Charge control circuit for a vehicle vacuum cleaner battery

Disclosed herein is a battery charger control circuit having a voltage detector to generate a signal indicative of a source voltage level to select one of a first charging mode and a second charging mode, and a charge controller coupled to the voltage detector to enable charging in accordance with one of the first charging mode and the second charging mode based on the signal from the voltage detector. The first and second charging modes establish charging at differing, non-zero rates. The source voltage level may be sampled at a sampling rate to minimize power consumed by monitoring the source voltage level.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention generally relates to battery charge control circuits and, more particularly, to a charge control circuit for an auxiliary battery charged by a primary vehicle battery.

2. Brief Description of Related Technology

Charge control circuits for regulating the charging of Nickel-Cadmium (NiCd), Nickel-Metal-Hydride (NiMH), and other rechargeable batteries from a DC source are well known. In fact, a portion of the circuitry involved in such circuits is made available commercially as a discrete integrated circuit (IC). Such ICs are often referred to as charge controllers. One commercially available charge controller IC is a pin-programmable, fast-charge controller from Maxim Integrated Products under product number MAX712 or MAX713 (www.maxim-IC.com, Sunnyvale, Calif.).

Circuits for charging batteries often employ a control or feedback scheme based on an evaluation of the voltage of the battery cell(s) being charged. For example, a battery charging circuit may facilitate low-current charging, or trickle charging, until the battery cell being charged reaches a threshold voltage. Other characteristics of the battery cell(s) being charged, such as charging current and battery temperature, have also been evaluated to control charging.

In many cases, these charge control circuits are applied in the context of a power source of effectively infinite capacity. The control circuit is thus designed without regard to whether the power source could be adversely affected by the charging operation. However, in cases where one battery is charging another, the discharging of the source battery may need to be regulated.

The discharging of a vehicle battery has been regulated to ensure sufficient capacity for engine start via, for example, the operation of a starter motor. Complicating matters somewhat, vehicle batteries exhibit large voltage swings based on whether the engine is running or, more specifically, whether an alternator is operating to convert mechanical power to electrical power. For instance, a typical 12-volt, automobile battery may be at approximately 13.8 Volts with the alternator operating.

Without the power supplied by the alternator, the voltage of the vehicle battery drops noticeably with the use of accessories requiring significant power or current. Once the vehicle battery drops below a threshold voltage, past discharge regulators have prohibited certain accessory use. But despite such voltage drops, the vehicle battery may nevertheless have sufficient capacity to power certain, low-power accessories, particularly if the power consumption of such accessories could be regulated in response to the condition of the vehicle battery. In this manner, power consumption by a vehicle accessory may be permissible well after the engine has stopped running and the voltage of the vehicle battery has decreased.

SUMMARY OF THE INVENTION

Disclosed herein is a battery charger control circuit having a voltage detector and a charge controller coupled to the voltage detector. The voltage detector generates a signal indicative of a source voltage level to select one of a first charging mode and a second charging mode, and the charge controller enables charging in accordance with one of the first charging mode and the second charging mode based on the signal from the voltage detector. The first and second charging modes establish charging at differing, non-zero rates.

In some embodiments, the voltage detector includes a window comparator such that the signal is one of first and second control signals generated by the window comparator to indicate whether the source voltage level is below, within, or above a voltage window set by the window comparator. The battery charger control circuit may further include a control switch to determine whether the charge controller should be powered based on whether the first and second control signals indicate that the source voltage level is below the voltage window. The battery charger control circuit may still further include an OR gate responsive to the first and second control signals to drive the control switch.

The charge controller may include a fast-charge controller integrated circuit such that the first and second charging modes correspond with trickle and fast charging, respectively. The signal indicative of the source voltage level may then be provided to a temperature threshold pin of the fast-charge controller integrated circuit to disable fast charging.

In some embodiments, the battery charger control circuit further includes a step-up, DC-DC controller coupled to the charge controller. The step-up, DC-DC controller may include a switching regulator.

The voltage detector may include an oscillator to enable sampling of the source voltage level.

The battery charger control circuit may be used in combination with a source battery that provides the source voltage level. The first and second charging modes may be two of a plurality of operational modes of the battery charger control circuit. The plurality of operational modes may include a non-charging mode, and the voltage detector may be connected to the source battery to monitor the source voltage level regardless of the operational mode of the battery charger control circuit.

In accordance with another aspect, a vehicle electrical system has primary and secondary batteries, and a control circuit coupling the secondary battery to the primary battery to control recharging of the secondary battery via the primary battery. The control circuit includes a voltage detector that generates a signal indicative of the terminal voltage of the primary battery to select one of a first charging mode and a second charging mode. The control circuit also includes a charge controller that enables charging of the secondary battery in accordance with one of the first and second charging modes based on the signal from the voltage detector. The first and second charging modes establish charging at differing, non-zero rates.

In some embodiments, the voltage detector uses the terminal voltage of the primary battery as a power supply.

In accordance with yet another aspect, a battery charger control circuit includes a window comparator to generate first and second signals collectively indicative of whether a source voltage level is below, within or above a voltage window to select one of a plurality of operational modes. The window comparator has an oscillator to establish a sampling rate such that the source voltage level is sampled at the sampling rate to minimize power consumed by monitoring the source voltage level. The battery charger control circuit further includes a charge controller coupled to the window comparator to enable charging in accordance with the selected operational mode.

In some embodiments, the plurality of operational modes includes first and second charging modes that establish charging at differing, non-zero rates. The plurality of operational modes may include a non-charging mode. The voltage detector may be in communication with the source voltage level such that the voltage detector monitors the source voltage level during the non-charging mode.

