Battery charging system that utilizes a full wave rectifier and maintains active error correction with a sample and hold circuit

A system and method for charging a battery to a threshold voltage. The battery has an output voltage that is lower than the threshold voltage when undercharged. A switch is provided that controls a current flow to the battery. The switch alternates between a closed state and an open state, wherein the current switch only connects the current source to the battery when in the closed state. A sample and hold circuit is provided that samples the output voltage of the battery each time the current switch is in its open state. This produces a saved voltage value. A comparator is used to compare the saved voltage value of the rechargeable battery to the threshold voltage. The charging current is stopped when the comparator determines that the output voltage of the rechargeable battery is at least as great as the threshold voltage.

BACKGROUND OF THE INVENTION

1. Field of the Invention

In general, the present invention relates to systems and methods used to rapidly charge a rechargeable battery. More particularly, the present invention relates to recharging systems that actively monitor battery output during a recharging cycle.

2. Prior Art Description

Rechargeable batteries come in many shapes and sizes. Small rechargeable batteries are used to power cell phones, cordless tools, and a large variety of everyday items. Large rechargeable batteries are used to power cars, trucks, boats, and the like. Racks of even larger rechargeable batteries are used to store power in electric vehicles, hybrid vehicles, commercial airliners, and many buildings that collect power using solar panels.

In order to charge a rechargeable battery, a current must be passed into the battery at a voltage that is greater than the output voltage rating of the battery. For example, in order to recharge a 12-volt battery, current must be supplied to the battery in excess of 12 volts. This voltage differential is required to ensure that electricity is flowing into the battery and not out of the battery during a recharging cycle. Determining the best charging voltage for a particular battery or series of batteries is complicated. If a battery is overcharged, it can damage the battery. The rechargeable battery may even overheat and catch fire. Battery fires due to overcharging have been reported in smartphones, laptop computers, electric vehicles, and commercial aircraft, even though these applications may use sophisticated systems to monitor the status of the rechargeable batteries.

The most common reason for batteries being overcharged is that many recharging systems are designed under the assumption that the battery and the wiring that leads to the battery have resistance values that are both low and constant. In reality, this assumption is not accurate. Rechargeable batteries are not static systems. Rather, they are dynamic systems that vary in resistance in response to many variables. For instance, the resistance of a rechargeable battery and its surrounding wiring are dependent upon the temperature of the battery, the age of the battery, the condition of the chemicals within the battery, port corrosion, internal electrode degradation, and the gauge of the wire leads.

In the prior art, battery resistance and lead resistance are assumed to be constants and a conditioned DC power source is designed in light of this assumption. The current is conditioned to a preselected charging voltage and charging current that are supplied to the rechargeable battery. The output of the battery charger is monitored using a voltage comparator. The voltage comparator compares the output voltage of the battery charger to a preselected reference voltage. For example, a rechargeable battery may be nominally rated with an output voltage of 12.2 volts. The reference voltage for the comparator may be set at 13.8 volts. Thus, when the output voltage of the battery reaches 13.8 volts, the battery is deemed fully charged and the recharging cycle is stopped.

Such a prior art recharging strategy assumes that the output voltage being received at the voltage comparator is equal to the actual voltage output being produced by the rechargeable battery. This can be a false assumption that creates significant problems. The wires that lead to and from the rechargeable battery have internal resistances. The battery itself has an internal resistance. The termination between the wires and the battery may be partially corroded and can provide significant resistance. The connections used within the recharging port may be partially corroded and can provide significant resistance. In accordance with Ohm's Law, when a current passes through a resistive element, a voltage is developed that varies as a function of the current multiplied by the value of the resistance. Consequently, the resistances of the wires, battery and contacts inflate the output voltage of the battery. The inflated voltage is read by the voltage comparator. The voltage comparator, therefore, believes that the battery is outputting more voltage than it actually is. The voltage comparator will, therefore, stop the recharging process before the battery has become fully charged.

The difference between the inflated voltage and the real voltage of the battery is the error voltage. The existence of an error voltage is often designed into the recharging system. However, the error voltage will vary with time, temperature, and battery degradation. Thus, the recharging system may overcharge the battery while attempting not to undercharge the battery. This can cause batteries to become damaged and present a fire risk.

In the prior art, battery rechargers tend to use sophisticated voltage regulators that condition the DC signal being supplied to the rechargeable battery. The voltage regulators contain smoothing capacitors and other filtering circuitry that provide a nearly constant DC signal during recharging. Such prior art is exemplified by U.S. Pat. No. 7,683,574 to Guang. The use of such voltage regulators does add to the complexity and cost of the overall recharging system. Furthermore, since the supply voltage is made nearly constant during recharging, the current is nearly constant. Consequently, the error voltage generated in the system never drops to zero. As such, the true voltage of the rechargeable battery is difficult to ascertain.

