Abstract:
An energy storage system is provided that includes input terminals for receiving input energy from a remote power source and an energy storage device coupled to the input terminals. The energy storage device is operative to store at least of portion of the input energy and to supply stored energy to the input terminals. A pump device is also coupled to the input terminal and to the storage device. The pump device is operative to cause the energy storage device to store at least a portion of the input energy when the input energy level is above a first threshold level, and to cause the energy storage device to supply energy to the input terminals when the input energy level is below a second threshold level. The energy storage system is optionally provided with a current limiter for added protection against short circuits. In one embodiment, the current limiter is bi-direction; it protects against short circuits originating at both the power source and within the energy storage system. Also, the energy storage system is optionally provided with a safety discharge circuit that discharges the energy storage system upon its removal from the energy storage system.

Description:
BACKGROUND OF THE INVENTION 
     The present invention is directed toward the field of energy storage systems. In particular, the invention is directed to energy storage systems for use with remotely powered electronic or telecommunication devices. 
     Remotely powered devices are devices that are provided power from a power source located some distance away through the use of power transmission wires. When the remotely powered device&#39;s load demands are low or average, the power transmission wires are capable of delivering sufficient current and voltage. But, during peak load demand periods, the power transmission wires may not be capable of delivering sufficient power because of, among other things, power losses in the transmission wires and the power source&#39;s power supplying limits. To counteract these limitations, remotely powered devices are often provided with energy storage systems that store energy during low and average load demand periods and supply energy to the remotely powered devices during peak load periods. 
     A specific type of remotely powered electronic device is known as an optical network unit (“ONU”). An ONU is a device that is used as an interface between fiber optic telecommunication lines and traditional wires used to provide telecommunication services such as cable television and telephonic services to homes or other buildings. The ONU has a power supply that typically includes: (i) input protection and filter circuitry; (ii) energy storage circuitry, (iii) input voltage monitors and threshold circuitry, (iv) D.C. to D.C. power converters; (v) ringing generators; and (vi) alarm and digital interface circuitry. 
     Power is supplied to the ONU from a central location through thin telephone wires. As a result, the available peak power is extremely limited. At an ONU, the load current demand varies depending on the customers&#39; telecommunication service usage. Peak loads occur, for example, when phone sets ring or when a coin-phone executes a coin-collection operation. The peak power requirement is substantially higher than the average requirement and typically exceeds the available power supplied over the power transmission wires. Thus, some form of local energy storage is needed that can supply energy to the ONU during peak load periods. 
     A few storage methods have been proposed to help meet the peak power requirement. In the past, batteries have been used for energy storage. Batteries, however, have limited service life and require periodic maintenance. They are not well accepted for use with modem remote telephone equipment. 
     Other methods include the use of a very large storage capacitor C to provide the energy storage, such as a 200V, 8000μF capacitor. When this method is used with an ONU, a capacitor is coupled across the input terminals of the ONU and is charged up, when the load conditions are low or average, to the input line voltage of typically 90V to 140V. During a peak load event, the input powering line will supply some of the power while the storage capacitor supplies a substantial portion of the load power by discharging its stored energy. 
     This method, however, is very inefficient. For example, the capacitor will only discharge enough energy to decrease the capacitor voltage from typically 90V, (V 1 ), the voltage level at which the capacitor is initially charged, to typically 70V, (V 2 ). In this example, the available energy is equal to ½*C[(V 1 ) 2 −(V 2 ) 2 ]=12.8 Joules. The energy stored in the capacitor before the discharge, however, was ½*C(V 1 ) 2 =32.4 Joules. Thus, only 40% of the stored energy in the capacitor was made available to supply peak loads. As a result, larger and more costly capacitors must be used to meet peak load demands. 
     Therefore, there remains a need in this art for a more efficient energy storage system that can make more efficient use of the stored energy to meet peak load power demands. There remains a more particular need for an efficient energy storage system that can utilize smaller storage elements resulting in a substantial reduction in cost and size. 
     SUMMARY OF THE INVENTION 