The battery charger control circuit may further include a control switch controlled by at least one of the first and second signals to determine whether the charge controller is powered. The battery charger control circuit may still further include an OR gate coupling the window comparator and the control switch such that either one of the first and second signals may activate the control switch to allow the charge controller to be powered.

While the disclosed system and circuit are susceptible of embodiments in various forms, there are illustrated in the drawing (and will hereafter be described) specific embodiments of the invention, with the understanding that the disclosure is intended to be illustrative, and is not intended to limit the invention to the specific embodiments described and illustrated herein.

DETAILED DESCRIPTION OF THE INVENTION

The invention generally relates to a system and circuit for charging a battery where another battery, such as a vehicle battery, is acting as the power source. Controlling such charging may be useful in circumstances where unregulated discharging of the source battery may be problematic. For instance, use of the disclosed system and circuit ensures that a vehicle battery will maintain sufficient capacity for an engine start, despite acting as the power source for charging a secondary, or auxiliary, battery. The secondary battery may be used in connection with powering an accessory installed in the vehicle, such as a vacuum cleaner system. Operation of the disclosed circuit or system is fully automatic, thereby maintaining the vacuum cleaner system in a ready-to-use condition, without the need for intervention from an operator or user.

While the disclosed system and circuit are described herein in a vehicular context, and in connection with a vacuum cleaner accessory application, practice of the disclosed system and circuit is not limited to any such context or application. Rather, the disclosed system and circuit may be useful in a number of different applications in which a secondary, or auxiliary, battery is charged by a primary, or main, battery acting as the power source.

Generally speaking, the voltage of the primary battery is monitored to control and adjust the power consumption effected by the charging of the secondary battery. The power consumption is adjusted via selection of one of several operational modes, which include multiple charging modes and a non-charging mode. The primary battery may be monitored such that power consumption is minimized (i.e., consumed in an energy-efficient manner), which, in turn, allows the voltage to be continually monitored. Continual monitoring of the primary battery voltage involves detecting the voltage repeatedly during all of the operational modes, and under all conditions. For instance, the voltage may be monitored after the primary battery capacity has decreased to a point where further charging of the secondary battery is not permitted (i.e., the non-charging mode), because the monitoring consumes such little power. Circuit elements detect whether the vehicle engine is running, to what condition the electrical system is loaded, and whether it is safe to divert power to the battery charging system for the accessory (e.g., the vacuum cleaner system). The safety of the diversion is premised upon the requirement that nothing within the disclosed system or circuit will compromise the vehicle engine starting function.

With reference now toFIG. 1, a vehicle electrical system indicated generally at10includes a primary battery12having a positive terminal14and a negative terminal16. The voltage across the positive and negative terminals14,16provides a power source for any number of vehicle accessories, including audio equipment, power windows, power door locks, and the like. (not shown). The primary battery12serves as the direct or indirect power source for most, if not all, of the electrical equipment and devices associated with the vehicle, though some vehicle accessories, such as security systems, may have a supplemental power source. One or more of these vehicle accessories may cause the terminal voltage to fluctuate as load conditions vary, or to decrease over time due to degradation in battery capacity. While an alternator (not shown) may assist in the recharging of the primary battery12via conversion of mechanical energy generated by the engine, there may be extended periods of time between instances of engine operation. In such cases, regulating the discharging of the primary battery12in accordance with the disclosed circuit and system may help ensure that sufficient capacity is available for the starter motor, which typically requires significant current to crank the engine.

The primary battery12in automotive vehicles is typically a lead-acid battery, but the type of battery used as the primary, or source, battery is not germane to the practice of the disclosed system and circuit. In fact, application of the disclosed system and circuit may be advantageous in connection with any power source susceptible to undesirable degradation in capacity. Lead-acid and other batteries often exhibit a decreased terminal voltage after an excessive or significant amount of use. In circumstances when the vehicle engine will not be running, the terminal voltage may also decrease with the operation of one or more vehicle accessories. For example, a standard, “12-Volt” automotive battery may rise to about 13.8 Volts or higher during engine operation, but voltage levels as low as 12.6 Volts are generally regarded as fully, or highly, charged with sufficient capacity for powering certain accessories. Terminal voltages between about 12.4 and about 12.6 Volts may indicate a decreased capacity for powering accessories, and voltages below that range may be avoided to reserve sufficient starting capacity. The disclosed system and circuit generally distinguishes between these voltages or voltage ranges to select an operational mode appropriate for the condition of the vehicle battery12. Of course, other voltages or voltage ranges may be used in alternative embodiments where, for instance, other source batteries are applicable, and where discharge, terminal voltage, or other characteristics may differ.

The vehicle electrical system10includes a secondary, or accessory, battery18coupled to the vehicle battery12. The secondary, or auxiliary, battery18may be any type of rechargeable battery suitable for the vacuum cleaner system or other vehicle accessory or load to be powered by the secondary battery18. The secondary battery18may be composed of any number of cells arranged in series, each of which may have any cell voltage. The secondary battery18and the vehicle battery12often have different voltages, but may, in certain embodiments, have approximately the same voltage when fully charged. In one embodiment, the secondary battery18is a set of series-connected, sub-C size, NiCd battery cells, although other cell sizes and battery types (e.g., Li-ion, Li-polymer) may be used. Generally, the size, type, and number of secondary battery cells are selected based on accessory operation requirements. Moreover, the number of cells is also selected in consideration of charging requirements. For instance, individual cell terminal voltage rises when fast-charging Ni—Cd cells, such that the charging voltage should be a total of 1.9 Volts per cell plus 1.5 Volts.

