Abstract:
A method and apparatus for identifying different types of energy sources used to charge a battery by receiving energy from at least one of the different types of energy sources at input terminals, identifying the type of energy source, and selecting a mode for charging the battery based on the type of energy source identified. A method and apparatus for protecting against certain energy sources used to charge a battery is also disclosed.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to U.S. Provisional Patent Application No. 61/276,418, filed on Sep. 12, 2009, entitled DECENTRALIZED ELECTRICITY GENERATION, STORAGE AND DISTRIBUTION DEVICES, which is incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     The subject matter disclosed herein relates to a method and apparatus for conversion of energy to electricity from a plurality of different sources for immediate use, storage of electricity for later use (e.g., charging a battery), and distribution of safe, regulated electricity for end-use to provide energy services. 
     Many of the devices used to conduct day-to-day activities (e.g., devices providing lighting, communication, entertainment, news, medicine, etc.) require electricity to either power the devices directly or to charge the batteries used to power those devices. It is estimated that nearly one in four people on the planet (more than 1.6 billion people) lack access to electricity, many of whom live in the developing world at the base of the economic pyramid. It is also estimated that an additional billion people struggle with unreliable access to electricity. While these populations have access to electricity, the power plants and distribution grid are overloaded, under-maintained and therefore unreliable, resulting in power outages, often several outages each day, many spanning several hours or even days. As a result, this negatively impacts the economy, the ability for these populations to rise up the economic pyramid and the ability to improve quality of life. 
     The framework of traditional centralized power generation and grid distribution is not meeting the needs of these people because large power plant installations are costly and prone to delays and the grid improvements required to distribute this electricity is expensive as well. Even alternatives such as petrol-powered generators (e.g., diesel or gasoline) and the current offering of solar photovoltaic home systems experience only limited success because of cost (e.g., high operating cost purchasing fossil fuel, or high upfront cost to purchase solar home systems) and reliability issues (e.g., inexpensive generators often experience malfunctions and the current offering of solar home systems require installation, maintenance and support). 
     And yet, despite these challenges, it is estimated that there are presently over 500 million off-grid mobile phone subscribers and countless other off-grid appliances (e.g., lights, radios, televisions, refrigerators, etc.). To power these appliances, millions of people at the base of the pyramid have re-appropriated 12V direct current (DC) lead-acid car batteries to serve the need of energy storage and electricity distribution. However, the practice of using jumper cables or twisting bare wires to connect appliances leaves much to be desired where such usage of unprotected batteries is dangerous, significantly reduces battery life, and frequently causes sparks and even electrocution. 
     Populations in areas that do have access to reliable sources of electricity under normal circumstances may be subject to natural disasters, blackouts, or planned recreational activities to remote locations (e.g., camping or hiking) that result in prolonged periods of time where users do not have access to electricity to operate or charge their appliances. Some users that do have access to a modern power distribution grid, for environmental reasons, may choose to use renewable energy sources when possible (e.g., solar power, kinetic power, etc.). In some cases, users may employ kinetic generators operated by rotating and driving a shaft with a device driven by natural forces (e.g., wind turbine, hydro turbine, etc.) or by human/animal forces (e.g., bicycle generator, hand-crank, animal generator, etc.) to create the required electricity. Often times, however, these systems do not supply sufficient power to supply the electricity necessary for powering or charging devices for prolonged periods of time without requiring excessive amounts of effort by the users or performance by the devices. In addition, many these systems can only accept one type of energy source and may be damaged if another source were inadvertently connected. 
     It would be advantageous to provide a method and apparatus for supplying electrical power for people who do not have access to reliable electricity to improve the quality of life for these individuals, provide a safer and more effective source of electrical power, and provide the flexibility to use a plurality of different types of energy sources, including renewable energy sources. 
     BRIEF DESCRIPTION OF THE INVENTION 
     In one embodiment of the invention, a method and apparatus are disclosed for identifying different types of energy sources used to charge a battery by receiving energy from at least one of the different types of energy sources at input terminals, identifying the type of energy source, and selecting a mode for charging the battery based on the type of energy source identified. 
