Patent Publication Number: US-8111039-B2

Title: Multiple function current-sharing charging system and method

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 12/763,721, filed Apr. 20, 2010, by Ryan M. Bayne, et al., entitled “Multiple Function Current-Sharing Charging System and Method,” (10933-US-CNT2-4214-00510) which is a continuation application of U.S. patent application Ser. No. 12/275,093, filed Nov. 20, 2008, by Ryan M. Bayne, et al, now issued as U.S. Pat. No. 7,714,534 on May 11, 2010 entitled “Multiple Function Current-Sharing Charging System and Method,” (10933-US-CNT-4214-00505) which is a continuation application of U.S. patent application Ser. No. 10/834,283, filed Apr. 29, 2004, by Ryan M. Bayne, et al, now issued as U.S. Pat. No. 7,471,059 on Dec. 30, 2008 and entitled “Multiple Function Current-Sharing Charging System and Method,” (10933-US-PAT-4214-00500), which is a filing under 35 U.S.C. 119 which claims priority to United Kingdom Patent Application No. GB 0309804.3 filed Apr. 29, 2003, published as GB2401258 B, by Ryan M. Bayne, et al., entitled “Multiple Function Current-Sharing Charging System and Method,” (10933-GB-PAT) which are incorporated by reference herein as if reproduced in their entirety. 
    
    
     BACKGROUND 
     This invention relates generally to charging of rechargeable power supplies such as batteries. 
     Providing an external source of power to a portable device, such as a personal digital assistant (“PDA”), a mobile communication device, a cellular phone, a wireless two-way e-mail communication device, and other types of device, requires design considerations with respect to both the device and the power source. For example, many portable devices provide a power interface for receiving power from a power source, for instance to recharge a battery installed in the device. Charging systems configured for charging rechargeable batteries or other rechargeable power supplies that have been removed from a device are also known. Another known type of charging system is a multiple function charging system configured to charge a power supply whether it is installed in a device or removed from the device. 
     Multiple function charging systems enabled for connection of more than one power supply at a time generally charge power supplies in a serial fashion. One power supply is typically charged at a time. Although more than one power supply, such as a device with a battery installed and a spare battery, may be connected to the charging system, charging current is applied to only one power supply at any time. As such, known multiple function chargers offer no charging time advantage over charging systems that accept only one power supply at a time. In the above example of a device and a spare battery simultaneously connected to a charging system, the battery inside the device is normally charged first, and only then is the spare battery charged. The total charging time for the device battery and the spare battery is substantially the same as the time required to charge each battery separately. 
     SUMMARY 
     According to an aspect of the invention, a multiple function current-sharing charging system comprises a power source interface configured to receive energy from a power source, a power converter connected to the power source and configured to regulate the energy received from the power source and to output charging current, a plurality of power supply interfaces configured for connection to respective rechargeable power supplies, and a charging controller connected to the power converter to receive the charging current and to the plurality of power supply interfaces, and configured to detect connection of a first rechargeable power supply to a first one of the plurality of power supply interfaces, to determine whether a second rechargeable power supply is connected to a second of the plurality of power supply interfaces, and to provide a first charging current to one of the first and second rechargeable power supplies and a second charging current to the other of the first and second rechargeable power supplies where a second rechargeable power supply is connected to a second of the plurality of power supply interfaces. 
     In accordance with another aspect of the invention, a current-sharing charging method for a multiple function charging system comprises the steps of detecting connection of a first rechargeable power supply to the charging system, determining whether a second rechargeable power supply is connected to the charging system, and where a second rechargeable power supply is connected to the charging system, providing a first charging current to the first rechargeable power supply, and providing a second charging current to the second rechargeable power supply. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In order that the invention identified in the claims may be more clearly understood, preferred embodiments thereof will be described in detail by way of example, with reference to the accompanying drawings, in which: 
         FIG. 1  is a block diagram of a multiple function current-sharing charging system. 
         FIG. 2  is a block diagram of a multiple function charging system connected to a mobile device and a battery. 
         FIG. 3  is a block diagram of a multiple function current-sharing charging system incorporating a Universal Serial Bus (USB) interface and a battery receptacle. 
         FIG. 4  is a flow diagram illustrating a current-sharing charging method. 
         FIG. 5  is a block diagram of a wireless mobile communication device. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  is a block diagram of a multiple function current-sharing charging system. The charging system  10  includes a power source interface  12 , a power converter  14 , a charging controller  16 , and rechargeable power supply interfaces  17  and  18 . 
