Abstract:
A system, apparatus, method, and manufacture for generating backup power in a wireless communications system such as a wireless communications service base station. The system includes a communications interface, a primary power interface, a generator, rectifiers, and a battery circuit. During normal operation, the communications interface is powered from the primary power interface. During a power outage, the communications interface is powered from either the generator or the battery circuit. The generator is cycled on and off during power outages to charge the battery circuit while conserving fuel. To decrease rectification loss, rectifiers are run near full load while rectifying the generator output.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
       [0001]    This application is a divisional application of U.S. patent application Ser. No. 14/251,539, filed on Apr. 11, 2014, and entitled “CELL SITE POWER GENERATION,” now U.S. Pat. No. 9,386,518, which is a divisional application of U.S. patent application Ser. No. 12/170,675, filed on Jul. 10, 2008, and entitled “CELL SITE POWER GENERATION,” now U.S. Pat. No. 8,729,732, both of which are hereby incorporated herein in their entireties by reference. This application is also related to U.S. patent application Ser. No. 12/365,165 filed Feb. 3, 2014, now U.S. Pat. No. 8,279,074, and entitled “BATTERY MONITORING SYSTEM, SUCH AS FOR USE IN MONITORING CELL SITE POWER SYSTEMS”, which is incorporated by reference herein in its entirety. 
     
    
     BACKGROUND 
       [0002]    The popularity of commercial wireless communications services (e.g., wireless telephony, wireless network access, and wireless email) has substantially increased during recent years. In many cases, users, such as consumers, mobile workers, emergency response personnel, and/or the like, now utilize these services for both personal and business communications. Likewise, users are also increasingly relying on these services. For example, some households forgo wired telephone service in favor of wireless telephone service, some government agencies rely on these services for both routine and emergency communications, and businesses rely on these services to communicate with customers and mobile workers. Correspondingly, the cost (both financial and nonfinancial) of outages is also increasing. 
         [0003]    Typical commercial wireless communications service (CMRS) providers rely on remote facilities to facilitate the provision of services. For instance, CMRS providers rely on base stations (e.g., cell sites, radio repeaters, wireless to backhaul interfaces, etc.) to facilitate some communications services. If a base station experiences a loss of commercially-provided electrical power, users near the base station may experience a service outage. Power outages are an example of a common cause for base station failures. For example, natural disasters, rolling brownouts, accidents, and/or the like may result in power outages. While most base stations include some form of backup power (e.g., generators and/or batteries), these forms of backup power may not provide sufficient power during lengthy power outages and may require servicing, monitoring, and on-site maintenance. During lengthy power outages, use of commercial wireless communications services may increase due to users&#39; needs and/or desires. Further, pending regulations may require commercial wireless communications service providers to provide base stations with at least seven days of backup power. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0004]      FIG. 1  is a block diagram of an embodiment of an environment for practicing the invention; 
           [0005]      FIG. 2A  is a block diagram of a base station in accordance with an embodiment of the invention; 
           [0006]      FIG. 2B  is a block diagram of components of a rectifier and switch circuit in accordance with embodiments of the invention; 
           [0007]      FIG. 3  is a block diagram of a power controller usable in the base station of  FIG. 2A  in accordance with an embodiment of the invention; 
           [0008]      FIG. 4  is a schematic diagram of a battery circuit usable in the base station of  FIG. 2A  in accordance with an embodiment of the invention; 
           [0009]      FIG. 5  is a schematic diagram of an opto-isolator usable in the battery circuit of  FIG. 4  in accordance with an embodiment of the invention; and 
           [0010]      FIG. 6  is a logical flow diagram of a process for conserving power in a wireless communications system in accordance with an embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0011]    The following description provides specific details for a thorough understanding of, and enabling description for, various embodiments of the technology. One skilled in the art will understand that the technology may be practiced without many of these details. In some instances, well-known structures and functions have not been shown or described in detail to avoid unnecessarily obscuring the description of the embodiments of the technology. It is intended that the terminology used in the description presented below be interpreted in its broadest reasonable manner, even though it is being used in conjunction with a detailed description of certain embodiments of the technology. Although certain terms may be emphasized below, any terminology intended to be interpreted in any restricted manner will be overtly and specifically defined as such in this Detailed Description section. 