     In another embodiment of the invention, a method and apparatus are disclosed for protecting against certain energy sources used to charge a battery by determining the voltage at input terminals isolated from the battery by a switch, continuously determining if the voltage of the energy source at the input terminals is less than a maximum source voltage threshold and greater than a minimum source voltage threshold, opening the switch to disconnect the energy source from the battery and resetting a counter to zero if these conditions are not met, incrementing the counter if the voltage of the energy source at the input terminals is greater than a minimum source voltage threshold, closing the switch to connect the energy source to the battery if the counter has exceeded a counting threshold, confirming that the energy source is not an alternating current (AC) source, and continuously determining if the voltage of the energy source at the input terminals is greater than a minimum source voltage threshold. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       So that the manner in which the features of the invention can be understood, a detailed description of the invention may be had by reference to certain embodiments, some of which are illustrated in the accompanying drawings. It is to be noted, however, that the drawings illustrate only certain embodiments of this invention and are therefore not to be considered limiting of its scope, for the scope of the invention encompasses other equally effective embodiments. The drawings are not necessarily to scale, emphasis generally being placed upon illustrating the features of certain embodiments of invention. Thus, for further understanding of the invention, reference can be made to the following detailed description, read in connection with the drawings in which: 
         FIG. 1  illustrates an electrical power system in an exemplary embodiment of the invention. 
         FIG. 2  illustrates an input cable connection to the power center in an exemplary embodiment of the invention. 
         FIG. 3  illustrates a bicycle mounted on a bicycle generator adapter in an exemplary embodiment of the invention. 
         FIG. 4  illustrates a bicycle generator adapter (without the bicycle) in an exemplary embodiment of the invention. 
         FIG. 5  illustrates a user interface of a power center in an exemplary embodiment of the invention. 
         FIG. 6  is a block diagram of a power center in an exemplary embodiment of the invention. 
         FIG. 7  is a block diagram of a charge circuit in an exemplary embodiment of the invention. 
         FIG. 8  is a flow diagram for charge input protection in one exemplary embodiment of the invention. 
         FIG. 9  is a flow diagram for energy source identification in one exemplary embodiment of the invention. 
         FIG. 10  is a flow diagram for the charging mode of a solar panel in one exemplary embodiment of the invention. 
         FIG. 11  is a flow diagram for the charging mode of a wall outlet adapter in one exemplary embodiment of the invention. 
         FIG. 12  is a flow diagram for the charging mode of a bicycle generator in one exemplary embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  illustrates an electrical power system  1  in an exemplary embodiment of the invention. The electrical power system  1  can include a plurality of different types of energy sources (e.g., a solar panel  30  providing solar power from solar cells  31 , a wall outlet adapter  33  (e.g., wall wart or power brick) providing grid power from a wall outlet  32 , a bicycle generator  52  providing human-generated power from riding a bicycle  40 , a wind turbine  34  providing wind-generated power, a hydro turbine (not shown) providing hydro-generated power, attachments to animals (not shown) providing animal-generated power, etc.). The plurality of different energy sources can be connected to a power center  10  using input cables  17 . In one embodiment, only one of the plurality of different energy sources can be connected to the power center  10  at a time, while in another embodiment, multiple energy sources can be connected to the power center  10  at the same time.  FIG. 2  illustrates an input cable  17  connection to the power center  10  in an exemplary embodiment of the invention in which the input cable  17  is terminated with input cable plugs  18  (e.g., banana plugs) that are inserted into the input terminals  16  (e.g., banana jacks or binding posts) at the rear of the power center  10 . In another embodiment, bare wires can be attached to the screw terminals located on the input terminals  16 , providing further flexibility for connecting a plurality of different energy sources. 
     The power center  10  can also have a plurality of outputs, including one or more 12V DC cigarette lighter outputs  12  (also known as cigar lighter adapters), one or more 5V DC USB outputs  14 , and one or more AC outputs (e.g., 90-260V, types A-L, etc) (not shown). The cigarette lighter outputs  12  can be used to power a plurality of devices (e.g., a light  60 , water purifier (not shown), refrigerator (not shown)) using a cigarette lighter output cable  13 , while the USB outputs  14  can be used to power a plurality of other devices (e.g., mobile telephone  62 , radio  64 , LCD television (not shown)) using a USB output cable  15 . In one embodiment, the light  60  can be a fluorescent or incandescent bulb. In other embodiments, the light  60  can be one or more LEDs or other alternative light sources. Use of AC outputs (not shown) would require a DC to AC converter to provide AC power from a DC battery. 
       FIG. 3  illustrates a bicycle  40  mounted on a bicycle generator adapter  50  in an exemplary embodiment of the invention.  FIG. 4  illustrates a bicycle generator adapter  50  (without the bicycle  40 ) in an exemplary embodiment of the invention. The bicycle  40  has a front gear  42  with a certain number of front gear teeth (GT F )  43  connected via a chain  41  to drive a rear gear  44  with a certain number of rear gear teeth (GT R )  45 . The outer tread of the rear tire  46  comes into contact with the roller  54  of bicycle generator adapter  50  and causes the roller  54  to rotate along with the rear tire  46  during cycling. The roller  54  can be mounted to the stand  51  of the bicycle generator adaptor  50  using a roller mount  55  that can rotate or pivot to allow the roller  54  to contact rear tires  46  of different dimensions. The roller mount  55  and all components mounted to the stand  51  of the bicycle adapter  50  using the roller mount  55  are firmly attached to the stand  51 , but can also be removed from the stand  51  when the bicycle generator  52  is not in use. The stand  51  can also be folded when not in use. 
