APC BK600EI - Manual (Page 17)

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4.2 Load transfer switch
The load transfer switch is actually an electro-dynamic relay which serves to rapidly
transfer your computer equipment (load) from the utility to the UPS’s alternate power
source in the event of a utility failure. When the utility is restored to within safe limits, the
switch acts to re-transfer the load to the utility. The time required for the relay to transfer
your load to either power source is much, much faster than that which is required by any
modern computer or computer peripheral device.
4.3 Battery Charger
The UPS's battery charger converts the alternating current (AC) supplied by the utility to
a direct current (DC) which is compatible with the battery. The charger maintains the
battery at a constant voltage to ensure that the battery will have the capacity to support
your load as often as possible. This charging method, known as “float” charging, provides
maximum battery service life and minimal internal heating. The battery is charged
whenever the UPS is plugged in, whether or not it is switched on.
4.4 Battery
The UPS’s battery is an energy source much like the battery in an automobile. Also, like
most automobile batteries, the UPS’s battery is a modern maintenance free lead-acid type;
it is sealed and leakproof. The battery has a typical service life of 3 to 6 years. The service
life is extended when the UPS is kept in a cool environment, below 30°C or 86°F.
4.5 Inverter
The UPS must convert the battery’s energy into a form that your computer equipment can
rely upon during a utility failure. This is the job of the UPS’s inverter. The UPS converts
the battery’s DC to AC using solid state devices (transistors), controlled using a technique
known technically as “pulse width modulation”. This technique is highly efficient which
means that little battery power is wasted in the conversion process. Hence, your
equipment can run for long periods from the UPS before the battery’s capacity is spent.