         [0012]      FIG. 1  is a block diagram of environment  190  in which the invention may be practiced. As shown, environment  190  includes base station  100  and wireless devices  197 - 199 . Base station  190  includes antenna  192  and is coupled to back-haul  194  and to primary power source AC over line  196 . Base station  190  and wireless devices  197 - 199  are configured to wirelessly communicate with each other. 
         [0013]    Base station  100  may include virtually any device for facilitating wireless network access. For example, base station  100  may be a wireless telephony base station, a wireless network access base station, a wireless email base station, and/or the like. In one embodiment, base station  100  is operated by a mobile telephony service provider. Generally, base station  100  is configured to provide a network interface for wireless devices  197 - 199  by providing an interface (via antenna  192 ) between wireless devices  197 - 199  and back-haul  194 . Base station  100  and wireless devices  197 - 199  may communicate using any wireless protocol or standard. These include, for example, Global System for Mobile Communications (GSM), Time Division Multiple Access (TDMA), Code Division Multiple Access (CDMA), Orthogonal Frequency Division Multiple Access (OFDM), General Packet Radio Service (GPRS), Enhanced Data GSM Environment (EDGE), Advanced Mobile Phone System (AMPS), Worldwide Interoperability for Microwave Access (WiMAX), Universal Mobile Telecommunications System (UMTS), Evolution-Data Optimized (EVDO), Long Term Evolution (LTE), Ultra Mobile Broadband (UMB), and/or the like. 
         [0014]    Back-haul  194  may be any connection that provides a network interface for base station  100 . For example, back-haul  194  may include one or more T-1 connections, T-3 connections, OC-3 connections, frame relay connections, Asynchronous Transfer Mode (ATM) connections, microwave connections, Ethernet connections, and/or the like. In addition, back-haul  194  may provide an interface to a telephone switch (e.g., to a 5ESS switch or a Private Branch Exchange switch) to a data network (e.g., to a router or network switch), and/or the like. 
         [0015]    Base station  100  is also powered from primary power source AC over line  196 . Primary power source AC may be provided by virtually any power source. For example, it may be provided by a public utility, from solar power, from a turbine, from a fuel cell, and/or the like. At times, however, primary power source AC may provide insufficient power for base station  100 . As discussed below, base station  100  also includes backup power sources. 
         [0016]    Wireless devices  197 - 199  may include virtually any devices for communicating over a wireless network. For example, wireless devices  197 - 199  may include mobile telephones (e.g., cellular telephones, GSM telephones, TDMA telephones, LTE telephones, etc.), wireless data devices (e.g., Personal Digital Assistants (PDAs), computers, pagers, etc.), and/or the like. 
         [0017]    One skilled in the art will appreciate that although illustrated in the context of a wireless telecommunications environment, the invention may be practiced in any environment in which backup power serves a commercial, public, or private operation or system reliant upon electrical power. 
         [0018]      FIG. 2A  is a block diagram of base station  200 . Base station  200  includes power controller  210 , rectifier and switch circuit  230 , primary power interface  250 , battery circuit  260 , generator  270 , and communications interface  280 . Base station  200  may be employed as an embodiment of base station  100  of  FIG. 1 . 
         [0019]    Power controller  210  is configured to control the power systems of base station  200 . As illustrated, power controller  210  is configured to receive or provide control signals  212  and status signals COM_STAT, RECT_STAT, and BAT_STAT, and to provide output/control signals RECT_CTL, GEN_CTL, and STATUS, as discussed below. 
         [0020]    In one embodiment, power controller  210  is configured to selectively enable and disable generator  270  and to control the operation of rectifier and switch circuit  230  based, at least in part, on the various status and control signal inputs. The operation of power controller  210  is discussed in further detail with regards to  FIG. 3 . 