     The roller mount  55  can be attached to the rear wheel axle  48  of the rear wheel  46  using a tensioning mechanism  58  including hooks  59  attached to the roller mount  55  on one end and attached to the rear wheel axle  48  on the other end to keep the roller  54  in contact with the rear tire  46  during cycling to help avoid slippage between the rear tire  46  and the roller  54 . The tensioning mechanism  58  can be made of various materials (e.g., elastic, bungee cord, non-stretchable cord (e.g., nylon), inner tube rubber). 
     The roller  54  is connected to a shaft  53  that drives the bicycle generator  52 . In one embodiment, the shaft  53  can extend beyond the bicycle generator  52  and support a fan  56  that can rotate along with the shaft  53  providing air cooling of the bicycle generator  52 , improving the capacity and performance of the device. The bicycle generator  52  can be a 36V brushed DC motor. Other voltage motors (e.g., 24V) and types of motors (e.g., brushless DC motors) can also be used in other embodiments. The power output of the bicycle generator  52  can be increased or decreased based on the speed of rotation (RPM) of the shaft  53  (e.g., the faster the rotation, the more power is generated). In one embodiment, the speed of rotation of the shaft (S RPM ) can be determined by the cadence (C RAM ) of the cyclist, gear ratio (R G ) (i.e., the ratio of the number of front gear teeth (GT F )  43  to the number of rear gear teeth (GT R )  45 ), and the rear tire/roller ratio (R TR ) (i.e., ratio of the diameter of the rear tire (D T ) to the diameter of the roller (D R )). 
     
       
         
           
             
               
                 
                   
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     In one exemplary embodiment, the bicycle  40  and bicycle generator adapter  50  can have the following parameters, with a user cycling at a cadence (C RPM ) of 40 RPM resulting in a shaft speed (S RPM ) of 2,352 RPM: 
     
       
         
           
             
               
                 
                   
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     As can be seen from these equations, varying one or more of these parameters can increase or decrease the resulting shaft speed (S RPM ), which will determine the amount of electrical power generated. For example, increasing the cadence (C RPM ), gear ratio (R G ), or rear tire/roller ratio (R TR ) will increase the shaft speed (S RPM ), which will increase the amount of electrical power generated (e.g., in the range of 30 W to 100 W). Various combinations of these parameters from different bicycles can produce shaft speeds (S RPM ) such as 3,000 RPM or more, with a desired power output such as 100 W or more. Accomplishing such high shaft speeds (S RPM ) and power output without the use of a gear box improves the reliability of the bicycle generator  52  and reduces the cost. 
     In one embodiment, the roller  54  can be a 1″ (25.4 mm) diameter extruded aluminum tube with no additional surface treatment. Rollers  54  of other materials (e.g., polypropylene) and dimensions can also be used. A roller  54  with a larger diameter (e.g., 1.25″ (31.75 mm)) will reduce the rear tire/roller ratio (R TR ) and require a higher cadence (C RPM ) to provide the same amount of power as a smaller diameter, while a roller  54  with a smaller diameter (e.g., 0.75″ (19.05 mm)) will increase the rear tire/roller ratio (R TR ) and only require a lower cadence (C RPM ) to provide the same amount of power as a larger diameter. Use of a smaller diameter roller  54 , however, can increase the slip (SL) that occurs between the rear tire  46  and roller  54  during cycling, leading to inefficiencies (i.e., cycling effort not resulting in rotation of the roller  54  and shaft  53 ). The smaller diameter roller  54  can also result in more voltage spikes of higher amplitude which can cause damage to the electronics of the power center  10 . Use of a low-friction polypropylene roller  54  can also increase the slip (SL) as compared with the use of extruded aluminum. It is desirable to keep slip to a minimum as the greater the amount of slip (SL), the slower the shaft speed (S RPM ) as shown in the following equation, modifying equation (3) to account for slip (SL) (shown as a percentage, where no slip under ideal conditions would be SL=0.00 and ten percent slip would be SL=0.10)
 
 S   RPM   =C   RPM   *R   G   *R   TR *(1 −SL )  (7)
 
     In order to maintain slip below acceptable levels (e.g., below 10%), the tensioning mechanism  58  with hooks  59  on each end can be adjusted (e.g., with straps) to provide varying amounts of pressure from the roller  54  onto the rear tire  46  (e.g., 4.5 kg, 6.0 kg, 8.5 kg, etc.). The use of higher pressure, however, can increase the amount of energy needed by the user to provide the necessary cadence (C RPM ) and increase the amount of wear on the rear tire  46 . Similarly, including texture on the roller  46  to increase the friction between the roller  54  and the rear tire  46  reduces slip but also increases the amount of wear on the rear tire  46 . 