With continuing reference to the exemplary embodiment ofFIG. 1, the secondary battery18powers a vacuum cleaner system depicted as a single motor20for ease in illustration. The motor20is coupled to the secondary battery18via a switch22made available to a user for controlling operation of the vacuum cleaner system. In this embodiment, the secondary battery18is coupled to the common element of the switch22, such that charging of the secondary battery18occurs when the motor20is not operating, i.e., when the switch22is in the off position. Conversely, charging is discontinued while the motor20is running. The switch22may be otherwise configured to allow for differing or additional operational modes, and switches other than the two-position switch22shown inFIG. 1may be used, as desired. Moreover, the switch22may be a relay or other device incorporating a switch or other control element remotely located from the system10, such as in a vehicle dashboard or other location convenient for the user. More generally, the switch22may be one of multiple switches controlling various motors or other aspects of the vacuum cleaner system. Additional switches, or alternative switch configurations, may be desirable in the event of additional operational modes, such as a mode during which the secondary battery18is charged while the motor20is powered.

The vacuum cleaner system or other accessories powered by the secondary battery18may be disposed in the vehicle in any manner, as desired. The vacuum cleaner system may be semi-detachably mounted such that the motor20or other vacuum cleaner components may be portable. To this end, one or more of the components of the system10may be integrated with a portion of the vehicle interior as a manufacturer-installed accessory.

In one exemplary embodiment, the vacuum motor20is designed to run on 18 Volts DC to provide a power level adequate for typical levels of vacuum cleaner performance. However, other motors of differing sizes and voltage requirements may be used as well. For instance, a vacuum cleaner system having a motor that runs on about 12 Volts may be selected in the interest of consistency with the typical voltage provided by the vehicle battery12. In the 18-Volt example, fifteen Ni—Cd cells are used to compose the secondary battery18, thereby requiring a fast-charging voltage of about 30 Volts with current levels of approximately 2300-2500 mA per hour.

To provide the requisite charging current at the appropriate voltage, a battery charger control circuit24couples the vehicle battery12to the secondary battery18when the switch22is in the charging position, as it is shown inFIG. 1. In the exemplary embodiment ofFIG. 1, the control circuit24is continuously connected to the vehicle battery12even though charging may be called for via the switch22only intermittently. Such continuous connection allows the vehicle battery12to be monitored under all conditions. In alternative embodiments, the control circuit24may be part of a detachable assembly such that the connection is temporarily lost. Other embodiments may have a switch or other coupling device to control the connection.

The control circuit24includes a voltage detector26to monitor the condition of the vehicle battery12. The voltage detector26intermittently samples the terminal voltage of the vehicle battery12, which may be particularly useful in embodiments where the control circuit24is continuously connected to the battery12, as shown inFIG. 1. In this manner, monitoring of the vehicle battery12does not present a significant drain on battery capacity. In one embodiment, the voltage detector26utilizes a strobing technique based on an oscillator28that sets a low sampling frequency, such as about 1-5 Hz. More generally, the sampling may occur at regular or irregular intervals, such that the sampling rate or frequency may be modulated in accordance with operating conditions or other considerations, as desired. Other embodiments need not involve voltage monitoring or detection schemes that rely on or include either strobing, such as where the control circuit24is not continuously connected to the vehicle battery12, or sampling, such as where the power drain from monitoring the source voltage continuously is not a significant concern.

A number of different commercially available voltage detection devices may be used as, or in connection with, the voltage detector26. Such devices may involve any combination of circuitry, other hardware, and software. Generally speaking, however, the voltage detector26generates one or more output signals carrying information indicative of the condition or voltage of the vehicle battery12. The one or more output signals are used to select one of a plurality of operational or charging modes for the control circuit24such that the charging mode is selected based on, or in accordance with, the condition (e.g., terminal voltage) of the vehicle battery12. Depending on the type of voltage detection device, the information provided by the voltage detector26may be supplied in digital or analog form and, in either case, may be provided over one or more lines.

In certain embodiments, the control circuit24includes mode-select logic circuitry30in communication with the voltage detector26. The mode-select logic circuitry30is responsive to the one or more signals generated by the voltage detector26to generate, in turn, control signals indicative of the operational or charging mode. The manner in which the circuitry30generates the control signals depends on whether the one or more signals generated by the voltage detector26are analog or digital, or incorporate one or more voltage threshold comparisons. The mode-select logic circuitry30may be particularly useful in embodiments where the signals generated by the voltage detector26are solely indicative of a voltage magnitude. The mode-select logic circuitry30may also be useful in connection with voltage detectors that generate one or more signals indicative both of voltage and a voltage comparison with certain thresholds. In such cases, the mode-select logic circuitry30may analyze or process the signal(s) to generate one or more control signals indicative of the charging mode. In alternative embodiments, the voltage detector26generates such control signals directly, such that the voltage detector26effectively has the mode-select logic integrated therein.

The different charging modes of the control circuit24provide a flexible approach to charging the secondary battery18. For instance, a charging mode having a lower charging rate may enable continued charging even though the voltage detector26has sensed that the capacity of the battery12has begun to degrade. With the voltage detector26continuing to monitor the battery12, the charging then may begin at a higher rate to support a quick charge cycle. If the secondary battery18does not reach full charge before detection of voltage level degradation, the charging then continues at the lower rate without the risk of reducing the vehicle battery capacity to a point that would compromise an engine start. To this end, the control circuit24includes the mode-select logic either in communication and/or integrated with the voltage detector26to toggle or switch between the multiple charging modes. Moreover, the disclosed circuit and system may involve any number of lower and higher charging modes, and is not limited to a two-charging mode approach.