     The present invention overcomes the problems noted above and satisfies the needs in this field for an efficient energy storage system. The pumped storage system of the present invention interposes a bi-directional pump, or alternatively known as a switching power converter, between the input powering terminals of the remotely powered device and a storage device. Whenever the available input power exceeds the load power demands, the bi-directional pump will pump energy into the storage device, charging it close to its voltage limit, which is higher than the level the storage device would be charged to under presently known methods. When the available input power is less than the load power demand, the bi-directional pump will reverse direction and pump energy from the storage device into the input terminal, supplying power to the load. The bi-directional pump is capable of supercharging the storage device, i.e., storing more energy into the storage device than presently known methods, making this form of storage much more space efficient. 
     The present invention provides additional features not found in the presently known energy storage systems. Not all of these features are simultaneously required to practice the invention as claimed, and the following list is merely illustrative of the types of benefits that may be provided, alone or in combination, by the present invention. These advantages include: (1) a bi-directional current limiter for added protection against short circuits; (2) more efficient use of the storage elements resulting in cost savings, physical space savings and the capability of using fewer, smaller, and less costly capacitors; and (3) a safety discharge circuit for added safety in handling the storage elements. 
     In accordance with the present invention, an energy storage system is provided that includes input terminals for receiving input energy from a remote power source and an energy storage device coupled to the input terminals. The energy storage device is operative to store at least a portion of the input energy and to supply stored energy to the input terminals. A pump device is also coupled to the input terminal and to the storage device. The pump device is operative to cause the energy storage device to store at least a portion of the input energy when the available input power exceeds the load power demands, and to cause the energy storage device to supply energy to the input terminals when the available input power is less than the load power demand. 
     In one embodiment the pump device uses power threshold levels to determine whether the available input power exceeds the load power demands. The power threshold levels are voltage threshold levels in one embodiment. In another embodiment, the power threshold levels are current threshold levels. 
     In yet another embodiment, the pump device is operative to cause the energy storage device to store at least a portion of the input energy when the available input power exceeds a first power threshold level, and to cause the energy storage device to supply energy to the input terminals when the available input power is less than a second power threshold level. In this embodiment, the power threshold levels may not correspond to whether the available input power exceeds the load power demands. 
     The energy storage system is optionally provided with a current limiter for added protection against short circuits. In one embodiment, the current limiter is bi-direction; it protects against short circuits originating at both the power source and within the energy storage system. Also, the energy storage system is optionally provided with a safety discharge circuit that discharges the energy storage system upon its removal from the remotely powered device. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will become more apparent from the following description when read in conjunction with the accompanying drawings wherein: 
     FIG. 1 is a block diagram of a remotely powered system; 
     FIG. 2 is a block diagram of a preferred embodiment of a pumped capacitive storage system according to the present invention; 
     FIG. 3A is a schematic diagram of a preferred embodiment of a pumped capacitive storage system according to the present invention; 
     FIG. 3B is a continuation of FIG. 3A; and 
     FIG. 4 is a block diagram of an alternate remotely powered system. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to the drawings, FIG. 1 sets forth a block diagram of a remotely powered system. The power source Vs provides power on power transmission lines  2 ,  3  to a remotely powered device  4 . In the illustrated embodiment, the power source Vs is a DC power source, but the system could be employed with an AC power source rectified to D.C. Due to resistance within the power transmission lines  2 ,  3 , the line voltage VL at the input T, R to the remotely powered device  4  is somewhat less than the voltage supplied by the power source Vs. The line voltage VL at the input T, R of remotely powered device  4  is supplied to local energy storage device  6  so that energy can be stored therein during low load periods for use by remotely powered device  4  during peak load periods or during periods when the power transmission lines are not supplying sufficient power to meet the power needs of remotely powered device  4 . The line voltage VL at the input T, R is also supplied to switching converters  8  which convert the line voltage VL to voltage levels usuable by the power loads  10  within remotely powered device  4 . 
     Now referring to FIG. 2, the local energy storage device  6  used in the remotely powered device  4  of the present invention is the pumped local energy storage device  6 . In the illustrated embodiment, the pumped local energy storage device  6  includes an input capacitor C 9 , a storage capacitor C 10 , and a bi-directional switching converter  16  which is also designated as the pump  16 . 