       FIG. 5  illustrates a user interface  11  of a power center  10  in an exemplary embodiment of the invention. The user interface  11  can include a plurality of elements designed to convey information about the power center  10  to the user (e.g., state of charge of the power center  10 , whether the battery of the power center  10  is charging or discharging, the rate of charging or discharging of the battery, whether maintenance is required, and whether an error or fault has occurred). 
     As shown in  FIG. 5 , the state/rate of charge indicator  20  can be a funnel or inverted cone shaped display of a plurality of LEDs or other visual indicators. LEDs can be used based on their reliability and long life as well as their low power consumption. The brightness and related current draw of the each of the LEDs can be controlled by selecting an appropriately series resister (e.g., 330 ohms). In one embodiment, the state/rate of charge indicator  20  can include a red first (low) state/rate LED  21 , an amber second state/rate LED  22 , an amber third state/rate LED  23 , and an amber fourth state/rate LED  24  to provide the required information. For example, if the red first (low) state/rate LED  21  were continuously illuminated, that would communicate to a user that the battery of the power center  10  has 25% or less of full capacity charge remaining, while if all four LEDs  21 ,  22 ,  23 ,  24  were continuously illuminated, including the amber fourth state/rate LED  24 , that would communicate to the user that the power center  10  has more than 75% of full capacity charge remaining. 
     When the power center  10  is charging/discharging, as indicated by the amber charge/discharge LED  25  (e.g., illuminated only when the battery is charging and turned off when the battery is not charging), the particular percentage range of remaining battery charge at that moment (e.g., 50% to 75%) can be communicated by flashing the corresponding charge/discharge LED (e.g., amber third state/rate LED  23 ) during charging/discharging. In addition, the rate of flashing of the corresponding charge/discharge LED can be determined by the rate of charging/discharging of the battery (e.g., the faster the rate of charging/discharging, the higher the frequency of flashing of the LED). In an alternative embodiment, the charge/discharge LEDs  21 ,  22 ,  23 ,  24  can be illuminated in a cascading or rippling fashion to indicate charging or discharging of the battery of the power center  10 . 
     The need for maintenance of the power center  10  can be communicated to the user by the red maintenance LED  26 , which can be illuminated to, e.g., indicate to the user that a full charge of the battery is required to optimize battery performance and life. In the event of a fault or other error, all of the LEDs can flash for a predetermined amount of time to alert the user of the existence of the error or fault, and that the outputs  12 ,  14  have been disconnected. In the event of a fault or other error, the multi-function pushbutton  28  can be pressed to reset the power center  10  to reactivate the outputs  12 ,  14  or to accept an energy source. In addition, the multi-function pushbutton  28  can be pressed during normal conditions to display the state of charge on the state/rate of charge indicator  20 . If the power center  10  is off, the multi-function pushbutton  28  can be pressed to energize the power center  10 . 
     In addition to the information about the power center  10  conveyed to the user by the user interface  11 , the power center  10  can also include an audible device to provide information and feedback. For example, a buzzer can be used that can play several different notes based on the switching frequency to provide audible notification when a particular event occurs (e.g., a positive sound to indicate the beginning of charging/discharging, a positive sound to indicate the completion of 1 watt-hour of charging, a negative sound if voltage is approaching dangerous levels, a beeping negative sound to indicate the presence of a fault, a continuous negative sound to indicate the use of an improper input device). 
     The power center  10  can also include a plastic enclosure with a dimple (not shown) on the underside of the unit at its center of gravity, shaped appropriately to rest on the top of a user&#39;s head during transport. 
       FIG. 6  is a block diagram of a power center  10  in one exemplary embodiment of the invention. The power center  10  includes a battery  2 , which can be a sealed, maintenance free 12V DC lead-acid battery. The power center  10  can include a microcontroller unit (MCU)  4  powered by the battery  2  (e.g., at 5V DC or 3.3V DC) that communicates relevant information about the power center  10  to the user through the user interface  11  discussed previously. For example, the MCU  4  can be directly connected to power the LEDs  21 ,  22 ,  23 ,  24 ,  25 ,  26  of the user interface  11  through a resistor without the need for driver circuitry. 
     The MCU  4  can also control the operation and interaction of the other components of the power center  10  to manage power measurements, and control the inputs and outputs to the power center  10  to optimize battery performance and life. The MCU  4  dynamically controls the charge circuit  6 , which is responsible for receiving power from a plurality of different types of energy sources attached to the input terminals  16  and using that power to charge the battery  2  in a safe and efficient way. The MCU  4  also dynamically controls the discharge circuit  8 , which supplies power to and monitors the outputs  12 ,  14  to prevent the user from drawing power out of the battery  2  in a manner that might damage the battery  2 . 