     The power source interface  12  is configured for connection to a power source from which rechargeable power supplies are charged. In one embodiment, the power source interface  12  is a plug unit that can be used to couple with a conventional power socket to receive power therefrom. For example, such a plug unit may be a two prong or three prong plug of the type used in North America that can couple to a North American AC power socket. Alternatively, the power source interface  12  can accept one or more types of plug adapters configured to couple the power source interface  12  to corresponding types of power sockets. The use of interchangeable plug adapters has the advantage of allowing the same charging system to be used with a variety of types of power sources, depending on availability. Thus, the power source interface  12  is configured to receive energy from a power source either directly or through the use of a plug adapter, and is operative to transfer the received energy to the power converter  14 . 
     A power converter such as  14  typically includes at least one of the following components: a switching converter, a transformer, a DC source, a voltage regulator, linear regulator, and a rectifier. The power converter  14  is operative to receive energy from a power source through the power source interface  12 , and to convert that received energy to a form that can be used to charge power supplies connected to the charging system  10 . For example, the power converter  14  can be of substantially conventional construction, such as a switching power converter that converts 115 VAC to 5 VDC. DC-to-DC converters or DC regulators, which convert DC inputs to DC outputs are also common in such power converters. In one embodiment, the power converter  14  is adapted to accept a wide range of input energy levels and frequencies from the power source interface  12 . Alternatively, the power converter  14  is adapted to accept a limited range of input energy levels and frequencies, and the power source interface  12 , or each plug adapter if any, is operable to convert the input energy levels and frequencies into a range that the power converter  14  can accommodate. The power converter  14  provides its energy output to the charging controller  16 . 
     The charging controller  16  controls the amount of charging current applied to each rechargeable power supply connected to the power supply interfaces  17  and  18 . Where only one power supply is connected to an interface  17  or  18 , the charging controller  16  outputs full charging current, which may vary between different power supplies or types of power supply, to that power supply. The charging controller  16  is also configured to implement current sharing between multiple connected power supplies, as described in further detail below. Although the charging controller  16  is preferably implemented in firmware, such as a microprocessor executing charging control software, those skilled in the art appreciate that hardware implementations of the charging controller  16  are also possible. 
     Each power supply interface  17  and  18  is compatible with a charging input on a corresponding power supply. The power supply interfaces  17  and  18  may be of the same or different types. For example, in one possible embodiment, the power supply interface  17  is configured for direct connection to a battery, whereas the power supply interface  18  is configured for connection to battery through other circuits or components in a device in which a battery is installed. Alternatively, a power supply interface  17  or  18 , or each of the interfaces, is configured to receive power supply adapters so as to indirectly connect to different types of power supplies. 
     In operation, the charging controller  16  detects the connection of a rechargeable power supply to a power supply interface  17  or  18 . If each power supply interface  17  and  18  is configured for connection to a particular type of power supply, then the charging controller determines the type of a power supply, and thus the appropriate charging currents for the power supply, by determining to which power supply interface  17  or  18  the power supply is connected. Otherwise, the charging controller detects the type of connected power supply, for example, by performing voltage and current tests on the power supply. 
     If only one power supply is connected to a power supply interface  17  or  18 , then a full charging current is output to that power supply by the charging system  10 . As described briefly above and in more detail below in the context of a battery and a device, full charging currents may be different for different types of device. In accordance with an aspect of the invention, where a power supply is connected to each power supply interface  17  and  18 , the charging controller shares available charging current between each power supply. Since the charging system  10  is typically capable of supplying more charging current than one power supply draws, full charging current is supplied to one of the connected power supplies, and any or all available excess charging current is output to the other connected power supply by the charging controller  16 . Thus, one power supply receives full charging current, and the other receives remaining available charging current, referred to herein primarily as “trickle” charging current. 
     Those skilled in the art will appreciate that an amount of trickle charging current available is dependent upon such factors as the output characteristics of the charging system  10  and the full charging current of the connected power supplies. In alternate embodiments, the charging controller  16  is configured to output a predetermined trickle charging current or a variable trickle charging current dependent upon the available excess charging current. 