         [0021]    Rectifier and switch circuit  230  may include switching devices of any type (e.g., power field-effect-transistors, power insulated gate bi-polar transistors, relays, etc.) that are configured to selectively switch (e.g., route) power from either primary power interface  250 , battery circuit  260 , or generator  270  to communications interface  280 .  FIG. 2B  is a block diagram of an example rectifier and switch circuit  230 . In  FIG. 2B , switching devices  231  route power to the communications interface  280  from the primary power interface  250 , battery circuit  260 , and generator  270 . Rectifier and switch circuit  230  may also be configured to selectively switch power from either primary power interface  250  or generator  270  to battery circuit  260  for providing a charging current. For example, referring again to  FIG. 2B , switching devices  232  routes power from either the primary power interface or the generator to the battery circuit. The switching between communications interface  280 , primary power interface  250 , battery circuit  260 , and generator  270  may be controlled via control signal RECT_CTL from power controller  210 . 
         [0022]    In addition, rectifier and switch circuit  230  may also include multiple rectifiers that are each configured to rectify power either from primary power interface  250  or from generator  270  before providing it either to communications interface  280  or to battery circuit  260 . For example, referring again to  FIG. 2B , a rectifier  233  rectifies power from the primary power interface and a rectifier  235  rectifies power from the generator. Each of the rectifiers in rectifier and switch circuit  230  may be controlled via control signal RECT_CTL from power controller  210 . 
         [0023]    For some common rectifiers, rectification efficiency increases when the rectifier is run at higher loads. However, continuous operation of rectifiers at higher loads may increase operating temperatures and may lead to premature failure of the rectifiers. Accordingly, when base station  200  is powered from primary power source AC, communications interface  280 ′s power draw may be balanced across each of the rectifiers of rectifier and switch circuit  230  to reduce the load on each rectifier. However, during a periods of primary power source reduced availability, the power drawn by communications interface  280  may be balanced across fewer rectifiers such that efficiency of each operating rectifier is increased. 
         [0024]    Rectifier and switch circuit  230  may also be configured to provide status signal RECT_STAT to power controller  210  to, for example, indicate the status of rectifiers, the status of switches, the status of primary power source AC, the status of generator  270 &#39;s output, failure conditions (e.g., failure of particular rectifiers, failure of particular switches, excessive current draw from communications interface  280 , out of range input voltages, etc.), and/or the like. Status signal RECT_STAT may be provided to power controller  210  to enable power controller  210  to adjust the operation of rectifier and switch circuit  230  or generator  270  based on these and other conditions. 
         [0025]    Primary power interface  250  is configured to couple primary power source AC to rectifier and switch circuit  230 . Primary power interface  250  may include a circuit breaker, line filter, surge protector, power meter, and/or the like. However, in one embodiment, primary power interface  250  may simply be a wire segment connecting primary power source AC to rectifier and switch circuit  230 . 
         [0026]    Battery circuit  260  is configured to store power that is provided by primary power source AC or by generator  270 . Battery circuit  260  may include any number of batteries arranged in any combination of series configurations, parallel configurations, and/or series and parallel configurations. In one embodiment, battery circuit  260  includes multiple strings of serially connected absorbed glass mat lead-acid batteries. However, any suitable type of battery may be employed. Further, battery circuit  260  is configured to provide status signal BAT_STAT to power controller  210 . For example, status signal BAT_STAT may be employed to indicate the output voltage of battery circuit  260 , a voltage of each individual battery, and/or the like. Also, status signal BAT_STAT may include multiple signals and be provided on one or more signal lines. 
         [0027]    Power controller  210  may be configured to determine a failure condition or approximate a charge percentage for battery circuit  260 , and/or the like, from status signal BAT_STAT. While battery circuit  260  is illustrated as being coupled to rectifier and switch circuit  230  by signals BAT_IN and BAT_OUT, in other embodiments, signals BAT_IN and BAT_OUT may be a single signal. 