     The basic operation of the preferred pumped local energy storage device  6  is as follows. In the illustrated embodiment, the input line voltage VL can range from 140V down to 70V. The input capacitor C 9  serves as transient bypass for all internal circuits of remotely powered device  4 . Whenever the input line voltage VL is above a first threshold, 80V in the preferred embodiment, bi-directional switching converter  16  will use any available current at input T, R to charge the storage capacitor C 10  up to the maximum storage level, 190V for the 200V capacitor used in the preferred embodiment. During the charging process, the input voltage VL is kept at the first threshold, 80V in the preferred embodiment, and all available current is used to charge storage capacitor C 10 . After storage capacitor C 10  is fully charged to the maximum storage level, 190V in the preferred embodiment, the input voltage VL will then rise to a level determined by the steady load conditions. This pumping and supercharging of the storage capacitor C 10  leads to increased efficiency and the capability of using a smaller capacitor. By supercharging, a greater percentage of the storage capacity of storage capacitor C 10  is utilized. 
     The input capacitor C 9  in the preferred embodiment is physically located within pumped storage device  6 , but could physically reside at another location within the remote device  4  without departing from the present invention. Alternatively, one of skill in the art could use existing capacitance in the remote device  4  in lieu of adding an additional input capacitor. 
     During occasional heavy load conditions, the load current required by remotely powered device  4  may exceed the available current from the input line at T, R. As a result, input capacitor C 9  will discharge and the input voltage VL will gradually decrease. When the input line voltage VL reaches the first threshold level, 80V in the illustrated embodiment, the bi-directional switching converter  16  will reverse direction and discharge the energy in storage capacitor C 10  into the input capacitor C 9 . Storage capacitor C 10  is sized to provide adequate energy storage for the intended surge loads. In the illustrated embodiment, both the input capacitor C 9  and the storage capacitor C 10  are 1000 μF, 200V capacitors. The preferred embodiment has been arranged such that the pump is triggered by the input voltage level. One skilled in the art, however, could also implement the present invention by triggering the pump based, alternatively, on other energy levels such as current levels without departing from the spirit of the invention. 
     The use of the pump  16  of the present invention makes more efficient use of the storage capacity, and allows a typically 9:1 reduction for the size of the storage capacitor. This affords a substantial reduction in cost and more importantly physical size. The idle power and conversion efficiency of the pump can be designed to consume a fraction of a Watt and represent a minuscule burden on the system. 
     The preferred pumped local energy storage device  6  is also provided with a bi-directional current limiter circuit  12  and a safety discharge circuit  20 . The bi-directional current limiting circuit  12  may be inserted in series between either input T or R and the bi-directional switching converter  16  to limit the charge/discharge currents that could occur during an unintentional short to GND or high-voltage source Vs. The safety discharge circuit  20  discharges the high-voltage storage capacitor C 10  to a lower safe voltage level, 4V in the preferred embodiment, when the pumped local energy storage device  6  is physically removed from active duty. 
     Referring now to FIGS. 3A and 3B, a functional description of the preferred circuit implementation follows. The description that follows is directed to an implementation of a preferred bi-directional switching converter or pump  16  that uses boost and buck converter configurations. One skilled in the art could, without departing from the spirit of the invention, implement the present invention using a different pump configuration such as a pump configuration using, for example, a bi-directional flyback converter configuration, a c{tilde over (u)}k converter configuration, a SEPIC converter, or a number of other switching converter configurations. The present invention could also be implemented using a bi-directional flyback converter configuration and a super capacitor as the pump and storage device, respectively. 
     The main components of the illustrated bi-directional switching converter or pump  16  include switches Q 7  and Q 8  and inductor L 1 , inductor L 1  having an input end  15  and a switching end  17 . The input line voltage VL is coupled to the input end  15  of inductor L 1  and switches Q 7  and Q 8  are coupled to the switching end  17  of inductor L 1 . Switches Q 7  and Q 8  are driven by a pulse width modulator  14  and alternately connect the switching end  17  of inductor L 1  between GND and storage capacitor C 10 . The illustrated bi-directional switching converter or pump  16  is recognized as a standard boost or buck converter. In the direction of the capacitor C 9  to the capacitor C 10 , it is a boost converter. In the direction of the capacitor C 10  to the capacitor C 9 , it is a buck converter. 
     Because switches Q 7  and Q 8  are actively driven switches, bi-directional switching converter or pump  16  operates in the continuous current mode at all times. In fact, the ratio of voltages on input capacitor C 9  and storage capacitor C 10  (V 9  and V 10  respectively) are determined by the switching, duty cycle signal D where: 
     
