Abstract:
A charging circuit for regulating current to a plurality of batteries is provided. The regulation is performed by a single control signal that is optionally scaled by resistor dividers. Switches coupled to the resistor dividers allow a microprocessor to actuate a particular resistor divider, thereby scaling the current flowing through a corresponding battery. As such, a single control signal, like a pulse width modulated signal, may be used to cause different currents to flow to different batteries, thereby leaving other output pins of the microprocessor open for other functions.

Description:
BACKGROUND 
     1. Technical Field 
     This invention relates generally to battery charging devices, and more particularly to multiple battery charging devices for electronic devices. 
     2. Background Art 
     Portable computers, personal digital assistants (PDA&#39;s), cellular telephones, pagers, calculators, and other such electronic devices are commonplace in today&#39;s mobile society. One of the reasons these electronic devices are so popular is that they are portable, i.e. they provide a user with virtual freedom regarding the location of their use. Although these devices may be powered by plugging them into a standard AC outlet, AC power is often not convenient or accessible. Hence, batteries, by offering a portable source of power, provide portability and added utility to these electronic devices. 
     Although battery technology has progressed greatly in recent decades, a single battery is sometimes unable to meet a user&#39;s demands. For example, many cellular telephone service providers are offering plans with 3000 plus minutes of talk time or more per month. Some business people talk on their cellular phones six or more hours per day. Typical cellular telephone batteries provide only three to four hours of talk time before needing to be recharged. Consequently, some users carry two or more batteries with them so that a spare is ready when the first battery dies. To remain on the go, users now demand shorter charge times in addition to extended battery life. They also want to be able to quickly charge two or more batteries at the same time. 
     One solution to the problem of how to quickly recharge two or more batteries is to charge the batteries serially. In other words, when the two batteries are placed in the same charger, the charger completely charges the first battery. The charger then switches to the second battery and charges it. Total charging time is the charging time of one cell multiplied by the number of cells. Another solution to the problem with quickly recharging multiple batteries was to charge each battery in a charging system with its own power line or charging system. 
     Both prior art solutions, however, present problems. A problem with the first solution is that it is not efficient, in that it takes quite a long time to fully charge both batteries. The second solution is prohibitive in terms of both cost and the amount of space required for multiple charging circuits. For these reasons, there is a need for an adaptive multiple battery charging apparatus that allows simultaneous charging of multiple batteries with a single battery charging system. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrates a prior art charger. 
     FIG. 2 illustrated a prior art charging circuit. 
     FIG. 3 illustrates a charging circuit in accordance with the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A preferred embodiment of the invention is now described in detail. Referring to the drawings, like numbers indicate like parts throughout the views. As used in the description herein and throughout the claims, the following terms take the meanings explicitly associated herein, unless the context clearly dictates otherwise: the meaning of “a,” “an,” and “the” includes plural reference, the meaning of “in” includes “in” and “on.” 
     Referring now to FIG. 1, illustrated therein is a prior art charger  1  having a front pocket  2  and a rear pocket  3 . Typically the front pocket  2  is for receiving a portable electronic device like a cellular telephone  6 , and the rear pocket  3  is for receiving just a spare battery module or pack  5 . As such, the user may charge the battery pack that is coupled to the phone  6  by inserting the base of the phone  6  into the first pocket  2 , while a spare battery pack  5  is installed within the second slot  3 . A connection  4  is provided for coupling the charger  1  to a source of electric power. Some phones are also known that contain a safety switch (SW) in series with the battery, where the switch may be opened and closed by the circuitry  7  within the phone. It is not required that both slots  2  and  3  be used simultaneously, as one slot or the other could be used at any given time. 
     The typical charging circuit for such a charger  1  is shown in FIG. 2. A power source  10  with enough capacity to fully charge two batteries  11 , 12  is provided and coupled to the charging circuit  13 . A microprocessor  14  having at least two analog outputs  15 , 16  drives a pair of corresponding current regulators  17 ,  18  to simultaneously charge the pair of cells. 
     This prior art charging circuit  13  is less than optimum, however, in that it is often difficult and expensive to dedicate two analog outputs  15 , 16  to current control. To begin, today&#39;s chargers are being asked to provide more and more functionality. As such, output pins for microprocessors are often required for fuel gauging, driving displays, channeling data, and other high-end functions. Thus, output pins are in high demand and are limited on any one microprocessor. 
     Second, microprocessors with multiple, on-board Digital to Analog (D/A) converters are expensive. While chargers are being asked to do more, customers are simultaneously demanding lower and lower prices. Thus, the addition of a microprocessor with multiple D/As is not conducive to manufacturing high performance, low cost chargers. 
     This invention solves this problem by providing a charging circuit capable of regulating charging current in two pockets simultaneously by using a single pulse width modulator (PWM). PWMs are switching outputs with variable duty cycles that are less expensive than analog outputs. For this reason, at least one PWM output is routinely found on even the least expensive microprocessors. 
     Referring now to FIG. 3, illustrated therein is one preferred embodiment of a charging circuit in accordance with the invention. The circuit includes a first and second current regulators  20 , 21  that manage the current delivered to a pair of rechargeable batteries  22 , 23 . The current regulators  20 , 21  may be as simple as a transistor operating in the ohmic, or linear, region. 