In one exemplary embodiment, the control circuit24and, in particular, the voltage detector26and the mode-select logic circuitry30, discriminate between three voltage ranges: (i) below 12.4 Volts; (ii) between 12.4-12.6 Volts; and, (iii) above 12.6 Volts. Thus, three operational modes are available. While one of the operational modes may, in fact, correspond with an absence of charging (i.e., a zero rate of charging), the other two modes establish charging at differing, non-zero rates. The specific levels of the two non-zero rates is a matter of design choice, but generally the differing rates may correspond with low and high charging rates, where the lower charging rate is designed to present a lower risk of detrimental vehicle battery discharge. In certain embodiments, the lower charging rate may be considered a trickle charge. Trickle charging rates may be about 150 mA, but may range both above and below that charging rate, depending on the number of secondary battery cells, vehicle battery capacity, and other considerations apparent to those skilled in the art given the primary battery12, the secondary battery18, the control circuit24, and other aspects of the system. More generally, the trickle charging rate may be set to any rate lower than the maximum charging rate, and need not be limited to industry standard rates such as C/16 (i.e., about 150 mA).

In certain embodiments, the mode-select logic circuitry30couples the voltage detector26to a charge controller32, which enables charging in accordance with the selected charging mode. The charge controller32drives the charging based on the control signals indicative of the charging mode, which may be generated by either the voltage detector26and/or the mode-select logic circuitry30. Thus, the charge controller32and the voltage detector26may be coupled via one or more control lines, which may, but need not, be processed by intermediate circuitry, such as the mode-select logic circuitry30.

The charge controller32may include a discrete IC, such as the aforementioned MAX713 fast-charge controller (as shown inFIG. 2) or, alternatively, the similar MAX712 controller, both of which are commercially available from MAXIM Integrated Products. More generally, the charge controller32may be any one of a number of different charge controllers either commercially available or known to those skilled in the art, and is not limited to any IC, circuit, or other configuration. For instance, the charge controller32may differ depending on the type, nature or capacity of the secondary battery18and, thus, should not be limited to the components or configuration described and shown herein.

In one embodiment, the charge controller32provides charge controller functionality known to those skilled in the art, including, without limitation, (i) fast and trickle charging rates, (ii) fast-charge cutoff based on voltage slope, temperature, and time, (iii) charging current regulation, and (iv) linear or switch-mode power control. However, one or more of these functions need not be provided by, or integrated into, an IC package. In fact, discrete ICs such as the MAX713 may rely on external components, such as current sense and other resistors, power transistors and diodes, as explained further below and in the MAXIM product specification entitled “MAX712/MAX713: NiCd/NiMH Battery Fast-Charge Controllers,” the disclosure of which is hereby incorporated by reference.

In alternative embodiments, the charge controller32includes circuitry for additional functionality, such as directly powering a load while charging the secondary battery18, NiMH charge control, adjustable trickle charging rates, and the capability of charging any number (e.g., 1 to 16) of cells. Some of these capabilities may be provided by the MAX713 IC when configured in a manner other than that shown inFIG. 2. In any event, the nature and details of the circuitry necessary to implement such charge control functionality are well understood by those skilled in the art given, for instance, the relevant product specification materials and the teachings of the present invention.

In embodiments where the secondary battery18has a different voltage than the vehicle battery12, the control circuit24also includes a boost converter34coupled to the charge controller32. The boost converter34also may be useful when a fast charging scheme provided by the charge controller32causes the cell voltages to rise during charging, as set forth above. The boost converter34, or step-up DC-DC controller or regulator, is generally configurable or adjustable to provide a charging current in accordance with the current driven or supplied by the charge controller32and at a voltage level appropriate for the secondary battery18. As is well known to those skilled in the art, the boost converter34may be a switching converter, such that the charging voltage (e.g., 30 Volts) may be provided in cyclical fashion.

With reference now toFIG. 2, where elements common to multiple figures are identified with like reference numerals, an exemplary embodiment of the control circuit24(FIG. 1) is shown in greater detail in connection with other components of the electrical system10, such as the vehicle battery12and the secondary battery18. In this embodiment, the components of the control circuit24shown inFIG. 1(i.e., voltage detector, charge controller, etc.) are established with respective, discrete ICs. Discrete ICs, however, are not necessary, but rather provide one convenient, off-the-shelf approach. In this exemplary embodiment, the voltage detector26includes a window comparator IC50that samples the voltage of the vehicle battery12to select the operational mode. The charge controller32includes a fast-charge controller IC52, and the boost converter34includes a DC-DC converter IC54. Apart from the respective ICs, the components of the control circuit24also include other circuit elements, as described below. For purposes of clarity and ease in description and illustration, the reference numerals ofFIG. 1will be used to refer to the IC and any related circuit elements collectively. The reference numerals50,52and54will refer to the ICs individually, it being understood that such related circuit elements may be integrated to a greater or lesser extent into the IC as a matter of design choice. For example, the voltage detector or window comparator26includes a number of circuit elements, including the window comparator IC50and several resistors that provide voltage divider and other functions. The resistors and other elements related to the window comparator26will be further described in connection with the operation of the window comparator26.

Using the window comparator26, the disclosed circuit and system discriminates between three voltage levels: (1) a terminal voltage level of 12.6 to 13.8 Volts and above enables a fast-charge mode over, for instance, a full, fast-charge cycle; (2) a terminal voltage level of 12.4-12.6 Volts places the system in a trickle charge mode; and, (3) a terminal voltage below 12.4 Volts places the system into a zero charging rate, or shutdown, mode. When in shutdown mode, the only power drawn by the disclosed system from the vehicle battery12is a negligible sampling current drawn at, for instance, one second intervals. These three voltage levels or ranges correspond with three operational modes, i.e., two charging modes and a non-charging mode. The output generated by the window comparator26helps to select one of the operational modes.