       
           V   9 = D*V   10 , or equivalently  V   10 = V   9 / D , 
       
     
     where duty cycle signal D is the fraction of the switching cycle that switch Q 7  is on and switch Q 8  is off. 
     Thus by controlling duty cycle signal D, the voltage ratio can be changed and the direction of the pump determined. 
     The preferred bi-directional switching converter  16  includes a control circuit  26  that controls the generation of signal D and the switching of switches Q 7  and Q 8 . The control circuit  26  functions as follows. When voltage V 9  is below 80V, signal D is set at a maximum duty-cycle, close to unity. Switch Q 7  is ON almost constantly. Storage capacitor C 10  is effectively connected to input capacitor C 9 . When voltage V 9  approaches and reaches 80V from below, signal D is slowly decreased. This causes the pump  16  to increase voltage V 10  by pumping current into storage capacitor C 10 , thus charging the storage capacitor C 10 . During the pump-up, all available input current is used to charge the storage capacitor C 10  and the input V 9  stays at 80V. 
     When voltage V 10  reach a predetermined level, 190V in the illustrated embodiment, a detector  30  stops any further decrease of signal D and thus stops the charging process. At this point, the available current will flow into input capacitor C 9  and voltage V 9  will increase until it reaches a voltage level determined by system load conditions. During the time that input capacitor C 9  is charging, signal D will adjust itself to accommodate the voltage-ratio equation above. 
     During overload condition, load current demand exceeds available line current and input capacitor C 9  will first discharge to supply the load. Voltage V 9  will decrease in the process. When voltage V 9  decreases to 80V, the pump control circuit  26  will increase the duty cycle D, thus pumping energy from storage capacitor C 10  into input capacitor C 9 , discharging storage capacitor C 10  in the process. The discharging of storage capacitor C 10  will continue until the overload condition stops. The available line current will then exceed the load current, and voltage V 9  will increase and approach 80V from below. The pump  16  will then operate in the charging mode as described above. 
     The preferred bi-directional switching converter  16  is also provided with the following support circuits. An oscillator  22  is provided to set the switching frequency. The pulse-width-modulator  14  is provided to generate the duty-cycle D signal. A dead-time control circuit  24  is provided to generate gapped gate-drive signals for switches Q 7  and Q 8 . The gap is necessary to insure that switches Q 7  and Q 8  are never on at the same instance. Also, the preferred bi-directional switching converter  16  is provided with an accurate 5V reference  28  for V 9  and V 10  sensing and threshold detection circuits, which will be described in more detail below. 
     The basic elements of the preferred pumped local energy storage device  6 , bi-directional switching converter  16 , input capacitor C 9 , storage capacitor C 10 , bi-directional current limiter circuit  12 , safety discharge circuit  20 , control circuit  26 , oscillator  22 , pulse-width-modulator  14 , dead-time control circuit  24 , and 5V reference  28 , have been generally described. The preferred energy storage device  6  also has inputs for the input line voltage VL, Ground, and a 10V supply to power the internal circuits. A more detailed description of the specific implementation of the preferred embodiment follows. 
     The preferred current limiter  12  consists of four transistors Q 10 -Q 13 , three resistors R 32 -R 34  and two diodes CR 7 -CR 8 . Transistors Q 10  and Q 13  are pass devices and preferably implemented with FETs. They are normally biased in the full conduction state. Normally, resistor R 33  and diode CR 7  keep the gate-to-source voltage of transistor Q 10  at 10V. Similarly, resistor R 34  and diode CR 8  keep the gate-to-source voltage of transistor Q 13  at 10V. Resistor R 32  senses the current flowing through the pass devices Q 10  and Q 13 . 
     The preferred current limiter  12  functions as follows. When the voltage across resistor R 32  exceeds a diode drop, either transistor Q 11  or Q 12  will start conducting depending on the direction of the current flow. When transistor Q 11  starts conducting, it will put the transistor Q 10  in a regulating state. The transistors Q 10  and Q 11  and resistor R 32  will form a constant current regulator in one direction. When transistor Q 12  starts conducting, it will put the transistor Q 13  in a regulating state. The transistors Q 12  and Q 13  and resistor R 32  will form a constant current regulator in the other direction. The resistance of resistor R 34  determines the magnitude of the current limit. 
     The preferred current limiter  12  uses the following components: 
     