     The charging controller  16  also detects a charge level of each connected power supply. Initially, the power supply that receives its full charging current charges faster than the other power supply. When the full charging power supply is charged to a predetermined level, which is detected by the charging controller  16  by monitoring charging current or terminal voltage of that power supply, for example, the charging controller  16  switches full charging current to the other power supply, and trickle charging current to the power supply that was previously receiving full charging current. The charging controller  16  compares voltage and/or current levels to respective thresholds, which may be different for different power supplies, to determine when to switch between full charging current and trickle charging current. In a firmware implementation, a microprocessor in the charging controller  16  receives voltage and/or current level indications on input pins connected to voltage and current sensing circuits and software executed by the microprocessor performs the comparison and initiates the switch between full and trickle charging currents. 
     In conventional multiple function charging systems, power supplies are charged serially, such that one connected power supply receives its full charging current while the other receives no charging current. When one power supply is fully charged, its charging current is turned off, and the other power supply receives its full charging current. Total charging time for two power supplies is therefore equal to the separate charging time for each power supply. Current sharing as described herein provides both full charging current to one power supply and trickle charging current to another power supply. As such, the other power supply is slowly charging even while the one power supply is receiving its full charging current. When the charging controller  16  switches full charging current to the other power supply, it has already partially charged, which thereby reduces the total charging time for the two power supplies relative to conventional charging systems. 
     Although reference is made to full charging current and trickle charging current in the foregoing description, it should be appreciated that the invention is in no way restricted to any particular charging current levels. Where multiple power supplies are connected to a charging system, the charging controller  16  provides a first charging current to a first power supply and a second charging current to a second power supply. When the first power supply has charged to a predetermined level, a third charging current lower than the first charging current is output to the first power supply and a fourth charging current higher than the second charging current is output to the second power supply. In the preceding description, the first and fourth charging currents are the full charging currents of each power supply, and the second and third charging currents correspond to the trickle charging current. 
       FIG. 2  is a block diagram of a multiple function charging system connected to a mobile device and a battery. In the charging system  20 , the components  22 ,  24 ,  26 ,  27 , and  28  are substantially the same as the similarly-labelled components in  FIG. 1 , except that the rechargeable power supply interfaces  27  and  28  are a battery interface and a device interface, respectively. In the embodiment shown in  FIG. 2 , the charging system  20  is configured for use with both a battery  30  and a device  32 . 
     The battery interface  27 , or an adapter configured for connection to the battery interface  27 , is compatible with connectors in the battery  30 . Where the battery  30  is a spare battery for the device  32 , for example, the battery interface  27  may be similar to an interface (not shown) associated with in the power distribution and charging subsystem  36 . Similarly, the device interface  28  is compatible with the device interface  34  or an interchangeable adapter compatible with both interfaces  28  and  34 . 
     The device  32  may be a wireless mobile communication device such as a dual-mode data and voice communication device, a mobile telephone with or without data communications functionality, or a data communication device, for example, or another portable device, with or without communications capabilities. Even though wireless communication devices are one of the most common types of devices with which charging systems are used, the present invention is in no way restricted to communication devices, or any other type of device. Current-sharing charging as described herein is applicable to other types of devices and rechargeable power supplies. 
     The battery  38  supplies power for the device  32  through the power distribution and charging subsystem  36 . The power distribution and charging subsystem  36  preferably uses the power provided by the charging system  20  to provide operating power to the device  32  and to charge the battery  38 . The particular design of the power distribution and charging subsystem  36  is dependent upon the type of the device  32 , as will be apparent to those skilled in the art, and is substantially independent of the current-sharing scheme implemented in the charging system  20 . 
     If only the battery  30  or the device  32  is connected to the charging system  20 , then the charging controller  26  determines which one of the battery  30  and the device  32  is connected, and outputs the corresponding full charging current to the appropriate interface  27  or  28 . Although the battery  30  and the battery  38  may be the same type of battery, the full charging currents may different, since the battery  38  is installed in the device  32 . As described above, the power distribution and charging subsystem  36  preferably uses power received from the charging system  20  to both power the device  32  and charge the battery  38 . As such, the device  32  may draw higher current from the charging system  20  due to the additional power requirements of other components in the device  32  to which power is distributed by the power distribution and charging subsystem  36 . For example, the battery  30  may be rated for a typical full charging current of 700 mA, whereas the device  32  is rated for a typical full charging current of 750 mA. 