In U.S. Pat. No. 9,570,933 to Sioma, a battery recharging system is shown that actively measures the error voltage created within a recharging system. The error voltage is then used dynamically to adjust current and voltage used by the recharging system. Although such a recharging system is effective, it can be improved. The need to calculate the error voltage can be removed, if a system can be designed that can both recharge a battery and accurately measure the true output voltage of the battery during the recharging process.

A need therefore exists for an improved charging system for a rechargeable battery that can eliminate error voltages and accurately measure the voltage output of a battery, while the battery is being actively recharged. In this manner, the true voltage output of the battery can be actively monitored during recharging to achieve a full charge of the battery without overcharging the battery. This need is met by the present invention as described and claimed below.

SUMMARY OF THE INVENTION

The present invention is a system for recharging a battery to a threshold voltage, wherein said battery has an output voltage that is lower than said threshold voltage when undercharged. The system utilizes a full wave rectifier that converts an incoming AC signal to an outgoing DC signal. The outgoing DC signal is not smoothed and, therefore, has a waveform that periodically drops to zero volts.

A charging switch is provided that controls the flow of the DC signal to the battery. The charging switch alternates between a closed state and an open state, wherein the charging switch enables said current flow to reach the battery only when in its closed state. A sample and hold circuit is provided that samples the output voltage of the battery each time the charging switch is in its open state, therein producing a saved voltage value.

A comparator compares the saved voltage value of the battery to the threshold voltage, wherein the outgoing DC signal is stopped when the output voltage is at least as great as the threshold voltage. The result is a charging system that cam measure the true output voltage of a battery without any significant voltage error. The battery can therefore be fully charged without the overcharging dangers that may occur due to voltage errors in the charging system.

DETAILED DESCRIPTION OF THE DRAWINGS

Although the present invention recharging system can be embodied in many ways to recharge a wide variety of batteries, only one embodiment has been selected for the purposes of illustration and discussion. The embodiment represents one of the best modes contemplated for utilizing the invention. The illustrated embodiment, however, is merely exemplary and should not be considered a limitation when interpreting the scope of the appended claims.

Referring toFIG. 1, a recharging system10is shown that is used to recharge a rechargeable battery12. The rechargeable battery12illustrated is intended to be representative of all rechargeable batteries, regardless of battery type, composition, or size. The rechargeable battery12has an output voltage Voutand an internal resistance, shown as R1. The value of the output voltage Voutand the internal resistance R1will vary over time as the rechargeable battery12ages. The output voltage Voutand the internal resistance R1will also vary as a function of other short-term variables, such as temperature and state of charge. For the purposes of discussion and scaling waveforms, the exemplary battery12is assumed to be a 12-volt battery, such as that used in an automobile.

The shown recharging system10has a charging port14that can selectively interconnect with the rechargeable battery12when the rechargeable battery12needs to be recharged. It will be understood that depending upon the size of the rechargeable battery12and the application, the rechargeable battery12may be wired to the recharging system10. In such a scenario, a separate charging port need not be provided.

The rechargeable battery12has two terminals16,18. The recharging system10connects the first terminal16to a power source and the second terminal18to ground. The power source is an AC power source19that produces the AC signal21shown inFIG. 2. By way of example, the AC power source19is considered to be drawn from the standard US power grid. That is, 120 volts at 60 Hz. The AC power source19is connected to a full wave rectifier20. The full wave rectifier20does not include any smoothing capacitors or any other filtering features. Accordingly, the full wave rectifier20converts the AC signal21ofFIG. 2into the DC signal23shown inFIG. 3. The DC signal23is used to charge the rechargeable battery12at a selected voltage. The selected voltage is greater than the output voltage Voutof the rechargeable battery12, so that electricity flows into the battery12during a recharging cycle.

The various leads and components of the recharging system10between the full wave rectifier20and the rechargeable battery12have a combined resistance that are represented by the resistance value R2. Furthermore, the leads and components between the rechargeable battery12and ground have a combined resistance that are represented by the resistance value R3. It will therefore be understood that the total resistance of the overall recharging system10and the rechargeable battery12are equal to the sum of the resistances R1+R2+R3.

