46
UPS SYSTEMS
life. Overcharging causes batteries to dry out which
depletes charging capacity, causing thermal runaway.
Thermal runaway occurs when a battery generates
heat more quickly than it is dissipated and its this rise
in internal battery temperature that dries out the cells
and can damage or rupture the plastic case.
Undercharging results in a battery block’s terminal
voltage falling below its rated value. If left in a
discharged state or stored for a prolonged period of
time, lead sulphate crystals start to form and act as
a barrier to recharging, impeding normal operation.
Depending on the degree of sulphation, a battery
can be recovered with a constant current charge at a
voltage higher than that used for standard charging,
with the current limited to one tenth of battery
capacity for 4 to 10 hours.
Because this process exceeds the normal
recommended charging voltage, the battery must
be carefully monitored for safety sake. If excess heat
is generated, it must be removed from charge. The
voltage required to ‘force’ the maximum current into
Fig. 1: Temperature/life characteristics of a typical VRLA battery
Ad
the battery reduces as the battery recovers and normal
charging is resumed. Sometimes a battery never
recovers from sulphation and it must be replaced.
An undetected undercharged condition can further
deteriorate into a state of deep discharge. A battery
is considered to be deeply discharged if its on-load
voltage falls below a predetermined critical level.
Severe sulphation results and the battery may be
unable to recharge and will require replacement.
Battery operation is an electrochemical process and
highly temperature-dependent. Most manufacturers
recommend a working temperature environment
of +20°C. As temperatures increase, the battery can
deliver more power. However long term exposure to
higher temperatures adversely affects service life. The
empirical equation is that for every 10 °c above the
recommended ambient temperature, battery life is
halved.
Conversely, if the ambient temperature falls below
optimum, the rate of chemical reaction and battery
performance reduces; reduced temperatures have
little impact on battery life. Any AC component or
’ripple’ that is superimposed onto the battery’s DC
charging voltage will adversely affect battery life.
Transformer less UPS systems utilise a DC/DC converter
for the battery charger which eliminates the possible
effects of this superimposed AC ripple associated with
transformer based UPSs. The ambient temperature
should always be carefully monitored.
Battery maintenance and testing
The battery maintenance schedule, once initiated,
should start with some sim