       FIG. 7  is a block diagram of a charge circuit  6  in an exemplary embodiment of the invention. The charge circuit  6  can include a charge input protection circuit  102  for protecting the power center  10  from being damaged by certain harmful or inappropriate energy sources connected to the input terminals  16 . A charge input switch  104  (provided hardware (mechanical (e.g., relay) or electrical (e.g., MOSFET)) or software) located after the charge input protection circuit  102  can isolate the energy sources from the power center  10  by serving as a gatekeeper to allow current (closed state) or block current (open state) from the sources from flowing into the remainder of power center  10  depending on whether those sources have been approved by the MCU  4 . A conventional buck regulator  106  can be used to step down source voltages for correct charging of the battery  2  and to modulate any excess power when the battery  2  charge is near full. The MCU  4  can use current and voltage sensor readings to identify the particular energy source attached to the input terminals  16 . Once the particular energy source has been identified, a charging mode  110  tailored to the particular energy source connected can be used to charge the battery  2 . In one embodiment, the charge circuit  6  can accommodate charging voltages from 16V DC (minimum) to 30V DC (maximum) and charge the battery  2  at up to 4 A or 50 W depending on the available input power from the energy source. Other embodiments could include different ranges for charging voltages, currents, and wattages. 
     Turning first to the protection offered by the charge circuit  6 , in one embodiment, the power center  10  should be able to withstand energy source voltages of up to 300V, alternating current (AC) sources, and inadvertent connections to the input terminals  16  with reverse polarity without damaging the power center  10 . In order to accomplish this protection, the charge input protection circuit  102  can include a conventional full bridge rectifier on the input terminals  16  that allows the MCU  4  to read input voltage safely in the case of an AC energy source. The charge input protection circuit  102  can also include a conventional rectifier diode, rated up to 300V, so that if energy source is inadvertently connected with reverse polarity to the input terminals  16 , no voltage will be sensed by the MCU  4 . The rectifier diode can be used in addition to the full bridge rectifier since the full bridge rectifier would make a energy source connected with reverse polarity to the input terminals  16  appear positive. The charge input protection circuit  102  can also include a voltage divider to scale down voltages of up to 300V. If the energy source connected to the input terminals  16  is determined to be safe, the current from the energy source will bypass the full bridge rectifier and rectifier diode so as not to incur any substantial power losses across those components. Protected by the full bridge rectifier, rectifier diode, and/or voltage divider, the MCU  4  can safely detect overvoltage and AC inputs of up to 300V. 
       FIG. 8  is a flow diagram for charge input protection performed by the MCU  4  with the charge circuit  6  in one exemplary embodiment of the invention. When the energy source is connected to the input terminals  16 , the current from the energy source first passes through the full bridge rectifier, rectifier diode, and/or voltage divider of the charge input protection circuit  102  as discussed previously, protecting the MCU  4 . The initial or reset state of the MCU  4  at step  200  of the charge input protection has an AC flag set to “UNKNOWN” since it is has not been determined whether there is an inappropriate AC source, an AC counter (C) in the MCU set to 0 (C=0), and the charge input switch  104  in an open state (i.e., the current from the charge source cannot flow into the remainder of the power center  10 ). 
     In one embodiment of the invention, the voltage of the energy source is checked periodically (e.g., every 1 ms or some other interval) at the overvoltage check step  202  to determine if the voltage is greater than a maximum source voltage threshold (e.g., 90V). If the voltage is greater than the maximum source voltage threshold, the power center  10  goes into to a fault condition and opens the charge input switch  104  at step  204 . If the voltage is not greater than the maximum source voltage threshold, the voltage of the energy source will continue to be checked periodically in a continuous loop. 