     When both the battery  30  and the device  32  are connected to the charging subsystem  20 , the charging controller  26  distributes full charging current to one, and trickle charging current to the other. In most cases, a user would prefer to charge the device  32  first so that the device can be disconnected from the charging system  20 . However, the charging controller  26  may alternatively be configured to designate the battery  30  as a primary power supply for initial full charging current and the device  32  as a secondary power supply for initial trickle charging. Manual selection of the primary power supply, using a switch on the charging system  20 , for example, or a configurable system in which a device provides to the charging system  20  an indication of its rank or precedence, are also contemplated. 
     In order to further illustrate current-sharing charging, consider an illustrative example in which the charging system  20  has a maximum output of 825 mA, the battery  30  is the secondary power supply and has a full charging current of 700 mA, and the device  32  is the primary power supply and has a full charging current of 750 mA. The charging controller  26  detects the connection of both rechargeable power supplies (i.e., the battery  30  and the device  32 ) to the interfaces  27  and  28 , and supplies the full charging current of 750 mA to the device  32 . The charging controller  26  also provides trickle charging current to the battery  30 . As described above, trickle charging current may be set at a predetermined level at or below the available excess charging current, which is 825 mA maximum output of the charging system  20  less the 750 mA drawn by the device  32 , or 75 mA. For the purposes of this example, trickle charging current is set at 50 mA. Therefore, initially, the device  32  draws its full charging current of 750 mA, and the battery  30  draws trickle charging current of 50 mA. 
     After the battery  38  in the device  32  has been charged to a predetermined level, as determined based on a measured terminal voltage, for example, the charging controller  26  provides the battery  30  with its full charging current of 700 mA and switches the device  32  to the trickle charging current of 50 mA. When both the battery  30  and the battery  38  are fully charged, the charging system  20  preferably enters an idle state and no further charging current is drawn from the charging system  20 . In some implementations, the charging controller  26  may be configured to continue to provide operating power to the device  32  so that battery power is conserved until the device  32  is disconnected from the charging system  20 . 
     Other switching schemes and current sharing schemes will also be obvious to those skilled in the art, and as such, are considered to be within the scope of the invention. For example, many known charging systems provide several charging phases. A constant current charging phase during which a constant full charging current is provided to a power supply until the power supply reaches a predetermined terminal voltage is common. After the power supply reaches the predetermined terminal voltage, a constant voltage charging phase provides decreasing levels of charging current to the power supply to maintain the terminal voltage. When the power supply draws less than a predetermined amount of current, a typically time-limited top-off charging phase completes the charging cycle. In the example above, full to trickle charging current switching is based on terminal voltage of a primary power supply, and thus may coincide with a transition from a constant current charging phase to a constant voltage charging phase. Alternatively, the full to trickle charging current switching may be dependent upon the charging current drawn by the primary power supply, coinciding with the constant voltage charging phase to top-off charging phase transition, for example. Where trickle charging current is not set to a predetermined current as above, the charging controller  26  could be configured for “gradual” switching between full and trickle charging current. In the above example, as charging current drawn by the device  32  decreases during a constant voltage charging phase, more charging current could be supplied to the battery  30 . After the device  32  draws less than 825−700=125 mA, the battery  30  receives its full charging current of 700 mA from the charging system  20 . 
       FIG. 3  is a block diagram of a multiple function current-sharing charging system incorporating a USB interface and a battery receptacle. In  FIG. 3 , dashed lines indicate power transfer, while solid lines are used for data connections. The charging system  40  is substantially the same as the charging systems  10  and  20 , except that the power supply interfaces are a battery receptacle  47  and a USB interface  48 . 
     The battery receptacle  47  is configured to receive the battery  52 , which is a spare battery for the device  54  in one embodiment. In this case, the battery receptacle  47  is substantially similar to the battery receptacle  60 , although the battery receptacle  47  need not necessarily transfer power from the battery  52  to the charging system  40 . 
     The device  54  is also substantially similar to the device  32 , including a USB interface  56  to the charging system  40 , a power distribution and charging subsystem  58 , and a battery  62 . Although an interface to the battery  38  is inherent in the power distribution and charging subsystem  36  in  FIG. 2 , the battery receptacle  60  is an example of such an interface. The device  54  also includes a USB port  64  and a microprocessor  66 . In the device  54 , the power distribution and charging subsystem  58  provides operating power to the microprocessor  66  and other device components. A data connection between the microprocessor  66  and the power distribution and charging subsystem  58  provides for software-based control and monitoring of the power distribution and charging subsystem  58 , so that the microprocessor  66  can determine a remaining charge level of the battery  62  and provide an indication of battery charge to a user, for example. 
