Redundant residential power sources

Abstract

Methods, systems, and apparatus, including computer programs encoded on a computer storage medium, for controlling a battery power source. In one aspect, a system includes a first MOSFET having a first gate, a first source, and a first drain. A second MOSFET having a second gate, a second source, and a second drain. The first source is connected to the second source, and the second drain is coupled to a ground. A control circuit connected to the first gate and the second gate and that provides control signals to the first gate and the second gate that cause the first and second MOSFETS to operate in saturation regions during a first operational state to cause the first power source to discharge and the first MOSFET operates in a linear region during a second operational state to limit a charging current that charges the first power source.

What is claimed is: 
     
         1 . A system, comprising:
 a bidirectional switching circuit having a first switch terminal and a second switch terminal, wherein the second switch terminal is coupled to a ground;   a first power source having a first power terminal and a second power terminal, wherein the first power terminal is connected to the first switch terminal and second power terminal is connected to a DC bus;   a second power source having a third power terminal and a fourth power terminal, wherein the third power terminal is connected to the ground and the fourth power terminal is connected to the DC bus; and   a control circuit connected bidirectional switching circuit and that provides control signals to the bidirectional switching circuit that cause:
 the bidirectional switching circuit to operate in a closed state during a first operational state to cause the first power source to discharge; and 
 the bidirectional switching circuit operates in a current controlling state during a second operational state to limit a charging current that charges the first power source. 
   
     
     
         2 . The system of  claim 1 , wherein:
 the first operational state occurs when a reference voltage is higher than a voltage of the first power source as measured between the first power terminal and the second power terminal; and   the second operational state occurs when:
 the voltage of the DC bus is greater than the voltage of the first power source as measured between the first power terminal and the second power terminal; and 
 the voltage of the first power source is less than a reference voltage. 
   
     
     
         3 . The system of  claim 1 , wherein the control circuit provides further control signals to the bidirectional switching circuit that causes the bidirectional switching circuit to regulate a voltage of the first power source. 
     
     
         4 . The system of  claim 3 , wherein the third operational state occurs when:
 the voltage of the DC bus is greater than the voltage of the first power source as measured between the first power terminal and the second power terminal; and   the voltage of the first power source is less than the reference voltage.   
     
     
         5 . The system of  claim 3 , wherein the control circuit comprises:
 a charging voltage control circuit that generates signals to regulate voltage; and   a charging current control circuit that generates signals to regulate current.   
     
     
         6 . The system of  claim 1 , wherein the bidirectional switching circuit comprises a first transistor and a second transistor, and the first and second transistors operate in saturation regions during the first operational state to cause the first power source to discharge, and operate in linear regions during the second operational state to limit the charging current. 
     
     
         7 . A method, comprising:
 monitoring a voltage of a DC bus, the voltage for the DC bus being provided by a primary power source;   determining that the voltage of the DC bus is below a battery voltage and in response operating a bidirectional switching circuit in a first state to cause the battery to discharge onto the DC bus;   determining that the voltage of the DC bus is above the battery voltage and the battery voltage is below a reference voltage and in response operating the bidirectional switching circuit to causes the battery to charge from the DC bus by a controlled current that is independent of control of the battery voltage; and   determining that the voltage of the DC bus is above the battery voltage and that the voltage of the battery meets the reference voltage and in response operating the bidirectional switching circuit to cause the battery to charge from the DC bus by a controlled voltage that is independent of control of the current.   
     
     
         8 . The method of  claim 7 , further comprising:
 detecting a fault condition on the DC bus; and   in response to detecting the fault condition, operating the bidirectional switching circuit in an open state to open a portion of a circuit that is connected to the battery disabling the flow of current within the portion of the circuit.   
     
     
         9 . The method of  claim 7 , wherein the bidirectional switching circuit comprises first and second transistors, and operating the bidirectional switching circuit to causes the battery to charge from the DC bus comprises:
 operating the first and second transistors in a linear region to limit the amount of current provided to the battery.   
     
     
         10 . The method of  claim 9 , further comprising:
 determining that the voltage for the DC bus meets a reference voltage; and   regulating the battery voltage using a charging voltage control circuit, wherein regulating the battery voltage comprises modulating a voltage between an input terminal and an output terminal on the first transistor.