     In addition to checking for overvoltage, the voltage of the energy source is checked to make sure that it is not providing an AC signal that would damage the power center  10 . The voltage is checked at step  206  to determine if the voltage of the energy source is greater than a minimum source voltage threshold (e.g., 5V) but still less than the maximum source voltage threshold. If the voltage of the energy source is not greater than the minimum source voltage threshold, the system returns to the initial or reset state step  200 , and the AC counter (e.g., a timer) timer (C) is reset to zero. Using this test, if the voltage of the energy source is an AC signal with a frequency of 60 Hz (i.e., a period of 16.6 ms), the voltage will go through a complete cycle and, therefore, drop below the minimum voltage threshold every 16.6 ms, resetting the AC counter (C) to zero at step  200 . If the voltage of the energy source is greater than the minimum source voltage threshold, the status of the AC flag is checked at step  208  to see if it is “UNKNOWN.” If the status of the AC flag is not “UNKNOWN,” the system returns to the initial or reset state step  200 , and the AC counter (C) is reset to zero. If the status of the AC flag is “UNKNOWN,” the AC counter (C) is incremented (e.g., by 1 ms or some other time interval) at step  210 . Next, the AC counter (C) is checked to see if it is greater than an AC counting threshold (e.g., 600 ms) at step  212 . If the AC counter (C) is not greater than the AC counting threshold, the system loops back and repeats the tests performed at steps  206  and  208  to check again for minimum voltage and the status of the AC flag. If the AC counter (C) is greater than the AC counting threshold, the AC flag is set to “SAFE” and the charge input switch  104  is closed (allowing current to flow from the charge source into the remainder of power center  10 ) at step  214  based on the fact that the voltage of the energy source cannot be an AC voltage since it would have reset the AC counter (C=0) with a voltage below the minimum voltage threshold before the counting threshold was reached (i.e., the AC counter (C) would have reset after 16.6 ms for a 60 Hz AC signal). 
     As shown in  FIG. 7 , after passing through the charge input protection circuit  102  and charge input switch  104 , the signal from the energy source is received by the conventional buck regulator  106 , which converts the voltage of the energy source down to a lower voltage (e.g., 12V) for charging of the battery  2 . In one embodiment, the buck regulator  106  can use a transistor (e.g., a MOSFET) as a switch to alternatively connect (switch closed) and disconnect (switch open) the energy source, (e.g., at a frequency of 25 kHz). By varying the duty cycle (i.e., time that the switch is closed and the charge source is connected) of the buck regulator  106  using pulse width modulation (PWM), the MCU  4  can control charging of the battery  2 . 
       FIG. 9  is a flow diagram for energy source identification performed by the MCU  4  with the charge circuit  6  in one exemplary embodiment of the invention. As discussed previously and as shown in  FIG. 1 , the electrical power system  1  can include a plurality of different types of energy sources (e.g., a solar panel  30 , wall outlet adapter  33 , windmill  34 , bicycle generator  52 ). In one embodiment of the invention, the power center  10  can identify the particular energy source connected to the input terminals  16 . At step  300 , the charge input switch  104  is open when the energy source is connected to the input terminals  16 . At step  302 , the initial source voltage (V I ) at the input terminals  16  is measured by the MCU  4 . At step  304 , the MCU  4  measures the voltage transition time (T VT ) that it takes to transition from a point where there is substantially no voltage on the input terminals  16  (e.g., just prior to connection of the energy source) to a point where the voltage of the energy source is substantially constant and/or at or above the minimum charging voltage (e.g., 16V DC). If the voltage transition time (T VT ) is greater than a voltage transition time threshold (e.g., 30 ms), indicating that the constant charging voltage was arrived at relatively slowly, the energy source is most likely a source that provides substantially varying voltage output (e.g., bicycle generator  52 , wind turbine  34 , hydro turbine, hand-crank, animal generator, etc.). Accordingly, at step  305  and after it has been determined that the energy source is safe, the charge input switch  104  can be closed and charging for a bicycle generator  52  (or wind turbine  34 , hydro turbine, etc.) can begin. 
     If the voltage transition time (T VT ) is less than a voltage transition time threshold (e.g., 30 ms), indicating that the constant charging voltage was arrived at almost instantaneously, the energy source is most likely a source that provides substantially constant voltage outputs (e.g., a solar panel  30 , wall outlet adapter  33 , etc.). Additional steps can be used in order to distinguish between the solar panel  30  and the wall outlet adapter  33 . At step  306  and after it has been determined that the energy source is safe, the charge input switch  104  can be closed. At step  308 , the MCU  4  can increment the charging current of the source (I C ) by increasing the duty cycle of the buck regulator until a charge current threshold (e.g., I C =500 mA) is reached. At step  310 , the energy source voltage (V C ) at the input terminals is measured by the MCU  4 . At step  312 , the MCU  4  determines if the difference between the energy source voltage (V C ) and the initial source voltage (V I ) is greater than a voltage sag threshold (e.g., 1V DC). If the voltage had decreased or sagged less than the voltage sag threshold, then the energy source is most likely a source that provides a substantially constant voltage output under both load and no load conditions (e.g., a wall outlet adapter  33 ). If the voltage has sagged more than the voltage sag threshold, then the energy source is most likely a source that experiences substantial voltage sag under load conditions (e.g, a solar panel  30 ). As shown in  FIG. 6 , after the energy source has been identified, the power center  10  can enter the appropriate charging mode  110  for that particular energy source. In other embodiments of the invention, different techniques can be used to identify the energy source, including monitoring manual switches, communication over power, a digital handshake, or the use of special tip that plugs into the power center  10  to indicate the particular source employed. 
