Power back-up unit with low voltage disconnects that provide load shedding

A method of extending the length of time a power back-up unit can supply a critical load with power, the method comprising:

• providing a first low voltage disconnect with a first voltage threshold;
• electrically connecting the first low voltage disconnect to a critical load;
• providing a second low voltage disconnect with a second voltage threshold, the second low voltage disconnect being external to the first low voltage disconnect;
• electrically connecting the second low voltage disconnect to a non-critical load;
• selecting the second voltage threshold such that it is greater than the first voltage threshold;
• disconnecting the non-critical load via the second low voltage disconnect prior to disconnecting the critical load via the first low voltage disconnect;
• and
• monitoring the voltage of the secondary power source and controlling the second low voltage disconnect via a first voltage control circuit internal to the second low voltage disconnect.

A power back-up unit, comprising:

• a primary power source connected to a plurality of loads including a critical load and a non-critical load;
• a secondary power source;
• a first low voltage disconnect electrically connected to the secondary power source and the critical load;
• a second low voltage disconnect electrically connected to the secondary power source and the non-critical load, the second low voltage disconnect disconnecting the non-critical load upon loss of the primary power source, the second low voltage disconnect being external to the first low voltage disconnect;
• and
• a first voltage control circuit, internal to the first low voltage disconnect, that monitors the voltage of the primary power source and controls the second low voltage disconnect.