Abstract: Disclosed is a standby power supply system appropriate to supply a set of signal amplifiers within a cable television network. The standby power supply normally operates off of utility company power and alternatively operates from a battery power supply during power failures. Circuitry is disclosed to use battery power during failure mode operation to generate a pulse width modulated alternating current of substantially constant true rms voltage and of substantially constant frequency. Circuitry is also disclosed to cycle charge the battery supply to a desired voltage and then stop the charging cycle and begin timing normal mode operation until another charging cycle. Further, isolating circuitry is disclosed to remove much of the load on the battery supply during normal mode operation. Various safety provisions including soft start provisions, a high current cut off, a low voltage cut off, and protection for the amplification stages is included.