       FIG. 10  is a flow diagram for the charging mode  110  of the solar panel  30  in one exemplary embodiment of the invention. If the energy source is a solar panel  30 , the MCU  4  can dynamically track the maximum power point of the solar panel  30 , which depends upon voltage and current and changes as the solar panel  30  receives different levels of sunlight. In order to track the maximum power point, the MCU  4  continually monitors the effects of incrementing or decrementing the charging current (I C ) of the solar panel  30  to see if there is a more optimal charging point. In one embodiment, the MCU  4  can measure the original power output of the solar panel  30  (P O ) (e.g., the charging current (I C ) of the solar panel  30  multiplied by the output voltage of the solar panel  30 ) at step  400 . Next, the MCU  4  can increment the charging current (I C ) of the solar panel  30  by a step (e.g., 0.05 A) by increasing the duty cycle of the buck regulator at step  402 . After incrementing the charging current (I C ), the MCU  4  can check if the charging current (I C ) is less than the maximum (I MAX ) allowable charging current (e.g., 4 A) at step  404 . If the charging current (I C ) of the solar panel  30  is not less than the maximum allowable charging current (I MAX ), the MCU  4  can decrement the charging current (I C ) of the solar panel  30  by a step at step  406  and repeat the process. If the charging current (I C ) of the solar panel  30  is less than the maximum allowable charging current (I MAX ), the MCU  4  can then measure the new power output (P N ) of the solar panel  30  at step  408  based on the new charging current of the solar panel  30  to determine if the new power output (P N ) is greater than the original power output (P O ) at step  410  (i.e., to determine if incrementing the charging current improved the power output of the solar panel  30 ). If the new power output (P N ) of the solar panel  30  is greater than the original power output (P O ), the MCU  4  can repeat the process starting at step  400 . If the new power output (P N ) of the solar panel  30  is not greater than the original power output (P O ), the MCU  4  can decrement the charging current (I C ) of the solar panel  30  by a step at step  406  and repeat the process. The rate of incrementing or decrementing the charging current (I C ) of the solar panel  30  can be done relatively slowly since a rapid change in the charging current (I C ) could result in an unstable input voltage of the solar panel  30  since the voltage is current dependent. For example, the rate of incrementing the charging current (I C ) of the solar panel  30  should be chosen to avoid the possibility of decreasing the input voltage of the solar panel  30  below the minimum charging voltage (e.g., 16V DC). 
       FIG. 11  is a flow diagram for the charging mode  110  of a wall outlet adapter  33  in one exemplary embodiment of the invention. If the energy source is a wall outlet adapter  33 , the MCU  4  can dynamically determine the maximum current rating of the wall outlet adapter  33 , which will provide a rated constant voltage up to a rated maximum current. Since the voltage provided by the wall outlet adapter  33  will remain constant until the maximum current rating is exceeded, at which point the voltage will decrease (or sag), by incrementing the charging current (I C ) of the wall outlet adapter  33  and monitoring the voltage of the wall outlet adapter  33 , the MCU  4  can determine when the maximum current of the wall outlet adapter  33  is reached. In one embodiment, the MCU  4  can measure the original voltage (V O ) of the wall outlet adapter  33  at step  500 . Next, the MCU  4  can increment the charging current (I C ) of the wall outlet adapter  33  by a step (e.g., 0.1 A) at step  502  by increasing the duty cycle of the buck regulator  106 . After incrementing the charging current (I C ), the MCU  4  can check if the charging current is less than the maximum allowable charging current (I MAX ) (e.g., 4 A) at step  504 . If the charging current (I C ) of the wall outlet adapter  33  is not less than the maximum allowable charging current (I MAX ), the MCU  4  can decrement the charging current (I C ) of the wall outlet adapter  33  by a step at step  514  and charge at this current level. If the charging current (I C ) of the wall outlet adapter  33  is equal to the maximum allowable charging current (I MAX ) (not shown), the MCU  4  charge at this current level. If the charging current (I C ) of the wall outlet adapter  33  is less than the maximum allowable charging current (I MAX ), the MCU  4  can then measure the new voltage (V N ) of the wall outlet adapter  33  at step  506  to determine if the new voltage (V N ) has decreased (or sagged) less than a voltage sag threshold (e.g., 0.5V DC) as compared with the original voltage (V O ) at step  508 . If the voltage sag is not less than the voltage sag threshold, the MCU  4  can decrement the charging current (I C ) of the wall outlet adapter  33  by a step at step  514  and charge at this current level. If the voltage sag is less than the voltage sag threshold, the MCU  4  can next determine the internal battery voltage (V B ) of the power center  10  as step  510 , and then determine if the internal battery voltage (V B ) is less than the maximum safe internal battery voltage (V M ) at step  512 . If the internal battery voltage (V B ) is not less than the maximum safe internal battery voltage (V M ), the MCU  4  can decrement the charging current (I C ) of the wall outlet adapter  33  by a step at step  514  and charge at this current level. If the internal battery voltage (V B ) is less than the maximum safe internal battery voltage (V M ), the MCU  4  charge at this current level, increment the charging current (I C ) of the wall outlet adapter  33  by a step at  502 , and repeat the process. The rate of incrementing or decrementing the charging current (I C ) of the wall outlet adapter  33  can be done relatively quickly as compared to the solar panel  30 . 