     Typically, USB devices can draw limited current from a USB host. In the case of a charging system, such a limit may be undesirable. Therefore, when the device  54  is connected to the USB interface  48 , an identification signal is preferably provided to the device  54  to notify the device  54  that it is connected to a power source that is not subject to the normal power limits imposed by the USB specification. Such an identification signal is provided, for example, by the charging controller  46 . The device  54 , or in most implementations the microprocessor  66 , recognizes the identification signal and enables the power distribution and charging system to draw power through Vbus and Gnd lines of the USB interface  56  without waiting for the normal USB processes of enumeration or charge negotiation. 
     The detection of the identification signal may be accomplished using a variety of methods. For example, the microprocessor  66  may detect the identification signal by detecting the presence of an abnormal data line condition at the USB port  64 . The detection may also be accomplished through the use of other device subsystems in the device  54 . One preferred identification signal results from the application of voltage signals greater than 2 volts to both the D+ and D− lines in the USB interface  48  by the charging controller  46 , which can then be detected at the device  54 . Further details of USB-based charging are provided in the following U.S. patent application Ser. Nos. 10/087,629, and 10/087,391, both filed on Mar. 1, 2002 and assigned to the owner of the instant application. The disclosure of each of these applications, including the specification and drawings thereof, is hereby incorporated in its entirety herein by reference. 
     Operation of the charging system  40  is substantially as described above. Either the battery  52  or the device  54  is provided with its full charging current, while the other receives trickle charging current. When full charging device is charged to a predetermined level, then the charging controller switches the trickle charging device to full charging current and vice-versa. However, the USB connection between the device  54  and the charging system  40  could be further exploited beyond charging the device  54 . For example, charging level determination for the battery  62  could be left to the device  54  instead of the charging controller  46 . A data connection (not shown) between the USB interfaces  56  and  48  allows the device microprocessor  66  to signal the charging controller  46  when the battery  62  reaches a predetermined charging level. As shown, the USB interface  56  may also be connected to other USB interfaces in other devices or systems, to support such extended functions as indirectly charging such other devices and systems through the device  54  through a conventional USB connection. The device  54  then provides an interface to a power supply that is not itself compatible with the charging system  40 . 
       FIG. 4  is a flow diagram illustrating a current-sharing charging method. The steps in the method have been described in detail above and are therefore described briefly below. 
     The method begins at step  72 , in which connection of a rechargeable power supply to an interface is detected. At step  74 , a determination is made as to whether another power supply is connected. If so, then full charging current is provided to one power supply, the primary power supply, and trickle charging current is provided to the other, secondary, power supply. Step  78  illustrates monitoring of the charging level of the primary power supply. When the primary power supply is charged to a predetermined level, the secondary power supply is switched to full charging current and the primary power supply is switched to trickle charging current. When the secondary power supply is charged, as determined at step  82 , charging is complete, as indicated at  84 . 
     Where no other power supply is connected, as determined at step  74 , the connected power supply is provided with its full charging current at step  86 . If it is determined at step  88  that another power supply is subsequently connected to the charging system, then the method reverts to step  76 . Otherwise, the power supply receives its full charging current until it is determined at step  90  that the power supply is charged. 
     The method shown in  FIG. 4  and described above is one illustrative example of a current-sharing charging method, Modifications of the method are possible without departing from the invention. For example, the current-sharing method in  FIG. 4  could be adapted to a multiple-phase charging cycle including a constant current phase, a constant voltage phase, and a time-limited top-off charging phase, as described above. In addition, just as the method reverts to step  76  from step  88  when a second power supply is connected, the method proceeds to step  86  if the primary power supply is disconnected while the secondary power supply is receiving trickle charge. Further, although the decision steps  78 ,  82 ,  88  and  90  are shown as separate steps, it should be appreciated that these steps may instead be monitoring operations that are performed during power supply charging. For instance, the charging at steps  76 ,  80 , and  86  need not be halted to check a power supply charge level at steps  78 ,  82 , and  90 . Charge level is preferably monitored during charging. Similarly, detection of another power supply at step  88  preferably interrupts the charging at step  86 , but the operation of detecting whether another power supply has been connected, such as by polling an interface or monitoring for a detection signal, preferably does not require the charging to be halted. 
