Maximizing battery reserve in high availability power systems

Traditionally, the safest approach to ensure availability of power has been to provide dual redundancy for everything-two completely separate systems, such that any single failure will not result in loss of overall function. This technique cannot be criticized in terms of integrity but there are significant cost and size penalties. Frequently, power systems operate with N+1 redundancy for many of the elements in order to offer high availability without some of the drawbacks of dual redundancy. Often, the biggest single cost and size element of a battery backed protective power system is the battery. As customers look for the ultimate solution of low cost and high availability, it is desirable to look at the implications of "sharing" the energy reserve in a dual redundant system. This technique, if deployed correctly, can provide significant cost and size reductions without unacceptable increases in risk to the system integrity. This paper looks at the energy flow in normal systems focusing on the types of "faults" that can occur in the real world. Solutions are presented to allow battery energy to be channeled to the real loads under these "fault" conditions, thus avoiding a situation where there is useable energy in a battery and a critical load cannot be maintained during an AC utility failure.