Monitoring of the voltage level of the vehicle battery12is accomplished whenever the control circuit24, specifically the voltage detector26or window comparator26, is connected. As described above, certain embodiments may provide the option of uninstalling the entire vacuum cleaner system from the vehicle, or providing a mechanism for disconnecting the vacuum cleaner system from the vehicle electrical system. Apart from such or similar instances, the control circuit24is connected to the vehicle battery12for continual monitoring.

A number of commercially available ICs may be used as the window comparator IC50, or as two comparator ICs working in tandem to form the window. In one embodiment, the window comparator IC50is a window comparator available from Linear Technology (Milpitas, Calif.; www.linear.com) under the product name LTC 1042N. This window comparator IC50places a very slight load (e.g., on the order of 0.48 μW) on the vehicle battery12by using a strobing technique to read the input voltage only during a short (e.g., 80 μsec) sampling period. During the sampling period, power is turned on to the window comparator IC50, input voltage values are read and stored in internal CMOS output latches, and then power to the comparator IC50is turned off until the next sample cycle. The sampling rate can be set as low as, for instance, about once every second to minimize power dissipation almost to the point of being arbitrary. In the exemplary embodiment ofFIG. 2, the sampling rate is set, in fact, at about 1.0 Hertz. Further details regarding this exemplary embodiment of the window comparator IC50may be found in the specification published by Linear Technology and entitled “LTC1042N Window Comparator,” the disclosure of which is hereby incorporated by reference.

The voltage detector26may alternatively be composed of two separate comparators, either integrated into respective chips or composed of non-integrated components. In either case, the comparators evaluate the voltage of the vehicle battery12based on two, respective voltage thresholds. Additional circuit elements to provide output signals similar to those generated from a window comparator will be apparent to those skilled in the art. For instance, alternative configurations or devices for the voltage detector or window comparator26may include one or more discrete IC voltage detectors providing single threshold functionality.

With continued reference to the exemplary embodiment ofFIG. 2, the sampling rate is established by an RC circuit having a resistor56and a capacitor58, which self-strobes an internal circuit of the IC50. The RC circuit is connected to the OSC pin (i.e., oscillator) of the window comparator IC50. The RC circuit operates with circuitry internal to the window comparator IC50to provide the oscillator28. Alternatively, the device may be externally strobed by driving the OSC pin of the IC50with a CMOS or other gate (not shown).

The window comparator26includes voltage-divider resistors60and62to set the width of the voltage window, and a resistor64and a Zener diode66to set the mid-point, or center, of the window. Two high-impedance inputs at the WC pin (i.e., window center) and at the WID2 pin (i.e., width/2) are supplied the voltages developed by these elements. For the exemplary voltage ranges given above, the window width is 0.2 Volts and the window center is 12.5 Volts. However, the window comparator26is configured to evaluate the vehicle battery12at half of the terminal voltage level. In other words, a voltage divider having resistors68and70divides the voltage level in half before being delivered to the VIN pin of the window comparator IC50. This allows the vehicle battery12to act as the power supply for the window comparator IC50despite being the monitored voltage as well. To that end, the terminal voltage of the vehicle battery12is applied to the V+ pin of the window comparator IC50. As a further consequence, the window width is set to about 0.1 Volts (i.e., half of the actual 0.2 Volts between 12.4 and 12.6 Volts) and the window center is about 6.25 Volts.

To set a window width of about 0.1 Volts, the exemplary embodiment ofFIG. 2includes the voltage divider provided by the resistors60and62, the voltage divider being configured to supply a voltage of approximately 0.04-0.05 Volts to the WID2 pin. In this case, the Zener diode66has a breakdown voltage of 6.2 Volts, which would effectively set the window center at 12.4 Volts. However, the window may be re-centered or otherwise adjusted to, for instance, 6.25 Volts using the resistor68. The Zener diode66may have a tolerance of 1.0%. The foregoing voltage window characteristics and corresponding component values are presented with the understanding that they may vary between different embodiments or applications of the teachings of the present invention.

In this embodiment, the window comparator26generates two output signals indicative of the voltage of the vehicle battery12. The first signal is indicative of whether the voltage is above the window, and is provided at the ABOV pin (i.e., above) of the window comparator IC50on a line72. The second signal is indicative of whether the voltage is within the window, and is provided at the WITH pin (i.e., within) of the window comparator IC50on a line74. In this embodiment, each of these output signals is digital in the sense that a high or active output (e.g., 5 Volts) indicates that the voltage is within the range in question. Taken together, the states of these two signals are utilized to determine the operational mode of the control circuit24. If both are low or inactive, then the vehicle battery12is below the voltage window. When one or the other goes active or high, the vehicle battery12is either within or above the window. In this manner, the vehicle battery terminal voltage determines the collective state of the signals, which, in turn, determine or select the operational mode.

The exemplary embodiment ofFIG. 2includes mode-select control logic (see, e.g., element30ofFIG. 1) used to evaluate the two control signals. Part of such logic is integrated in the window comparator26in this embodiment, insofar as the two control signals on the lines72and74already represent the logic involved in the window comparisons. As a result, the signal on the line74is essentially supplied directly to the charge controller IC52, with the exception of signal conditioning in the form of a voltage divider based on resistors76and78. In alternative embodiments, the voltage detector26may generate an analog or digital representation of the vehicle battery terminal voltage, which then must be processed by comparator logic to generate the two control signals. The degree to which the voltage detector26incorporates, or does not incorporate, the mode-select logic is a matter of design choice. In the embodiment ofFIG. 2, the voltage detector26includes the comparator logic, but leaves a remainder of the mode-select logic30for external components. That is, a portion of the mode-select logic30is external to the window comparator26in the sense that additional logic circuit elements couple the window comparator26and the charge controller32. More specifically, the mode-select control logic30includes (i) a diode OR gate formed by first and second diodes80and82, and (ii) a control switch84. The diodes80and82may be any type of low-power diode, such as the 1N4148 diode IC commercially available from Diodes, Inc. (Westlake Village, Calif.). The diodes80and82may be packaged as two separate, discrete diodes or, alternatively, packaged in a single IC device. The control switch84may be a power MOSFET such as the IRLR2905Z MOSFET commercially available from International Rectifier (El Segundo, Calif.).