       
         
               
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 Q10 
                 1RF9620 
               
               
                   
                 Q11 
                 2N5807 
               
               
                   
                 Q12 
                 2N5807 
               
               
                   
                 Q13 
                 IRF9620 
               
               
                   
                 R32 
                 1Ω, 1W 
               
               
                   
                 R33 
                 1M, .1W 
               
               
                   
                 R34 
                 1M, .1W 
               
               
                   
                 CR7 
                 1N5240 
               
               
                   
                 CR8 
                 1N5240 
               
               
                   
                   
               
             
          
         
       
     
     The 5V reference  28  consists of resistors R 1 -R 3 , capacitor C 3 , and diode CR 1 . Diode CR 1  is a 2.5V shunt-regulator IC. Resistor R 1  provides the bias current for diode CR 1 . Resistors R 2  and R 3  scales the 2.5V reference to 5V. Capacitor C 3  provides feedback compensation for the shunt regulator IC CR 1 . 
     The preferred 5V reference  28  uses the following components: 
     
       
         
               
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 R1 
                 4020Ω, .1W 
               
               
                   
                 R2 
                 24.9K, .1W 
               
               
                   
                 R3 
                 24.9K, .1W 
               
               
                   
                 C3 
                 .01μF, 50V 
               
               
                   
                   
               
             
          
         
       
     
     The oscillator  22  consists of timer U 1 , resistor R 4  and capacitor C 4 . Timer U 1  is the industry standard 555-timer. The frequency is determined by resistor R 4  and capacitor C 4 . The square-wave output of timer U 1  is integrated by resistor R 5  and capacitor C 5  to provide a 100KHz-sawtooth for use by the pulse-width-modulator  14 . 
     The preferred oscillator  22  components are the following: 
     
       
         
               
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 R4 
                 4990, .1W 
               
               
                   
                 C4 
                 1000pF, 50V 
               
               
                   
                 R5 
                 4990Ω, .1W 
               
               
                   
                 C5 
                 1000pF, 50V 
               
               
                   
                   
               
             
          
         
       
     
     The pulse-width-modulator  14  (“PWM”) consists of a comparator U 2   a,  transistors Q 1  and Q 2  and resistor R 6 . The comparator detects the intersection of its two input signals, the 100khz-sawtooth and the output from control circuit  26 , and forms a 10V binary logic output signal with a duty cycle D. 
     The preferred PWM  14  uses the following components: 
     
