A system for regulating primary power and back-up power to a main power bus, the system comprising; a primary power source: a primary power regulator; a plurality of back-up power regulators (70, 72), each back-up power regulator interfacing with: a battery (66, 68); characterised by the system further including a local bus control amplifier (74, 80) configured to regulate power provided by the battery to the main power bus when the voltage on the main power bus drops below a first voltage set-point; a main bus control amplifier connected to the local bus control amplifier in each of the back-up power regulators wherein: said main bus control amplifier is further coupled to said primary power regulator for establishing a second voltage set-point on the main power bus when the primary power source is available, the first voltage set point being lower than the second voltage set point; said main bus control amplifier is configured to compare a main power bus voltage sense signal with a main bus reference voltage signal to generate positive and negative control signals; said main bus control amplifier is configured to provide the negative control signal to said primary power regulator for bringing down the second voltage set-point on the main power bus to within a regulation range when the primary power source is available thereby activating battery charging; said main bus control amplifier is configured to provide the positive control signal to the local bus control amplifier in each of the back-up power regulators for raising up the first voltage set-point on the main power bus to the same regulation range when the primary power source is not available thereby inhibiting battery charging; and a discharge current sharing amplifier for causing the discharge current of the battery of one of the back-up power regulators to be substantially equal to the average discharge current of the batteries of all of the back-up power regulators. a primary power regulator; a plurality of back-up power regulators (70, 72), each back-up power regulator interfacing with: a battery (66, 68); characterised by the system further including a local bus control amplifier (74, 80) configured to regulate power provided a battery (66, 68); characterised by the system further including a local bus control amplifier (74, 80) configured to regulate power provided by the battery to the main power bus when the voltage on the main power bus drops below a first voltage set-point; a main bus control amplifier connected to the local bus control amplifier in each of the back-up power regulators wherein: said main bus control amplifier is further coupled to said primary power regulator for establishing a second voltage set-point on the main power bus when the primary power source is available, the first voltage set point being lower than the second voltage set point; said main bus control amplifier is configured to compare a main power bus voltage sense signal with a main bus reference voltage signal to generate positive and negative control signals; said main bus control amplifier is configured to provide the negative control signal to said primary power regulator for bringing down the second voltage set-point on the main power bus to within a regulation range when the primary power source is available thereby activating battery charging; said main bus control amplifier is configured to provide the positive control signal to the local bus control amplifier in each of the back-up power regulators for raising up the first voltage set-point on the main power bus to the same regulation range when the primary power source is not available thereby inhibiting battery charging; and a discharge current sharing amplifier for causing the discharge current of the battery of one of the back-up power regulators to be substantially equal to the average discharge current of the batteries of all of the back-up power regulators. said main bus control amplifier is further coupled to said primary power regulator for establishing a second voltage set-point on the main power bus when the primary power source is available, the first voltage set point being lower than the second voltage set point; said main bus control amplifier is configured to compare a main power bus voltage sense signal with a main bus reference voltage signal to generate positive and negative control signals; said main bus control amplifier is configured to provide the negative control signal to said primary power regulator for bringing down the second voltage set-point on the main power bus to within a regulation range when the primary power source is available thereby activating battery charging; said main bus control amplifier is configured to provide the positive control signal to the local bus control amplifier in each of the back-up power regulators for raising up the first voltage set-point on the main power bus to the same regulation range when the primary power source is not available thereby inhibiting battery charging; and a discharge current sharing amplifier for causing the discharge current of the battery of one of the back-up power regulators to be substantially equal to the average discharge current of the batteries of all of the back-up power regulators.