In operation, the diodes80and82provide a high or active signal to the gate of the control switch84when the terminal voltage of the vehicle battery12is either within or above the voltage window established by the window comparator26. The diodes80and82thus assure that a gate drive is provided to the control switch, or MOSFET,84in both the trickle charge and fast-charge operational modes. Consequently, a voltage above 12.4 Volts activates the control switch84, which establishes a ground connection for the charge controller32and the DC-DC converter34along a line86. Without the ground connection, these components of the control circuit24are not powered, and charging of the secondary battery18does not occur, meaning that the control circuit24is in the shutdown (or non-charging) mode. When those components are powered, the signal generated on the line74is determinative of which one of the two charging modes the control circuit24will operate in. If the terminal voltage of the vehicle battery12is above the voltage window, then the output on the line74is inactive, and the resistor78pulls a line88coupling the window comparator26to the charge controller IC52to ground.

The mode-select logic circuit30also includes a resistor89that helps to stabilize the signal provided to the gate of the switch84. Without the resistor89, small voltage fluctuations due to noise in the control circuit24may be sufficiently sizeable to provide a false signal that activates the MOSFET and undesirably causes the control circuit24to charge the secondary battery18, thereby discharging the vehicle battery12during periods when the charge controller32should be in the shutdown or non-charging mode.

With continued reference to the exemplary embodiment ofFIG. 2, the charge controller IC52is coupled to the window comparator26for charging in accordance with the selected charging mode. The charge controller IC52is a pin-programmable IC having a number of control input ports, including pins for programming the number of cells. (PM0, PM1), fast-charge timeout (PM2, PM3), trickle/fast-charge current ratio (PM3), and full charge detection scheme (TMP). The charge controller IC52may also be programmed to switch from the fast-charge mode to the trickle charge mode via an under-temperature comparison (pin TLO), an over-temperature comparison (pin THI), and a battery voltage level threshold detection (pins BT+, BT−). The programming of the charge controller IC52may vary considerably based on the charging and other requirements of the secondary battery18, as well as the load powered thereby. Further details regarding its operation and pin-programming options will be well known to those skilled in the art based on the above-referenced specification publication available from the manufacturer. Consequently, the operation of the charge controller IC52will not be described in detail herein.

Of the several ways in which the charge controller IC52may be directed to switch between charging modes, the exemplary embodiment ofFIG. 2utilizes the temperature threshold pin, TLO (despite not utilizing a temperature-indicative control signal). Instead, the control signal on the line88is delivered to the TLO pin of the charge controller IC52. In operation, the TLO pin is pulled to ground by the resistor78unless driven by the control signal on the line74from the window comparator IC50, meaning that the default mode for the charge controller IC52is the fast charge mode. However, if the TLO pin is above 2.00 Volts (a reference provided internally in the charge controller IC52), fast charging is disabled. This is the case when the WITH pin of the window comparator IC50is high or active, indicating the vehicle battery is between 12.4 and 12.6 volts. Alternately, if the TLO pin starts out low or goes low any time after power up, then fast charging is enabled. For example, when the ABOV pin of the window comparator IC50goes high, indicating battery voltage is 12.6 or above, the WITH pin will coincidentally go low, enabling fast charge.

The charge controller IC52provides a negative slope detection option for detecting when the secondary battery18is fully charged. Other available approaches include options that utilize an external thermistor connected to the TEMP pin, which is not utilized in this embodiment. The negative slope detection involves a voltage-slope detecting analog-to-digital converter internal to the charge controller IC52and made available between the BT+ and BT− pins, which are coupled to the positive and negative terminals of the secondary battery18, respectively. Throughout the charging process, the voltage across the secondary battery18will rise until a peak is reached. Once that peak is reached, the charge controller IC52switches to the trickle mode. More specifically, when Ni-Cd cells are fully charged, their terminal voltage will dip slightly below the peak level. This voltage dip, or negative slope, is detected by the charge controller IC52, causing the fast charge mode to terminate until power is cycled. The fast charging process may otherwise last up to 66 minutes, i.e., the default timeout period that may be modified via pin programming.

The charge controller IC52also provides a mechanism for providing operational feedback to a user. When in the fast-charge mode, the pin FSG of the charge controller IC52acts as a current sink, enabling current to flow from the positive terminal14of the vehicle battery12through a single LED status indicator90and a current-limiting resistor92. The indicator90is ON only during the fast-charge cycle. While any number of status indicators or other information may be displayed to a user, the number of status indicators may be limited to minimize any additional burden on the vehicle battery12while the vehicle is not running.

The manner in which the charge controller32enables charging in accordance with one of the charging modes will now be described. With the charge controller IC52ofFIG. 2, the charge controller32may be configured in a switch-mode configuration or a linear mode configuration. In the exemplary embodiment ofFIG. 2, the charge controller32is configured in the linear mode to facilitate simulation of a single BJT power transistor as described in the above-referenced specification publication for the charge controller IC52. Alternative embodiments may be configured in the switch mode when use of a single BJT is not practical due to excessive heat or power dissipation, such as when higher output currents are required or when directly charging a battery in a non-boost mode.

