fig1 shows an improved power system that provides energy to a load 10 . preferably , the load is one that requires low voltage such as a computer system or the like . the ibm 4683 point - of - sale system is an example of a low voltage load that could be coupled to the power system . the ibm 4683 point - of - sale system is a computer system that is used in several types of business establishments . the improved power system includes a battery 12 which provides stand - by power in the event that the ac voltage from ac power source 14 is disrupted . preferably , the battery is a sealed lead acid battery such as panasonic pn lcl12v24p . of course , other types of similar batteries could be used without deviating from the scope or spirit of the present invention . the positive terminal of battery 12 is connected to node n2 . node n2 is connected by deep discharge prevention circuit means 15 to a voltage step - down converter means 16 . as stated previously , the battery 12 is subject to damage if discharged below a preset voltage level . the function of the deep discharge prevention circuit means 15 is to disconnect battery 12 from load 10 when the voltage level at node n2 reaches the preset level . the voltage step - down circuit means 16 is a dc - to - dc converter circuit which accepts the voltage from the deep discharge prevention circuit means 15 and generates a voltage whose level is compatible with the requirements of load 10 . in case load 10 is a computer system , the output voltage from the voltage step - down converter means 16 is within the range of 5 volts . still referring to fig1 the improved power system includes a conventional power supply 18 and a battery charging generating means 20 . the power supply 18 is connected through a connector plug assembly 22 to ac power source 14 . the conventional power supply 18 accepts at its input an ac line voltage and generates a dc voltage on conductor 24 . in the preferred embodiment of the invention , the dc voltage on line 24 is within the range of 22 - 34 volts dc . the conventional power supply 18 comprises a step - down transformer means 18 &# 39 ; and full wave rectifier means 18 &# 34 ;. because the use of a step - down transformer and a full wave rectifier , for generating a desired dc voltage , is well known in the prior art technology , further description of the power supply 18 will not be given . still referring to fig1 the battery charging generating means 20 includes a voltage step - down converter circuit means 26 , battery charge circuit means 28 and battery feedback circuit means 30 . the details of each of these components will be given subsequently . suffice it to say at this point that the voltage step - down converter 26 accepts a dc voltage on conductor 24 and generates a lower voltage which is supplied on conductor 32 . in the preferred embodiment of this invention , the voltage on conductor 32 is within the range of from 12 - 16 . 3 volts dc . similarly , the battery charge circuit 28 accepts the voltage on conductor 32 and generates a lower voltage which is outputted on conductor 34 . the voltage on conductor 34 is substantially equivalent to the voltage on battery 12 . in the preferred embodiment of this invention , the voltage at node n2 is within the range of from 11 . 4 volts to 14 . 7 volts dc . since the voltage at node n2 varies within a relatively wide range , the battery voltage feedback circuit means 30 adjusts the voltage on node n1 relative to a reference voltage ( to be described subsequently ) in the step - down converter 26 . as a result of the battery voltage feedback circuit means 30 , the energy dissipation in the battery charger is significantly reduced . fig2 shows a circuit schematic for battery charger circuit means 28 . for simplicity , elements which are common to fig1 are labeled with the same numeral . it should be noted that the dc supply voltage v i is a symbolic representation of the dc voltage which is generated on conductor 24 ( fig1 ). the voltage step - down converter 26 is a dc to dc converter . this type of voltage converter is well known in the prior art . essentially , an output voltage v o is generated from an input voltage v i . v o is inherently less than v i . the output voltage v o is generated on capacitor c . the positive plate of capacitor c is tied to node n1 . thus , whatever voltage is on capacitor c is also present at node n1 . node n1 is connected to ground potential through series connected resistors r1 and r2 . a transistor q1 has it emitter connected via coil l to the positive plate of capacitor c and through diode 36 to the negative plate of capacitor c . the collector terminal of transistor q1 is coupled to the dc voltage v i and the base terminal of transistor q1 is coupled through base drive circuit 38 operational amplifiers 40 and 42 to node n3 . still referring to fig2 the battery charge circuit 28 includes series pass device q3 and base drive circuit means 44 . even though q3 can be an fet device , in the preferred embodiment of this invention , q3 is a bi - polar transistor . the emitter electrode of q3 is connected via conductor 34 to node n2 . the collector electrode of q3 is connected to node n1 and the base electrode of q3 is connected to base drive circuit means 44 . the series pass device q3 accepts an input voltage on node n1 and provides a regulated output current to node n2 . as stated previously , the step - down converter circuit 26 is a dc to dc converter which provides an output voltage v o which is approximately 16 . 3 volts on node n1 . the battery charger circuit 28 uses the voltage on n1 as its input and charges the battery which is connected to node n2 to a voltage which varies from say 11 . 