       FIG. 12  is a flow diagram for the charging mode  110  of a bicycle generator  52  in one exemplary embodiment of the invention. If the energy source is a bicycle generator  52 , the MCU  4  can dynamically determine the internal battery voltage (V B ) to make sure that it is less than the maximum safe internal battery voltage (V M ), which changes based on the state of charge of the battery  2 . In one embodiment, the MCU  4  can increment the charging current (I C ) of the bicycle generator  52  by a step (e.g., 0.1 A) at step  600  by increasing the duty cycle of the buck regulator  106 . After incrementing the charging current ( 0 , the MCU  4  can check if the charging current is less than the maximum allowable charging current (I MAX ) (e.g., 4 A) at step  504 . If the charging current (I C ) of the bicycle generator  52  is not less than the maximum allowable charging current (I MAX ), the MCU  4  can decrement the charging current (I C ) of the bicycle generator  52  by a step at step  608  and repeat the test at step  602 . If the charging current (I C ) of the bicycle generator  52  is less than the maximum allowable charging current (I MAX ), the MCU  4  can next determine the internal battery voltage (V B ) of the power center  10  as step  604 , and then determine if the internal battery voltage (V B ) is less than the maximum safe internal battery voltage (V M ) at step  606 . If the internal battery voltage (V B ) is not less than the maximum safe internal battery voltage (V M ), the MCU  4  can decrement the charging current (I C ) of the bicycle generator  52  by a step at step  608  and repeat the test at step  602 . If the internal battery voltage (V B ) is less than the maximum safe internal battery voltage (V M ), the MCU  4  can increment the charging current (I C ) of the bicycle generator  52  by a step at  600  and repeat the process. 
     In one exemplary embodiment of the invention, a thermistor is used to measure ambient temperature. If the temperature falls outside of a safe operating range for the battery (e.g., −20° C. to 60° C.), the inputs and outputs are disabled, and an error message is displayed. Otherwise, if the ambient temperature is above 25° C., the maximum battery voltage for charging is reduced (e.g., by 24 mV/° C.) to protect the battery from damage due to overcharging, because the battery capacity is elevated at temperatures above 25° C. 
     As discussed previously and shown in  FIG. 5 , the need for maintenance of the power center  10  can be communicated to the user by the red maintenance LED  26 , which can be illuminated to indicate to the user that a full charge of the battery  2  is required to optimize battery performance and life. Repeated charging of the battery  2  without fully charging the battery  2  can significantly reduce the life of the battery  2 . In one embodiment of the invention, a counter or timer can be used to track the number of charges between full charges and/or the time between full charges and compare those numbers to thresholds based on proper maintenance of the battery  2 . For example, if it is advisable to complete a full charge after every seven incomplete charges, a counter can be used to count each of the incomplete charges. When that counter reaches seven, red maintenance LED  26  can be illuminated and reset when the MCU  4  determines that the battery  2  has been fully charged. Similarly, if it is advisable to complete a full charge after every seven days, a timer can be used to track the time. When the timer reaches seven days, red maintenance LED  26  can be illuminated and reset when the MCU  4  determines that the battery  2  has been fully charged. The MCU  4  can also measure cumulative power throughput and, after a certain threshold has been reached (e.g., 200 watt-hours), illuminate red maintenance LED  26 . 
     Returning to  FIG. 6 , the MCU  4  controls the discharge circuit  8 , which regulates power to and monitors the outputs  12 ,  14  to prevent the user from drawing power out of the battery  2  in a manner that might damage the battery  2 . The MCU  4  monitors the output currents and disables the outputs  12 ,  14  if the user attempts to draw too much power or if a short circuit occurs. In one embodiment, the 12V DC cigarette lighter outputs  12  are powered directly from the battery  2 , and the 5V DC USB outputs  14  are powered from the battery  2  through a 5V switching regulator. The outputs  12 ,  14  can be equipped with current sensors to measure the current draw. 
     This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal language of the claims.