With continued reference toFIG. 2, the linear-mode configuration of the charge controller32may use a current mirror to simulate the load of the BJT power transistor. The current mirror is coupled to an output port (i.e., the DRV pin) of the charge controller IC52, where either trickle or fast charge output is generated. The current mirror includes two PNP transistors94and96, which may be housed in a transistor package such as 2N3906 available from National Semiconductor (Santa Clara, Calif.). In embodiments having PNP transistors, the output of the charge controller32is, in fact, a current sink. Alternative embodiments may include other transistor types or configurations, together with any accompanying circuitry to accommodate such transistor types or configurations. Returning to the exemplary embodiment ofFIG. 2, the output port of charge controller IC52is the DRV pin and the transistors94and96present a current follower for the current controlled or generated at the output port. The respective currents in the two branches of the current mirror having resistors98and100are thus equal, and a line102taps the current mirror to drive a power MOSFET104via a gate resistor106. The current flowing through the transistor96and a resistor108sets the voltage on the line102, thereby determining the on-resistance of the MOSFET104. In this manner, the drive current controlled by the charge controller IC52in accordance with the selected charging mode controls the charging current generated by the step-up, DC-DC converter34. For example, when the control circuit24is in the trickle mode, a low current (e.g., 30 mA) is provided to the current mirror, which produces a low voltage in the collector of transistor96, such that MOSFET104will conduct lightly (i.e., high on-resistance). As will be described further below, the current flowing through the MOSFET104of the step-up converter34is the charging current for the secondary battery18.

In alternative embodiments, the charge controller32does not include a current mirror, but rather the single PNP pass transistor referred to hereinabove. This charging circuit, however, is better suited for a non-boost scheme, i.e., one in which the functionality provided by the boost converter34, as described further hereinbelow, is not utilized.

In embodiments where the charging voltage is higher than the source voltage level provided by the vehicle battery12(e.g., 12 Volts), the charge controller IC52is used in conjunction with the DC-DC step-up converter34such that the maximum battery voltage of the secondary battery18may rise above the power supply voltage provided to the charge controller IC52. The use of the charge controller IC52in the linear mode and in connection with the above-described current mirror arrangement supports coupling it to the boost converter34. In alternative embodiments, the drive current from the charge controller IC52may be directly supplied to the secondary battery18, such as when the secondary battery18reaches a maximum charging voltage 1.5 Volts less than source voltage for the charge controller IC52, which is set at the V+ pin via a resistor109and stabilized via a capacitor110. The above-referenced specification for the charge controller IC52may be consulted for further information regarding such instances.

The boost converter34is coupled to the charge controller IC52via the MOSFET104. As a switching boost converter, the boost converter arrangement also includes a switching power MOSFET111, charging inductor112, Schottky diode114, and capacitor116, coupled in the manner customary for providing a pulsed charging current at a frequency of, for instance, 300 kHz. At such high frequencies, these components may be selected for the capability of turning off quickly during the portions of the charging cycle. For instance, the capacitor116may consist of a solid electrolyte type and, thus, have a very low impedance. Moreover, these and other components of the boost converter34may have component values to support various levels of charging capacity, as desired. More generally, the boost converter34may include a high-current converter (i.e., a boost converter having high current capacity) in embodiments where the secondary battery18is charged during operation of the motor20(or other load).

As the switching aspect of the boost converter34of the exemplary embodiments shown in the figures is well known to those skilled in the art, its operation will only be briefly described herein and in connection with the configuration of the DC-DC converter IC54. In short, whenever the step-up switching controller IC54is powered, it attempts to drive the MOSFET111at maximum output via its output port (pin EXT). The output port of the DC-DC converter IC54provides an oscillating output on a line118to the gate of the MOSFET111. During the portion of the cycle that the MOSFET111is on, current is flowing effectively to ground through the inductor112, which consequently quickly energizes. At this point, the diode114is preventing the secondary battery18from discharging to ground. The diode114also isolates the accumulated charge (from a prior cycle) on the capacitor116from a similar discharge path. The capacitor116and its accumulated charge instead charges the secondary battery18with a smooth charging current (i.e., with a reduced ripple). The other portion of the cycle then occurs once the switching output causes the MOSFET111to open. At that point, the voltage across the inductor112collapses, and the voltage between the inductor112and the diode114goes to the positive terminal of the vehicle battery12(e.g., 12-14 Volts). With the diode114forward biased, the current flowing through the inductor112rapidly charges the capacitor116, storing sufficient energy to charge the capacitor116to a voltage higher than the level of the vehicle battery12. In this manner, the boost converter34boosts the charging voltage to a level (e.g., 30 Volts) higher than the source voltage level.

The step-up, DC-DC converter IC54may be a MAX1771 controller available from Maxim Integrated Products configured in a conventional fashion. Further details regarding its operation may be found in the manufacturer-published specification entitled “MAX1771: 12V or Adjustable, High-Efficiency, Low IQ, Step-Up DC-DC Controller,” the disclosure of which is hereby incorporated by reference. In short, the DC-DC converter IC54is configured to drive the MOSFET111at maximum power by setting a sufficiently low threshold for full-on operation via the feedback input pin (i.e., FB pin). More specifically, resistors120and122form a voltage divider to set the low threshold.

Despite being driven at full-on operation, the current through the MOSFET111is controlled via the on-resistance of the MOSFET104, which, in turn, is set via the current mirror and the output from the charge controller32. When the control circuit24is in trickle mode, the on-resistance of the MOSFET104is high, which limits the amount of current flowing through the charging inductor112, which, in turn, limits the amount of charge stored on the capacitor116during each cycle.

