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4.0 Principles of operation
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 PowerCell’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. Except
for the user control switches, the transfer switch is the only moving part in the
PowerCell. 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
In the event of utility failure, the PowerCell supplies power to your equipment
derived from energy taken from a battery. The PowerCell’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 charger operates whenever the PowerCell is plugged in, whether or not
the UPS is turned on.
4.4 Battery
The PowerCell’s battery is an energy source much like the battery in an automo-
bile. Also, like most automobile batteries, the PowerCell’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 PowerCell
is kept in a cool environment, below 30°C or 86°F.
4.5 Inverter
The PowerCell 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 PowerCell 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 PowerCell before the battery’s capacity is spent.