         [0028]    Generator  270  is configured to generate power under the control of power controller  210  for powering communications interface  280  and for charging battery circuit  260 . Generator  270  is further configured to enable and disable power generation based on control signal GEN_CTL. The output of generator  270  is provided to rectifier and switch circuit  230  via signal GEN_PWR. 
         [0029]    With certain generators, fuel consumption is primarily a function of run-time. For these generators, the increase in fuel consumption is relatively insignificant when the load current is increased. Accordingly, the total power produced from a given amount of fuel is greater when operating a generator at high loads for short durations as compared to continuously operating a generator at lower loads. In addition, operating a higher output wattage generator may produce a greater the total power output from a given amount of fuel than a lower output wattage generator. 
         [0030]    Thus, to conserve fuel, generator  270  may be enabled when the charge on battery circuit  260  is relatively low and disabled when the charge is relatively high. While running, generator  270  may be employed to power communications interface  280  and to charge battery circuit  260 . While generator  270  is stopped, communications interface  280  may be powered from battery circuit  260 . In this way, generator  270  produces more power for a given amount of fuel. In one embodiment, generator  270  is enabled when the voltage of battery circuit  260  represents that less than a  20  percent charge remains on battery circuit  260  and is disabled when the voltage represents that battery circuit  260  has an 82 percent charge. Although, any other suitable percentages, voltages, charge conditions, and/or the like may be employed. Thus, power controller  210  controls generator  270  to operate intermittently, with high load, when the charge on battery circuit  260  is below a threshold; otherwise, battery circuit  260  provides backup power to communications interface  280 . Also, generator  270  may be selected such that while powering communications interface  280  and charging battery circuit  260  near a maximum charge rate (e.g., a manufacturer specified maximum rate, a manufacturer specified recommended rate, an operator specified rate, etc.) generator  270  operates at high load. This selection may be based on the DC load requirements of communications interface  280 , the power available to charge battery circuit  260 , the number of batteries in battery circuit  260 , the ampere hour ratings of the batteries in battery circuit  260 , expected efficiency, anticipated future power requirements, operating margins, and/or the like. In one embodiment, the maximum charge rate for battery circuit  260  is approximately twice the recommended discharge rate for battery circuit  260 . 
         [0031]    As one example, a 20 kilowatt water cooled Generac generator and a 370 ampere hour battery circuit may be employed to power a Nokia UltraSite communications interface having 15 to 18 radio units and an additional two rectifiers in the rectifier and switch circuit. In this example, while powering the communications interface, approximately 80 amperes are available to charge and cool the battery circuit. Also, a charged battery circuit has approximately 220 ampere hours (after accounting for internal resistance and other losses) available to power communications interface  280  between charging cycles. Under normal circumstances, 220 ampere hours provides 5.5 to 7 hours of power to communications interface  280 . After the generator is enabled, it spends approximately 6 hours charging the battery circuit, first at a high load, and later at a lower load while cooling off. 
         [0032]    Such operation may increase the service life and decrease maintenance for both battery circuit  260  and generator  270 . Likewise, such operation may increase the length of time that communications interface  280  may operate from backup power. In one embodiment, a 250 gallon propane tank may be sufficient to power communications interface  280  for 6 days with intermittent generator operation as compared to 3.5 days with continuous generator operation. 
         [0033]    Generator  270  may include a gasoline generator, a diesel generator, a propane generator, a natural gas generator, a methanol generator, an ethanol generator, and/or the like. Moreover, generator  270  may be either air-cooled or liquid-cooled. 
         [0034]    Communications interface  280  is configured to interface (via antenna  292 ) wireless devices to back-haul  294 . Communications interface  280  typically includes both digital and radio frequency (RF) electronics. In one embodiment, communications interface  280  includes a RF transceiver and digital control circuitry. However, other components may also be associated with a transceiver and/or other circuits. Communications interface  280  is powered from rectifier and switch circuit  230  via line LOAD_PWR and is configured to provide status signal COM_STAT to indicate an operational status such as failure of back-haul  294 , the number of wireless devices associated with base station  200 , power consumption data, and/or the like. 
