0043] The circuit 400 also includes safety features. Comparator U6 functions as over-voltage protection and outputs Vo to a microprocessor (not shown). Comparator U4 functions as over-current protection and outputs OC to the microprocessor. If either comparator U6 or U4 indicates a fault, the microprocessor can directly shut off linear regulator U5 and prevent charging of battery BTl. The circuit 400 includes capacitors C 14, C49, Cl 5, and Cl 6 for noise filtering purposes.

[0044] The circuit 400 was tested to determine whether the charge algorithm operated to float charge the battery BTl. The test supplied a discharged lithium cell battery to the charging circuit, which immediately began float charging at 45 mA of current to the lithium cell. As the lithium cell's voltage increased, the charging current decreased. The test took approximately 30 hours to charge the battery fully. The circuit was also subjected to power surges and transients according to IEC 61000-45. During the test, the circuit's over-current, over- voltage, and overall functionality remained as expected. In a test designed for an external short circuit, the circuit disconnected the battery (through operation of R25 as a fuse), thereby preventing an explosion or an otherwise dangerous increase in temperature. In other safety tests, a thermistor appropriately tracked the temperature of the battery. The over-current protection and over-voltage protection also were tested, both of which operated appropriately to open the float charging circuit as a safety measure.

[0045] Different types of batteries, such as lithium cells, have different optimum float charge voltages, which can be based on manufacturer's recommendations for the batteries' storage voltage. The exemplary embodiments illustrated in Figures 1-4 can accommodate multiple types of batteries. In particular, the reference voltage VSl and certain current-limiting resistors can be adjusted to any level appropriate for the target voltage designed to maximize the life of a particular battery. A person having ordinary skill in the art will understand ways to adapt the exemplary embodiments to optimize performance for different types of batteries.

[0046] Exemplary types of lithium cells include lithium polymer cells, lithium ion cells, and lithium phosphate cells, each of which is suitable for use with the exemplary methods and circuits described herein.

[0047] Many other modifications, features, and embodiments of the invention will become evident to those of ordinary skill in the art. It should be appreciated, therefore, that many aspects of the invention were described above by way of example only and are not intended as required or essential elements of the invention unless explicitly stated otherwise. Accordingly, it should be understood that the foregoing relates only to certain embodiments of the invention and that numerous changes may be made therein without departing from the spirit and scope of the invention as defined by the following claims. It should also be understood that the invention is not restricted to the illustrated embodiments and that various modifications can be made within the scope of the following claims.

Claims

CLAIMSWhat is claimed is:

1. A method for float charging a battery, comprising charging a battery to a reference voltage at a rate equal to a rate of discharge of the battery, wherein the reference voltage is less than a maximum voltage of the battery.

2. The method of claim 1 , further comprising the steps of: determining whether a voltage of the battery exceeds the maximum voltage; and discontinuing the charging step in response to determining that the voltage of the battery exceeds the reference voltage.

3. The method of claim 1 , further comprising the steps of: determining whether a current through the battery exceeds a maximum current for the battery; and discontinuing the charging step in response to determining that the current through the battery exceeds the reference current.

4. The method of claim 1 , further comprising the steps of: determining whether a temperature of the battery exceeds a predetermined temperature; and discontinuing the charging step in response to determining that the temperature of the battery exceeds the predetermined temperature.

5. The method of claim 4, further comprising the step of measuring the temperature of the battery with a thermistor.

6. The method of claim 1, further comprising the steps of: providing a power source that provides power to charge the battery to the reference voltage, and outputting current from the battery to a load.

7. The method of claim 6, wherein the load comprises a faulted circuit indicator.

8. The method of claim 7, wherein the power source comprises a current transformer associated with the faulted circuit indicator.

9. The method of claim 6, wherein the power source comprises at least one of a solar cell, a fuel cell, a battery, and a current transformer associated with a faulted circuit indicator.

10. The method of claim 1, wherein the battery comprises one of a lithium polymer cell, a lithium ion cell, and a lithium phosphate cell.

11. The method of claim 1 , wherein charging the battery is performed using an operational amplifier.

12. The method of claim 11, further comprising preventing a backflow of current from the battery to the operational amplifier.

13. The method of claim 1, wherein charging the battery is performed using a linear regulator.

14. The method of claim 1, wherein the reference voltage is a voltage that will charge the battery to not less than forty percent of a maximum capacity of the battery and not greater than seventy percent of the maximum capacity of the battery.

15. A circuit for float charging a battery, comprising a power source; a battery; a device that charges the battery to a reference voltage at a rate equal to a rate of discharge of the battery, wherein the reference voltage is less than a maximum voltage of the battery.

16. The circuit of claim 15, further comprising: a comparator that determines whether a voltage of the battery exceeds the maximum voltage; and a microprocessor that controls the device to discontinue charging the battery in response to determining that the voltage of the battery exceeds the reference voltage.

17. The circuit of claim 15, further comprising: a comparator that determines whether a current through the battery exceeds a maximum current; and a microprocessor that controls the device to discontinue charging the battery in response to determining that the current through the battery exceeds the reference current.

18. The circuit of claim 15 , further comprising: for a temperature gauge for measuring a temperature of the battery; and a microprocessor that controls the device to discontinue charging the battery in response to determining that the measured temperature of the battery exceeds a predetermined temperature.

19. The circuit of claim 18, wherein the temperature gauge comprises a thermistor.

20. The circuit of claim 15, wherein the battery outputs current to a load.

21. The circuit of claim 20, wherein the load comprises a faulted circuit indicator.

22. The circuit of claim 15, wherein the power source comprises a current transformer associated with the faulted circuit indicator.

23. The circuit of claim 15, wherein the power source comprises at least one of a solar cell, a fuel cell, a battery, and a current transformer associated with a faulted circuit indicator.

24. The circuit of claim 15, wherein the battery comprises one of a lithium polymer cell, a lithium ion cell, and a lithium phosphate cell.

25. The circuit of claim 15, wherein the device that charges the battery comprises an operational amplifier.

26. The circuit of claim 15, wherein the device that charges the battery comprises a linear regulator.

27. The circuit of claim 15, wherein the reference voltage is a voltage that will charge the battery to not less than forty percent of a maximum capacity of the battery and not greater than seventy percent of the maximum capacity of the battery.