An apparatus, comprising: a system circuit comprising a positive lead for receiving power at a first node from a first output of an external power source; at least one battery comprising a positive lead for receiving power at a second node from a second output of the external power source; a controlled resistor comprising a first lead coupled to the first node and a second lead coupled to the second node, the controlled resistor being configured to cause a high impedance path between the first node and the second node when the controlled resistor is off, the controlled resistor being configured to cause a low impedance path between the first node and the second node when the controlled resistor is on, the controlled resistor further comprising an input for turning the controlled resistor on or off; and a control circuit having a first input coupled to the first node, a second input coupled to the second node, a logical input coupled to a component configured to cause a logical signal at the logical input indicating that the external power source is active or inactive, and the control circuit comprising a control output coupled to the input of the controlled resistor, the control circuit being configured to cause the controlled resistor to be on when the logical signal at the logical input indicates that the external power source is inactive, the control circuit being configured to cause the controlled resistor to be off when the logical signal at the logical input indicates that the external power source is active, and when a first signal at the first input and a second signal at the second input fulfill first criteria, wherein the first criteria are fulfilled when a voltage difference between a voltage of the first signal and a voltage of the second signal is below a first threshold, and the control circuit being configured to cause the controlled resistor to be on when the logical signal at the logical input indicates that the external power source is active, and when the first signal at the first input and the second signal at the second input fulfill second criteria, wherein the second criteria are fulfilled when the voltage of the first signal is below the voltage of the second signal by at least a second threshold. a system circuit comprising a positive lead for receiving power at a first node from a first output of an external power source; at least one battery comprising a positive lead for receiving power at a second node from a second output of the external power source; a controlled resistor comprising a first lead coupled to the first node and a second lead coupled to the second node, the controlled resistor being configured to cause a high impedance path between the first node and the second node when the controlled resistor is off, the controlled resistor being configured to cause a low impedance path between the first node and the second node when the controlled resistor is on, the controlled resistor further comprising an input for turning the controlled resistor on or off; and a control circuit having a first input coupled to the first node, a second input coupled to the second node, a logical input coupled to a component configured to cause a logical signal at the logical input indicating that the external power source is active or inactive, and the control circuit comprising a control output coupled to the input of the controlled resistor, the control circuit being configured to cause the controlled resistor to be on when the logical signal at the logical input indicates that the external power source is inactive, the control circuit being configured to cause the controlled resistor to be off when the logical signal at the logical input indicates that the external power source is active, and when a first signal at the first input and a second signal at the second input fulfill first criteria, wherein the first criteria are fulfilled when a voltage difference between a voltage of the first signal and a voltage of the second signal is below a first threshold, and the control circuit being configured to cause the controlled resistor to be on when the logical signal at the logical input indicates that the external power source is active, and when the first signal at the first input and the second signal at the second input fulfill second criteria, wherein the second criteria are fulfilled when the voltage of the first signal is below the voltage of the second signal by at least a second threshold. the control circuit being configured to cause the controlled resistor to be on when the logical signal at the logical input indicates that the external power source is inactive, the control circuit being configured to cause the controlled resistor to be off when the logical signal at the logical input indicates that the external power source is active, and when a first signal at the first input and a second signal at the second input fulfill first criteria, wherein the first criteria are fulfilled when a voltage difference between a voltage of the first signal and a voltage of the second signal is below a first threshold, and the control circuit being configured to cause the controlled resistor to be on when the logical signal at the logical input indicates that the external power source is active, and when the first signal at the first input and the second signal at the second input fulfill second criteria, wherein the second criteria are fulfilled when the voltage of the first signal is below the voltage of the second signal by at least a second threshold.