         [0035]      FIG. 3  is a block diagram of power controller  310 . Power controller  310  includes processor  314 , battery circuit interface  316 , generator control interface  318 , and operation, management, and control (OMC) interface  320 . Power controller  310  may be employed as an embodiment of power controller  210  of  FIG. 2A . 
         [0036]    As illustrated, processor  314  is configured to control the operations of the rectifier and switch circuit (e.g., via control signal RECT_CTL) and the generator (e.g., via control signal GEN_CTL) and to provide a status signal to a remote system (e.g., STATUS). In one embodiment, processor  314  is configured to selectively enable and disable the generator based, at least in part, on the primary power source status and a battery circuit status. For example, the generator may be enabled when there is reduced availability of the primary power source and the battery circuit voltage falls below a threshold value. Likewise, the generator may be disabled when the primary power source provides sufficient power, when the battery circuit voltage is above another threshold value, when there is a failure of the battery circuit, when an over-current condition is detected, when a battery circuit temperature is above a threshold value, when there is a failure in the rectifier and switch circuit, when processor  314  is in reset, and/or the like. 
         [0037]    In one embodiment, processor  314  detects a failure of the battery circuit based on a rate of change of the battery circuit output voltage or of the voltages of the individual batteries of the battery circuit. 
         [0038]    To determine these and other conditions, processor  314  receives various status signals as illustrated in  FIG. 3 . For example, signals COM_STAT and RECT_STAT may be employed to respectively represent the status of the communications interface and of the rectifier and switch circuit. Likewise, signal BAT SNS may be employed to represent the output voltage of the battery circuit, signals BAT_MON_ 1  to BAT_MON_N may be employed to represent the voltage across the individual batteries of the battery circuit, and signal BAT_TEMP may be employed to represent a temperature of the battery circuit. Also, signal RESET may be employed to reset and/or hold processor  314  in reset. Finally, control signal OVER_CURRENT may be employed to represent an over-current condition of the battery circuit, of the rectifier and switch circuit, of the generator, and/or the like. Likewise, processor  314  may be configured to control the rectifier and switch circuit based, at least in part, on the availability status of the primary power source, as discussed above. 
         [0039]    As illustrated, processor  314  is also configured to provide watchdog signal WD to a watchdog circuit (not shown). The watchdog circuit may be arranged to reset the processor via the RESET signal if, for example, the watchdog signal WD remains unchanged for a predefined duration. In other embodiments, internal watchdog circuits, and/or the like, may also be employed. 
         [0040]    Processor  314  is further configured to receive configuration signal CONFIG to represent a hardware configuration, to set various threshold levels, and/or the like. Any number of configuration signals may be provided. In one embodiment, configurations signals are employed to represent the number and/or types of rectifiers in the rectifier and switch circuit, the design voltage of the battery circuit, the type of generator, the number of battery strings in the battery circuit, and/or the like. For example, a configuration signal may be provided to indicate whether the generator is air-cooled so that an air-cooled generator may be run for a cooling off period prior to being disabled by processor  314 . As another example, a configuration signal may be provided to indicate the load capacity of the rectifiers so that processor  314  may more accurately determine the number of active rectifiers for providing efficient rectification. As yet another example, a configuration signal may be provided to indicate the design voltage of the battery circuit so that processor  314  may more accurately estimate the charge on the battery circuit from the battery circuit output voltage. 
         [0041]    Configuration signal CONFIG may be provided from a switch (e.g., a DIP switch), from pull-up resistors, from pull-down resistors, from jumpers, and/or the like. Alternatively, similar configuration information may be read by processor  314  from a memory or be received from another processor. 
         [0042]    Processor  314  may be a microprocessor, a microcontroller, a digital signal processor (DSP), and/or the like. However, in other embodiments, digital logic, analog logic, combinations of digital logic and analog logic, and/or the like may also be employed instead of a processor. For example, such embodiments may be implemented in a field-programmable gate array (FPGA), in an application-specific integrated circuit (ASIC), in other programmable logic devices (PLDs), and/or the like. 