       
         
               
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 U2a 
                 LM2901 
               
               
                   
                 Q1 
                 2N5210 
               
               
                   
                 Q2 
                 2N5087 
               
               
                   
                 R6 
                 10K, .1W 
               
               
                   
                   
               
             
          
         
       
     
     The output of PWM  14  is low pass filtered by the combination of resistor R 7  and capacitor C 6  and compared to a 3.3V and a 6.7V threshold. The threshold voltages are provided by resistors R 8 -R 10 . Comparators U 2   b  and U 2   c  with output pull-up resistors R 11  and R 12 , produce two pulse-width-modulated waveforms with appropriate gaps for driving the high-voltage switches Q 7  and Q 8 . Transistors Q 3  and Q 4 , capacitor C 7 , diode CR 2 , and resistors R 13  and R 14  perform the gate-drive and level shifting for driving switch Q 7 . Transistors Q 5  and Q 6 , capacitor C 8 , diode CR 3 , and resistors R 15  and R 16  perform the gate-drive function for switch Q 8 . 
     The preferred component values are shown below: 
     
       
         
               
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 U2b 
                 LM2901 
               
               
                   
                 U2c 
                 LM2901 
               
               
                   
                 C6 
                 100pF, 50V 
               
               
                   
                 R7 
                 2K, .1W 
               
               
                   
                 R8 
                 10K, .1W 
               
               
                   
                 R9 
                 10K, .25W 
               
               
                   
                 R10 
                 10K, .25W 
               
               
                   
                 R11 
                 10K, .1W 
               
               
                   
                 R12 
                 10K, .1W 
               
               
                   
                 Q3 
                 2N5210 
               
               
                   
                 Q4 
                 2N5087 
               
               
                   
                 Q5 
                 2N5210 
               
               
                   
                 Q6 
                 2N5087 
               
               
                   
                 C7 
                 .1μF, 200V 
               
               
                   
                 C8 
                 .1μF, 200V 
               
               
                   
                 CR2 
                 1N4745 
               
               
                   
                 CR3 
                 1N4745 
               
               
                   
                 R13 
                 10K, .1W 
               
               
                   
                 R14 
                 20Ω, .1W 
               
               
                   
                 R15 
                 20Ω, .1W 
               
               
                   
                 R16 
                 10K, .1W 
               
               
                   
                   
               
             
          
         
       
     
     The pump control  26  senses voltages V 9  and V 10 . Voltage V 9  is sensed and scaled down by the resistive divider  32  made up of resistors R 17 -R 19 . The scaling is set such that when voltage V 9 =80V, the divider  32  output=5V. The divider  32  output is compared to the 5V reference  28 . The Operational Amplifier U 3   a  and capacitor C 12  form an integrator whose output ramps up when voltage V 9 &lt;80V and ramps down when voltage V 9 &gt;80V. This is the direction control for the pump  16 . 
     Voltage V10 is sensed and scaled down by the resistive divider  32  made up of resistors R 20 -R 22 . The scaling is set such that when voltage V10=190V, the divider  32  output=5V. The divider  32  output is compared to the 5V reference  28 . The Operational Amplifier U 3   b  and capacitor C 11  form an integrator whose output ramps up when voltage V 10 &lt;190V and ramps down when voltage V 10 &gt;190V. 
     When voltage V 10 &lt;190V, diode CR 6  is back biased and does not conduct. It has no effect on the pump direction. When voltage V 10 &gt;190V, diode CR 6  is forward bias and bleeds current from the voltage V 9  resistive divider  32 . This renders the pump-direction integrator in a regulation state, and regulates voltage V 10  at 190V, as voltage V 9  changes according to load conditions. Diode CR 5  keeps the output ofthe voltage V 10  sense OpAmp U 3   b  from saturation toward the 10V supply. 
     The preferred pump control  26  uses the following components: 
     
       
         
               
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 R17 
                 374K, .1W 
               
               
                   
