US Patent 7566988 Method and apparatus for monitoring UPS power sources

An uninterruptible power supply (UPS) system includes a first UPS module that has a first power source, a second power source and a controller operable to selectively connect the first or second power source to a load. A second UPS module is connected in parallel with the first UPS module. The second UPS module also includes a first power source, a second power source and a controller operable to selectively connect the first or second power source to a load. The controller of the first UPS module gradually transfers power from the first power source of the first UPS module to the first power source of the second UPS module to detect the presence of the first power source of the second UPS module.