The power MOSFETS104and111may be any type of switching transistor that is suitable for use at switching frequencies and current levels encountered in step-up, DC-DC applications, such as the MOSFET available from International Rectifier (El Segundo, Calif.) under the product name, IRL1104. The power MOSFETS104and111may, but need not, be the same transistor type, and may be disposed on the same integrated circuit. The power MOSFETs may also have an intrinsic ON resistance, or Rds, suitable for significant power handling.

The fast-charge mode current may be supported by selecting components having ratings suitable for the fast-charge current (e.g., 2.5 A). For instance, the inductor112, diode114and capacitor116may be rated for 2.5 A or higher current levels. For instance, in the exemplary embodiment ofFIG. 2, the inductor112may be the inductor commercially available from Sumida (Chuo-ku, Tokyo, Japan) under the product name CDRH127-470MC, which is rated for current levels of 2.5 A due to suitable wire gauge and insulation. The diode114may be rated for current and voltage levels as high as 3.0 A and 40 Volts, respectively.

The boost converter34also includes a 30-Volt Zener diode122(1N4751A) to limit the maximum voltage produced by the converter34during the portions of the charging cycle when the secondary battery18is not connected. Any one of a number of commercially available diodes may be used for the Zener diode.122, such as the 1N4751A available from Diodes, Inc. (Westlake Village, Calif.). The Zener diode122may be rated for 1 Watt.

The charge controller32and the boost converter34may have several other circuit elements used to control, condition, or generate the signals provided to, or generated by, one or more input or output ports or transistor gates. For example, capacitors124,126,128,130,132, and134of the exemplary embodiment ofFIG. 2are used to, for instance, stabilize the signals on respective lines leading to the input ports or gates. These capacitors may also have one or more additional purposes, as will be apparent to one skilled in the art. The above-identified circuit components or elements may have the following values or specifications, it being understood that the values or specifications are exemplary only and may vary from those shown and still embody the disclosed circuit and system.

The charge controller IC52may regulate the charging current by monitoring and regulating the voltage across a sense resistor136(e.g., 0.1 Ohms, rated for 1 Watt). The sense resistor136helps to set the fast-charge current level and, in the exemplary embodiment having the MAX713 IC as the charge controller IC52, the voltage drop across the sense resistor136is regulated to 250 mV.

With reference to the exemplary embodiment ofFIG. 2, the number of cells in the secondary battery18makes it possible for the maximum voltage to exceed the rating for the BT+ pin. Specifically, the BT+ pin of the charge controller IC52may be rated for handling voltages associated with, for instance, only 11 NiCd cells when the IC52is not powered (e.g., when the control switch84disconnects the IC52from the power source). Consequently, the IC52may be protected during such periods via a transistor138. With the IC52not grounded, the transistor138will be turned off because its return is blocked. Once the IC52is powered, the BT+ pin can be connected to the positive terminal of the secondary battery18via the transistor138. The connection is enabled by the application of the primary battery voltage to the base of the transistor138via a resistor140. The transistor 2N3904 from National Semiconductor (Santa Clara, Calif.) may be used as the transistor138.

In alternative embodiments, the diode-based OR gate of the mode-select logic circuitry may be replaced with a transistor-based configuration. Such embodiments may, but need not, involve coupling the voltage detector to the charge controller in a different manner, and one that does not establish power via a switched connection to ground. For instance, the power connection may be established by coupling the positive terminal14of the vehicle battery12to the charge controller IC52and boost converter34via one of the transistors in the mode-select circuitry. Instead of using the low temperature comparison to toggle between charging modes, such alternative embodiments may use a control signal provided to the high temperature comparison input port of the charge controller IC52.

In other alternative embodiments, the logic provided by the OR gate and control switch84may be integrated into the voltage detector26, or the window comparator IC50, to any extent, as desired.

Other types of boost converters or DC-DC converters known to those skilled in the art may be used to step up the charging voltage in connection with the battery charger circuit given the teachings of the present invention, including single-ended pulse inverting converters (or SEPIC converters) and flyback converters.

In accordance with the above-described embodiments, the disclosed circuit and system regulates the discharge of the vehicle battery12by monitoring the terminal voltage across the battery terminals14and16and regulating the charging of the secondary battery18based on the measured terminal voltage. In this manner, the charging rate may be adjusted based on the condition of the vehicle battery12. A high charging rate may enable a charging of the secondary battery18from depletion to full charge in a reasonable time period, such as one hour. An adjustment to a lower charging rate may allow continued charging of the secondary battery18, where higher rates may risk compromising vehicle battery capacity for engine start.

Practice of the disclosed system and circuit is also not limited to the types of batteries used as the primary and secondary batteries. Furthermore, in certain embodiments, the power source need not be a battery. Thus, the disclosed system and circuit may be applied in connection with any rechargeable battery to be recharged under circumstances where the condition of the source voltage should be monitored to ensure that the power source is not adversely affected by the charging operation.

The control circuit24may be built or manufactured in accordance with any circuit fabrication or design methodology and materials. In one embodiment, the circuit24is built on a standard FR-4 PC board with very small surface mount components such that the board dimensions are approximately 2″ by 3″ by 1″. However, practice of the disclosed circuit and system is not limited to any particular circuit board implementation, nor is it limited to embodiments having surface mount components. Alternative embodiments may integrate some or all of the components in an application-specific integrated circuit (ASIC) or similar integrated configuration of either current or future design. Some embodiments may also utilize software executed by a general-purpose or other processor to implement any portion of the control logic embodied in the circuits and/or circuit components shown in connection with the voltage detector26, the charge controller32, or other elements of the battery charging control circuit. For instance, although the mode select logic circuitry30is shown and described as implemented in IC and component hardware, any combination of circuitry, other hardware, and software may be utilized, as will be understood by those skilled in the art.