       FIG. 5  is a block diagram of a wireless mobile communication device, which is one type of device for which the current-sharing charging schemes disclosed herein is applicable. The wireless mobile communication device (“mobile device”)  100  is preferably a two-way communication device having at least voice or data communication capabilities. Preferably, the mobile device  100  is also capable of communicating over the Internet, for example, via a radio frequency (“RF”) link. 
     The exemplary mobile device  100  comprises a microprocessor  112 , a communication subsystem  114 , input/output (“I/O”) devices  116 , a USB port  118 , and a power subsystem  120 . The microprocessor  112  controls the overall operation of the mobile device  100 . The communication subsystem  114  provides the mobile device  100  with the ability to communicate wirelessly with external devices such as other mobile devices and other computers. The I/O devices  116  provide the mobile device  100  with input/output capabilities for use with a device user. The USB port  118  provides the mobile device  100  with a serial port for linking directly with other computers and/or a means for receiving power from an external power source, as described above. The power subsystem  120  provides the mobile device  100  with a local power source. 
     The communication subsystem  114  comprises a receiver  122 , a transmitter  124 , antenna elements  126  and  128 , local oscillators (Ws)  130 , and a digital signal processor (DSP)  132 . The particular design of the communication subsystem  114  and the components used therein can vary. It would be apparent to one of ordinary skill in the art to design an appropriate communication subsystem using conventional methods and components to operate over a communication network  134  based on the parameters necessary to operate over that communication network. For example, a mobile device  100  geographically located in North America may include a communication subsystem  114  designed to operate within the Mobitex™ mobile communication system or DataTAC™ mobile communication system, whereas a mobile device  100  intended for use in Europe may incorporate a General Packet Radio Service (GPRS) communication subsystem  114 . 
     Network access requirements will also vary depending upon the type of network  134 . For example, in the Mobitex and DataTAC networks, mobile devices  100  are registered on the network using a unique personal identification number or PIN associated with each device. In GPRS networks however, network access is associated with a subscriber or user of a mobile device  100 . A GPRS device therefore requires a subscriber identity module (not shown), commonly referred to as a SIM card, in order to operate on a GPRS network. Without a SIM card, a GPRS device will not be fully functional. Local or non-network communication functions (if any) may be operable, but the mobile device  100  will be unable to carry out any functions involving communications over the network  134 , other than legally required functions such as ‘911’ emergency calling. 
     When required, after the network registration or activation procedures have been completed, a mobile device  100  may send and receive communication signals over the network  134 . Signals received by the antenna element  126  are input to the receiver  122 , which typically performs such common receiver functions as signal amplification, frequency down conversion, filtering, channel selection, and in the exemplary system shown in  FIG. 5 , analog to digital conversion. Analog to digital conversion of a received signal allows more complex communication functions such as demodulation and decoding to be performed in the DSP  132 . Similarly, signals to be transmitted are processed, including modulation and encoding for example, by the DSP  132  and input to the transmitter  124  for digital to analog conversion, frequency up conversion, filtering, amplification, and transmission over the communication network  134  via the transmitter antenna element  128 . The DSP  132  not only processes communication signals, but also provides for receiver and transmitter control. For example, signal gains applied to communication signals in the receiver  122  and transmitter  124  may be adaptively controlled through automatic gain control algorithms implemented in the DSP  132 . 
     In implementing its device operation control function, the microprocessor  112  executes an operating system. The operating system software used by the microprocessor  112  is preferably stored in a persistent store such as the non-volatile memory  136 , or alternatively read only memory (ROM) or similar storage element. The microprocessor  112  may also enable the execution of specific device software applications, which preferably are also stored in a persistent store. The operating system, specific device applications, or parts thereof, may also be temporarily loaded into a volatile store such as in RAM  138 . The non-volatile memory  136  may be implemented, for example, as a flash memory component, or a battery backed-up RAM, for example. 
     A predetermined set of software applications which control basic device operations, including at least data and voice communication applications for example, will normally be installed on the mobile device  100  during manufacture. One such application loaded on the mobile device  100  could be a personal information manager (PIM) application. The PIM application is preferably a software application for organizing and managing user inputted data items such as e-mail, calendar events, voice mails, appointments, and task items. The PIM data items may be stored in the RAM  138  and/or the non-volatile memory  136 . 