         [0043]    Battery circuit interface  316  is configured to interface processor  314  to a battery circuit. For example, battery circuit interface  316  receives signal BAT_STAT from the battery circuit and provides discrete signals to processor  314 . For example, signal BAT_STAT may be a multiplexed signal or may be provided on multiple signal lines. In one embodiment, battery circuit interface  316  includes an array of analog to digital converters (ADCs) that are configured to digitize each of signals BAT_SNS, BAT_MON_ 1  to BAT_MON_N, and BAT_TEMP for processor  314 . However, multiplexers, drivers, buffers, logic gates, analog circuits, and/or the like may also be suitably employed. 
         [0044]    Generator control interface  318  is configured to interface processor  314  to a generator such as generator  270  of  FIG. 2A . In one embodiment, generator control interface  318  includes a relay, a level-shifter, a driver, a buffer, an inverter, logic gates, and/or the like that is configured to provide control signal GEN_CTL based, at least in part, on the output of processor  314 . Also, generator control interface  318  and/or processor  314  may be configured such that a failure of either generator control interface  318  or processor  314  results in the generator being enabled. In this way, a failure of processor  314  and/or generator control interface  318  is less likely to cause a power interruption at the communications interface. 
         [0045]    OMC interface  320  is configured to interface processor  314  to a remote system and to provide operational data regarding the base station and/or the base station power system to the remote system. OMC interface  320  may include drivers, buffers, inverters, logic gates, network interface units, multiplexers, and/or the like. Likewise, OMC interface  320  may be configured to multiplex the STATUS signal onto the back-haul or may provide the STATUS signal as a discrete signal. 
         [0046]      FIG. 4  is a schematic diagram of battery circuit  460 . Battery circuit  460  includes batteries  462 , jumpers  464 , current limiting resistors  466 , and opto-isolator circuits  468 . Battery circuit  460  may be employed as an embodiment of battery circuit  260  of  FIG. 2A . 
         [0047]    As illustrated, battery circuit  460  is configured as a negative 48-volt battery circuit having one string of serially connected absorbed glass mat lead-acid batteries. In this embodiment, batteries  462  are serially connected by jumpers  464  to form a single battery string. However, other battery circuits may be configured with any number of batteries and any number of battery strings and may provide any positive or negative output voltage. In addition, battery circuit  460  includes opto-isolator circuits  468  to sense the voltage across each of the batteries and associated jumpers. Current limiting resistors  466  are also included to limit the current to opto-isolator circuits  468 . By coupling opto-isolator circuits  468  across both the battery and the associated jumper, the effects of loose or corroded connections may be included in the opto-isolator circuit output and may be detected by the power controller. 
         [0048]    In other embodiments, other isolated or non-isolated sense circuits may be employed instead of opto-isolator circuits  468 . For example, ADCs, sense resistors, and/or the like may be employed. 
         [0049]      FIG. 5  is a schematic diagram of opto-isolator circuit  568 . Opto-isolator circuit  568  includes diode D 1 , transistor Q 1 , and resistor R 1 . Opto-isolator circuit  568  may be employed as embodiments of opto-isolator circuits  468  of  FIG. 4 . 
         [0050]    Opto-isolator circuit  568  is configured to provide an output signal that is based on the voltage applied across diode D 1 . In operation, the voltage across diode D 1  causes current flow through diode D 1  and causes diode D 1  to provide an emitted light with an intensity that is related to the magnitude of the current flow. The emitted light is received by transistor Q 1  to provide an output signal. Resistor R 1  biases transistor Q 1  and may be of any suitable value. 
         [0051]      FIG. 6  is a logical flow diagram of process  600  for conserving power in a wireless communications system. For clarity, process  600  is described below as being performed by base station  200  of  FIG. 2A . However, process  600  may also be performed by processor  314  of  FIG. 3  and may be stored in non-volatile memory. Process  600  may also be performed by other processors, by other components, or in other systems, whether or not such processors, components, or systems are described herein. 