                 R18 
                 374K, .1W 
               
               
                   
                 R19 
                 49.9K, .1W 
               
               
                   
                 R20 
                 1M, .125W 
               
               
                   
                 R21 
                 815K, .1W 
               
               
                   
                 R22 
                 49.9K, .1W 
               
               
                   
                 C11 
                 .1μF, 50V 
               
               
                   
                 C13 
                 1000pF, 50V 
               
               
                   
                 CRC 
                 1N914 
               
               
                   
                 CR5 
                 1N914 
               
               
                   
                   
               
             
          
         
       
     
     The safety discharge circuit  20  consists of resistors R 23 -R 27 , transistor Q 9  and diode CR 4 . Normally, an external short circuit between terminals DISCHARGE1 and DISCHARGE2 keeps transistor Q 9  in a non-conducting state. When the energy storage device  6  is physically removed from the remote device  4 , the external short circuit no longer be present. Resistor R 27  and diode CR 4  will provide 16V gate drive for transistor Q 9  and put transistor Q 9  in the full conduction state. Resistors R 23 -R 26  are power resisters that will be connected across storage capacitor C 10 , discharging storage capacitor C 10  in the process to a safe level of 4V. The discharge time-constant is 0.2 Sec. This should be fast enough to dissipate the stored energy in storage capacitor C 10  and to prevent service craftsmen from accidentally contacting potentially hazardous high voltage. 
     The preferred component values are shown below: 
     
       
         
               
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 R23 
                 51Ω, 5W 
               
               
                   
                 R24 
                 51Ω, 5W 
               
               
                   
                 R25 
                 51Ω, 5W 
               
               
                   
                 R26 
                 51Ω, 5W 
               
               
                   
                 R27 
                 2M, .125W 
               
               
                   
                 Q9 
                 1RF840 
               
               
                   
                 CR4 
                 1N4745 
               
               
                   
                   
               
             
          
         
       
     
     Now referring to FIG. 4, another alternative embodiment of the present invention is shown. The remote device  4  in this embodiment utilizes two power detection circuits  36  &amp;  38  which are external to the pumped storage device  6 . The input power detection circuit  36  senses a characteristic of the available input power to device  4 . The load power detection circuit  38  senses a characteristic of the load power demanded by device  4 . Output signals  37  and  39  from the input power detection circuit  36  and the load power detection circuit  38 , respectively, are provided to a decision unit  40 . The decision unit  40 , based on the signals  37  and  39  received from the input power detection circuit  36  and the load power detection circuit  38 , provides a signal  41  to pumped energy storage system  6  indicating whether pump  16  should cause the energy storage element C 10  to store energy or provide power to the device  4 . The decision unit  40  and the power detection circuits  36  and  38  in the preferred embodiment are arranged in the same configuration as pump control  26 , but could be arranged in other configurations. 
     The characteristics sensed by the power detection circuits  36  and  38  in the preferred embodiment are voltage levels but could be the actual available input power and power demand, the input and output currents, the voltage levels, or any other signals related to power. The power detection circuits in the preferred embodiment comprise voltage divider networks but could comprise other circuits capable of measuring the power characteristics. 
     The decision unit  40  compares the characteristics sensed by power detection circuits  36  and  38  either to each other or to threshold levels to determine whether the energy storage unit C 10  should store energy or provide power and outputs a signal  41  to the pumped energy storage device  6  indicating the decision. The decision unit  40  preferably utilizes comparators without hysteresis, but could utilize comparators with hysteresis or other devices without departing from the spirit of the invention. The pump  16 , based on the signal  41  from decision unit  40 , functions as previously described to cause the energy storage element C 10  to store energy or provide power to the device  4 . 
     Having described in detail the preferred and alternate embodiments of the present invention, including preferred modes of operation, it is to be understood that the operation could be carried out with different elements and steps. The preferred and alternate embodiments are presented only by way of example and are not meant to limit the scope of the present invention which is defined by the following claims.