     The amount of charge current flowing through the batteries  22 , 23  is determined by a pair of reference signals  24 , 25  coupled through linear amplifiers  26 , 27  to the current regulators  20 , 21 . The reference signals  24 , 25  derived from an adjustable control signal  28 . The control signal  28  preferably comprises PWM signal  28  generated by the microprocessor  30  and coupled through a D/A converter  29 . Note that while the D/A converter  29  may be any of a number of types known in the art, in this preferred embodiment the D/A converter comprises a series resistor and parallel capacitor. This R-C filter converts the modulating output to a “ripply” DC level. Once the control signal  28  has been converted to a DC signal, it then becomes reference signals  24 ,  25  for use by the current regulators  20 , 21 . 
     Adjustment of the magnitude of the reference signal  24  is achieved by way of a means for scaling the adjustable control signal. In this preferred embodiment, the means for scaling comprises a resistor divider  32  and a switch  34 . The same is true for reference signal  25  by way of resistor divider  33  and switch  35 . In this preferred embodiment, the switch  34  (or corresponding switch  35 ) comprises a transistor, although other applications may find it more suitable to employ other devices known in the art, like relays, circuit breakers, positive temperature coefficient devices, fuses, etc. Field Effect Transistors (FETs) are preferable due to their specified on impedance. If, for example, a bipolar transistor were used, the microprocessor  30  would need to compensate for any DC offset caused by current flowing through the bipolar transistor. 
     By opening and closing the switches  34 , 35 , the microprocessor  30  may establish varying levels of current flowing through the batteries  22 , 23  for any given PWM duty cycle. The varying level is due to the scaling of the reference signal by way of voltage division created by the resistor dividers  32 , 33 . To see how this is applied in practice, consider the following exemplary scenarios: 
     The first scenario occurs where there is a single battery present. When only one battery is present, all power supply current may be directed to that battery. For instance, if battery  22  is present and battery  23  is not, the PWM is set to a duty cycle that corresponds to the full power supply current. If the power supply can source 525 mA, for example, it is desirable for all 525 mA to flow through current regulator  20 . When switch  34  is open, the output voltage  36  of the D/A converter  29  passes directly through to amplifier  26  (due to its high input impedance) resulting in a 525 mA current flowing through battery  22 . 
     Another scenario occurs when two batteries are present, each with equal charging priority. (Some chargers may be programmed such that the battery coupled to the phone has a higher priority, and thus receives more charging current.) For instance, if battery  22  and battery  23  are both present with equal priority, these batteries should share the charging current equally. Using the 525 mA supply from above, the PWM would be set to a duty cycle half of that in the first scenario. Thus, by opening both switch  34  and switch  35 , each battery  22 , 23  would receive 262 mA of charging current. 
     A third scenario occurs when two batteries are present and one of the batteries takes priority over the other. For example, assume that both battery  22  and battery  23  are present. Further assume that battery  23 , being coupled to a radio  37 , takes priority. (This can be the case because the radio  37  acts as a load. Consequently, the charger must provide more current to battery  23  to satisfy both charging requirements and radio load.) 
     In this scenario, the resistor dividers  32 , 33  provide a weighting factor that facilitates a higher level of current flowing through battery  23 . Staying with the 525 mA exemplary power supply  31 , the microprocessor  30  may set PWM  28  so as to request a current greater than half of the maximum. For discussion purposes assume that this level is 350 mA. By turning on switch  34 , and turning off switch  35 , more current will flow through battery  23  than battery  22 . Battery  23  would receive the full 350 mA. Battery  22  would receive a current scaled down by a factor set by the resistor divider  32 . This scaling factor may be any value between 0 and 1 of the designer&#39;s choosing. 
     Yet another scenario occurs when either of the batteries is in a state that requires a low current. One such example is known as an undervoltage condition. The undervoltage condition occurs when a battery has been discharged too deeply, such that the voltage of the battery has fallen below the manufacturer&#39;s recommended limits. In an undervoltage condition, a small current is to be applied until the voltage across the battery reaches the manufacturer&#39;s minimum limit. In such a scenario, the current in that battery may be reduced by actuating its corresponding switch. In so doing, the actuated switch causes the corresponding resistor divider to scale the reference signal, thereby reducing the current flowing through the corresponding battery. Meanwhile, the switch that has not been actuated allows a current equal to that requested by the control signal to flow through the corresponding battery. 
     For example, assume that battery  22  is in an undervoltage condition requiring no more than 100 mA. If all the resistors in the resistor dividers  32 , 33  are of equal value, the PWM  28  may be set to request a current of 200 mA. By closing switch  34  and opening switch  35 , 100 mA flow through battery  22  and 200 mA flow through battery  23 . Note that in each of the scenarios mentioned above, the batteries are always charged in parallel, and not sequentially. Note also that all current regulation is done by way of a single control signal. 
     While the preferred embodiments of the invention have been illustrated and described, it is clear that the invention is not so limited. Numerous modifications, changes, variations, substitutions, and equivalents will occur to those skilled in the art without departing from the spirit and scope of the present invention as defined by the following claims.