Using Ohm's Law, it will be understood that as current flows to recharge the battery12, a voltage error is generated. The voltage error is equal to the incoming current multiplied by the combined resistance values. The true output voltage Voutof the rechargeable battery12becomes inflated by the voltage error. To accurately measure the true output voltage Vout, the voltage error created by the sum of the resistances must be eliminated. In the present invention recharging system10, this is accomplished by periodically dropping the recharging current to a value of zero while taking a simultaneous measurement of the battery's output voltage Vout. Due to Ohm's Law, when the value of the current is zero, the voltage drops and the value of the resistances become inconsequential, since there is no current to resist. As a result, no error voltage is produced and the true value for the output voltage Voutcan be directly measured.

The DC signal23of the full wave rectifier20flows through a solid state charging switch24. The charging switch24operates between an open state and a closed state. In the open state, current does not pass through the charging switch24. Conversely, in the closed state, current flows through the charging switch24. The duration of the open state and the closed state are controlled in a manner later described.

Referring toFIG. 3in conjunction withFIG. 1, it can be seen that within the DC signal23produced by the full wave rectifier20, there are periods28of the closed state, where current flows, that are significantly longer than the interposed periods30of the open state, where no current flows. For the DC signal23shown inFIG. 2, the interposed periods30have a duration of approximately 8.33 ms.

Returning toFIG. 1, it will be understood that the DC signal23is directed to the charging port14and the rechargeable battery12. A diode27or similar current flow restrictor is provided to prevent any backflow from the rechargeable battery12to the charging switch24. When the charging switch24is in its closed state, the rechargeable battery12receives the current that is output by the full wave rectifier20. The resistances inherent in the system produce an error voltage. When the error voltage is incorporated into the DC signal23of the full wave rectifier20, the corrected signal25ofFIG. 4is created. As can be seen, when the charging switch24is in its open state, no current is received by the rechargeable battery12and no error voltage is produced.

A sample and hold circuit32is provided. The sample and hold circuit32has a voltage input34, a voltage output36and a command signal terminal40. The command signal terminal40receives the DC signal23that is shown inFIG. 3. As such, the operation of the sample and hold circuit32is synchronized with the DC signal23produced by the full wave rectifier20.

The voltage input34used for the sample and hold circuit32is tapped from a point between the diode27and the rechargeable battery12. Accordingly, the sample and hold circuit32receives the corrected signal25ofFIG. 4. The sample and hold circuit32samples the voltage of the corrected signal25only when the current waveform26is in a period of zero current. That is, the sample and hold circuit32samples the corrected signal25only when the charging switch24is in its open state and no current is flowing. At such moments, the only voltage received at the voltage input34is the true output voltage Voutof the rechargeable battery12. The sample and hold circuit32holds the output voltage value until that value is updated the next time the charging switch24opens. The voltage value being held during any one cycle of the sample and hold circuit32is the latest output voltage Voutof the sample and hold circuit32.

A voltage comparator42is provided. The voltage comparator42receives the output of the sample and hold circuit32. As such, the voltage comparator42receives the last measured output voltage Voutof the rechargeable battery12. The voltage comparator42compares the output of the sample and hold circuit32to a preselected reference voltage44. An appropriate reference voltage is selected for the specifications of the rechargeable battery12being charged. If the output of the sample and hold circuit32, i.e. the battery output voltage, is below the reference voltage44, then the recharging operation is continued.

To prevent an overcharge event or a damaged battery from corrupting the operations of the recharging system10, the output of the voltage comparator42is read to a logic circuit50. The logic circuit50ensures that voltage held by the sample and hold circuit32does not surpass a selected maximum voltage. The output of the logic circuit50triggers the charging switch24. However, when the output of the sample and hold circuit32, i.e. the last battery output voltage, reaches the reference voltage44, or when the output of the sample and hold circuit32reaches a selected maximum, the charging switch24stops the recharging cycle.

Referring now toFIG. 5in conjunction withFIG. 1andFIG. 3, the full method of operation for the recharging system10can be understood. If a rechargeable battery12needs recharging, the rechargeable battery12is connected to the recharging system10. See Block50. The rechargeable battery12is supplied with a cyclic recharging current from a full wave rectifier20. See Block51. The true output voltage Voutof the rechargeable battery12is sampled and held, using the sample and hold circuit32. See Block52. The sampling is conducted when the recharging current is at a zero value. The output voltage of the rechargeable battery12that was sampled is compared to a reference voltage44. See Block54. If the true output voltage is less than the reference voltage44, then the charging switch24is triggered and current is supplied to the rechargeable battery12. See Block lead56and Block58.

If the output voltage of the rechargeable battery12is equal to, or greater than, the reference voltage44, the charging switch24stops the current and the rechargeable battery12is considered recharged. See Block59.

It will be understood that the embodiment of the present invention that is illustrated and described is merely exemplary and that a person skilled in the art can make many variations to the embodiment. All such embodiments are intended to be included within the scope of the present invention as defined by the claims.