         [0052]    Flowing from a start block, processing begins at step  610  where power controller  210  disables generator  270 . For example, power controller  210  may disable generator  270  to conserve fuel while primary power source AC provides power to primary power interface  280 . At step  620 , primary power interface  280  receives power from primary power source AC. At step  630 , rectifier and switch circuit  230  employs a first number of rectifiers to rectify the received power. For example, to decrease the operating temperature of the operating rectifiers, the first number of rectifiers may include all or almost all of the rectifiers in rectifier and switch circuit  230 . In one embodiment, base station  200  may employ the rectified power to power communications interface  280  or to charge battery circuit  260 . 
         [0053]    At decision block  640 , power controller  210  determines whether power from primary power source AC is available. Power controller  210  may perform this determination based on status signal RECT_STAT, based on a status signal from primary power interface  250 , and/or the like. If power controller  210  does detect availability of power from primary power source  250 , it returns processing to step  620 . Otherwise, power controller  210  continues processing at decision block  650 . 
         [0054]    At decision block  650 , power controller  210  determines if the battery charge is under a lower threshold. For example, power controller  210  may estimate battery circuit  260 &#39;s remaining charge based on the battery circuit  260 &#39;s output voltage. If this remaining charge is not under the lower threshold, power controller  210  returns processing to decision block  640 . Otherwise, power controller  210  continues to step  660 . 
         [0055]    Power controller  210  enables power generation at step  660 , for example, by starting generator  270 . After power controller  210  enable power generation, processing flows to step  670  where rectifier and switch circuit  230  employs a second number of rectifiers to rectify the generated power. In one embodiment, the second number of rectifiers is less than the first number of rectifiers such that each of the operating rectifiers operates at a higher load and with higher efficiency. From step  670 , processing flows to decision block  680 . 
         [0056]    At decision block  680 , power controller  210  determines if the battery charge is above an upper threshold. For example, power controller  210  may estimate battery circuit  260 &#39;s remaining charge based on the battery circuit  260 &#39;s output voltage. If the remaining charge on battery circuit  260  is not above the upper threshold, power controller  210  returns processing to decision block  640 . Otherwise, power controller  210  continues processing at step  690  where power controller  210  disables generator  270 . From step  690 , power controller  210  returns processing to decision block  640 . 
         [0057]    Unless the context clearly requires otherwise, throughout the description and the claims, the words “comprise,” “comprising,” and the like are to be construed in an inclusive sense, as opposed to an exclusive or exhaustive sense; that is to say, in the sense of “including, but not limited to.” As used herein, the term “connected,” “coupled,” or any variant thereof means any connection or coupling, either direct or indirect, between two or more elements; the coupling of connection between the elements can be physical, logical, or a combination thereof. Additionally, the words “herein,” “above,” “below,” and words of similar import, when used in this application, shall refer to this application as a whole and not to any particular portions of this application. Where the context permits, words in the above Detailed Description using the singular or plural number may also include the plural or singular number, respectively. The word “or,” in reference to a list of two or more items, covers all of the following interpretations of the word: any of the items in the list, all of the items in the list, and any combination of the items in the list. 
         [0058]    The above detailed description of embodiments of the system is not intended to be exhaustive or to limit the system to the precise form disclosed above. While specific embodiments of, and examples for, the system are described above for illustrative purposes, various equivalent modifications are possible within the scope of the system, as those skilled in the relevant art will recognize. For example, while processes or blocks are presented in a given order, alternative embodiments may perform routines having steps, or employ systems having blocks, in a different order, and some processes or blocks may be deleted, moved, added, subdivided, combined, and/or modified to provide alternative or subcombinations. Each of these processes or blocks may be implemented in a variety of different ways. Also, while processes or blocks are at times shown as being performed in series, these processes or blocks may instead be performed in parallel, or may be performed at different times. Further, any specific numbers noted herein are only examples: alternative implementations may employ differing values or ranges. 
         [0059]    From the foregoing, it will be appreciated that specific embodiments of the invention have been described herein for purposes of illustration, but that various modifications may be made without deviating from the spirit and scope of the invention. Accordingly, the invention is not limited except as by the appended claims.