     The PIM application preferably has the ability to send and receive data items, via the wireless network  134 . The PIM data items are preferably seamlessly integrated, synchronized and updated, via the wireless network  134 , with corresponding data items stored or associated with a host computer system (not shown) used by the device user. The synchronization of PIM data items is a process by which the PIM data items on the mobile device  100  and the PIM data items on the host computer system can be made to mirror each other. 
     There are several possible mechanisms for loading software applications onto the mobile device  100 . For example, software applications may be loaded onto the mobile device  100  through the wireless network  134 , an auxiliary I/O subsystem  140 , the USB port  118 , a short-range communications subsystem  142 , such as an infrared (“IR”), Bluetooth™, or 802.11 communication system, or any other suitable subsystem  44 . Those skilled in the art will appreciated that “Bluetooth” and “802.11” refer to sets of specifications, available from the Institute for Electrical and Electronics Engineers (IEEE), relating to wireless personal area networks and wireless local area networks, respectively. 
     When loading software applications onto the mobile device  100 , the device user may install the applications in the RAM  138  or the non-volatile memory  136  for execution by the microprocessor  112 . The available application installation mechanisms can increase the utility of the mobile device  100  by providing the device user with a way of upgrading the mobile device  100  with additional and/or enhanced on-device functions, communication-related functions, or both. For example, a secure communication application may be loaded onto the mobile device  100  that allows for electronic commerce functions or other financial transactions to be performed using the mobile device  100 . 
     The I/O devices  116  are used to accept inputs from and provide outputs to a user of the mobile device  100 . In one mode of operation, a signal received by the mobile device  100 , such as a text message or web page download, is received and processed by the communication subsystem  114 , forwarded to the microprocessor  112 , which will preferably further process the received signal and provides the processed signal to one or more of the I/O devices  116  such as the display  146 . Alternatively, a received signal such as a voice signal is provided to the speaker  148 , or alternatively to an auxiliary I/O device  140 . In another mode of operation, a device user composes a data item such as an e-mail message using a keyboard  150  in cooperation with the display  146  and/or possibly an auxiliary I/O device  140 . The composed data item may then be transmitted over a communication network  134  using the communication subsystem  114 . Alternatively, a device user may compose a voice message via a microphone  152 , or participate in a telephone call using the microphone  152  and the speaker  148 . 
     The short-range communications subsystem  142  allows the mobile device  100  to communicate with other systems or devices, which need not necessarily be similar to device  100 . For example, the short-range communications subsystem  142  may include an infrared device, a Bluetooth module, or an 802.11 module, as described above, to support communications with similarly-enabled systems and devices. 
     The USB port  118  provides the mobile device  10  with a serial port for linking directly with other computers to exchange data and/or to receive power. The USB port  118  also provides the mobile device  100  with a means for receiving power from an external power source. For example, in a personal digital assistant (PDA)-type communication device, the USB port  118  could be used to allow the mobile device  100  to synchronize data with a user&#39;s desktop computer (not shown). The USB port  118  could also enable a user to set parameters in the mobile device  100  such as preferences through the use of an external device or software application. In addition, the USB port  118  provides a means for downloading information or software to the mobile device  100  without using the wireless communication network  134 . The USB port  118  provides a direct and thus reliable and trusted connection that may, for example, be used to load an encryption key onto the mobile device  100  thereby enabling secure device communication. 
     Coupled to the USB port  118  is a USB interface  154 . The USB interface  154  is the physical component that couples the USB port to the outside world. In the exemplary mobile device  100 , the USB interface  154  is used to transmit and receive data from an external data/power source  156 , receive power from the external data/power source  156 , direct the transmitted/received data from/to the USB port  118 , and direct the received power to the power subsystem  120 . 
     The power subsystem  120  comprises a charging and power distribution subsystem  158  and a battery  160 , which have been described above. In conjunction with a charging system connected as the data/power source  156 , current-sharing charging of the mobile device  100  and a spare battery therefor, in accordance with aspects of the present invention, is supported. 
     This written description may enable those skilled in the art to make and use embodiments having alternative elements that correspond to the elements of the invention recited in the claims. The intended scope of the invention thus includes other structures, systems or methods that do not differ from the literal language of the claims, and further includes other structures, systems or methods with insubstantial differences from the literal language of the claims. 
     For example, it would be obvious to implement current-sharing charging for more than two power supplies. In this case, a primary power supply receives its full charging current, and available excess charging current is then distributed among one or more secondary power supplies.