4 volts to 14 . 7 volts depending upon the charge level of battery 12 . at full charge , q3 charges the battery with a constant current , say of 1 . 5 amps . the power dissipated in q3 without and with the battery voltage feedback circuit is as follows : ## equ1 ## as can be seen from the above equations , the power dissipated in q3 without the battery voltage feedback circuit 30 is 7 . 4 watts . this large power dissipation in q3 is undesirable . therefore , without the inclusion of battery voltage feedback circuit means 30 ( to be described subsequently ) q3 would have to be selected and provided with heat sinks which is able to handle the maximum power 7 . 4 watts . moreover , this amount of power dissipation would require an expensive transistor device and sinks and may even require the use of fans , etc ., for cooling purposes . the unnecessary expenses and design limitations are averted by the battery voltage feedback circuit means 30 which will now be described . referring again to fig2 the battery voltage feedback circuit means 30 samples the voltage at node n2 and adjusts the input voltage at n1 accordingly . thus , the power dissipation across q3 is significantly reduced and over design of the transistor and related components are avoided . the battery feedback circuit means 30 includes transistor q2 with its base terminal connected through r5 to ground potential . similarly , the base electrode of q2 is coupled by series connected diode 46 and resistor r4 to node n2 . the emitter of transistor q2 is coupled through r3 to node n1 . the collector of transistor q2 is connected to node n3 . in operation , r1 and r2 form a voltage divider such that the voltage across r2 is approximately equal to the reference voltage on the positive input of op amp 42 . the control circuit ( including op amp 40 with its ramp voltage on the positive terminal and base drive circuit 38 ) adjusts the duty cycle of transistor q1 and thus , v o in an effort to maintain the two inputs of op amp 42 approximately equal . since the reference voltage on the positive terminal of op amp 42 is constant and r1 and r2 are constant , the output voltage v o , at node n1 , would also remain approximately constant . however , resistors r4 and r5 form a voltage divider of the voltage at node n2 to that at the base of q2 . as the voltage at node n2 increases due to the battery 12 , the voltage at q2 base also increases . this causes the current through r3 to decrease , since the current through r2 unit remain constant then the current through r1 has to increase . this is done with an increase in voltage at node n1 . thus , when the voltage at n2 rises , so does the voltage at node n1 . if the voltage at node n2 decreases due to the battery 12 being discharged , then the voltage at q2 base decreases . this causes an increase in current through r3 , thus the current through r1 must decrease . this is accomplished with a decrease in voltage at node n1 . fig3 shows a circuit diagram for the deep discharge prevention circuit means 15 ( fig1 .). the function of this circuit is to attach node n2 so that of supplies power to computer system 10 , through means 16 when the voltage at node n2 reaches a preset turn - on level and disconnects node n2 when the voltage level drops to a preset on level . by disconnecting node n2 at a selected voltage level from the load , the battery 12 is prevented from deep discharging . to this end , the deep discharge prevention circuit includes fet device q5 which is controlled to switch when the voltage at node n2 reaches one of two preset levels . in the preferred embodiment of this invention , q5 is a p - channel fet device . likewise , the device switches to its on state and thus connects the battery to the load when the voltage level at n2 is approximately 12 . 2 volts . this is called the turn - on voltage . similarly , the fet device q5 switches to an off state when the voltage at node n2 drops to 11 . 4 volts . this is called the turn - off voltage . the turn - on voltage is set by zenner diodes cr1 and r6 . likewise , the turn - off voltage is set by zenner diodes cr2 and r8 . the gate electrode of fet device q5 is connected to the collector electrode of q4 . the emitter electrode of q4 is connected to ground potential . also r7 is connected from the base electrode of q4 to ground . r10 ties the source electrode of q5 to the gate electrode of q5 . the bi - polar transistor q4 and r10 provide hysteresis so that once q5 begins to on , it is forced into its turn - on state . likewise , when q5 begins to turn off , it is forced into its off state . it is worthwhile noting that since zenner diodes do not come in 1 % steps like resistors , r6 is used is series with cr1 to add the proper voltage from one zenner value to the next . similarly , r8 is used for the same purpose in series with cr2 . thus , cr1 and r6 are used to control the exact battery voltage at which q5 turns on . similarly , cr2 and r8 are used to determine the exact battery voltage at which q5 turns off . it should be noted that with proper selection of q5 and the respective voltage setting devices , any load can be switched on / off from a battery or any other dc power source . also , the switching is done instantaneously due to the latching feature of the circuit . also , the circuit uses low power , low cost components while having the capability to switch high currents . also , this circuit does not load the battery when the load is disconnected from it since all active devices are turned off . while the invention has been particularly shown and described with reference to a preferred embodiment thereof , it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention .