n of their UPS battery system, since
it was originally sized to provide backup
power for some specified time period.
Many battery owners perform careful
maintenance on their system year after
year, feeling confident that by keeping
their battery in peak condition they will
be protected from a power outage. But
it turns out that their well-maintained
battery may leave them just as unpre-
pared for an outage as it would if their
battery was run-down, improperly main-
tained or undersized.
This potential problem exists be-
cause external changes that occur over
time at a battery site, although they may
seem trivial and minor when they hap-
pen, may cumulatively push the battery
system to the point of being insufficient.
This article reviews the factors that are
considered when a battery is originally
sized, and explains how changes in these
factors can critically affect the batterys
performance.
Required Duty Cycle
The most obvious factor that can af-
fect the batterys performance is the re-
quired duty cycle. The effect of a change
in current on the batterys performance
is often underestimated. To illustrate this
point, consider the example where the
load on a UPS battery has increased by
20% over the years. The battery user
commonly assumes that the batterys
runtime will simply be reduced by 20%.
Unfortunately, the reality is much more
dramatic. Table 1, examines the effect
on a typical UPS battery installation:
Table 2 details the effect on run time,
due to an increase in discharge current
for this battery. As the table shows, the
effect of an increase in current on the
run time is quite dramatic.
This effect on run time is also evi-
dent as a battery ages. As the capacity
of a battery decreases with age, its run
time decreases at a much higher rate.
The result is similar to the one described
above: a 15-minute battery that is near-
ing its end-of-life, and has lost 20% of
its initial capacity, will lose 50% of its
original run time. The result is even
more severe for a 5-minute battery.
When the same battery used in the pre-
vious example is sized for a 5-minute
load, it will not be able to sustain its load
at all if it has lost more than 12% of
its initial capacity.
Load Change
If the load changes over the years to
a level significantly greater than the
original installation, the increased
power loss in the connecting cables must
also be reconsidered. Resistive power
A
well-maintained and monitored battery system will pro-
vide the owner with the most reliable and cost effective
power source one that isnt likely to be an expensive dis-
appointment, due to the inevitable changes that occur at a
battery site. To guarantee that a facilitys backup battery
power system will operate as designed, routine maintenance
should also include procedures to examine external factors
to the battery system. This article outlines many of the key
factors external to the battery itself that will increase the
performance and the reliability of your battery-based power
system.
Table 1. Specifications of a typical battery. Table 2. The effects of increased current on the run time.
loss in a connector or cable is a func-
tion of the square of the current. This
means that if the load has doubled over
the years, the power loss and tempera-
ture rise in these connectors is not sim-
ply doubled, but will rise by a factor of
four. In high current discharges, this
power loss may be significant. This is
especially true for long cable runs,
where the UPS system is not located im-
mediately adjacent to the battery. In all
cases, whenever a batterys performance
is being evaluated, the total cable run
of the entire UPS and battery loop (in-
cluding cross-aisle connections) must be
considered.
Battery Type
In terms of battery type, the change
in cycle activity may also be significant.
If the original expectation was for long
periods of float, with a moderate cycle
(50% depth-of-discharge) every two
months, a light-duty, flooded battery
may have been specified. These battery
types typically have insufficient active
material retention for frequent, high-rate
cycles. If the cycle activity has increased
over the years, the plate without reten-
tion may no longer be appropriate. In
this case, a more appropriate battery
design would be one with a thicker glass
mat for active material retention (ex.
0.020 inch vs. 0.010 inch thick), in ad-
dition to a separator that will shield the
bottoms of the plates. If the original
battery continues to be cycled at the
higher rate, it is destined for early fail-
ure. Signs of failure would be excessive
typical flooded battery is shown in Fig-
ure 1. For VRLA applications which re-
quire frequent, high-rate discharges, the
gelled-electrolyte type of VRLA battery
is often selected. These gelled cells are
inherently better at heat dissipation due
to the larger volume of electrolyte mass
and the improved heat transfer to the cell
container. Local spill containment regu-
lations may also be relaxed with the
gelled-type electrolytes.
Temperature
Aside from changes in the load, the
most important factor in the batterys
performance is the temperature. Batter-
ies are often installed in mechanical
rooms with other maintenance
equipment. As a company grows and re-
crease in temperature (Figure 2). If dis-
charged at this higher temperature, the
battery will deliver more energy. How-
ever, what is much more important than
this short-term gain, is the long-term
loss in life that results from this higher
temperature. More specifically, a bat-
tery will lose one-half of its life if it is
kept at 95°F instead of 77°F (and half
again for every 18°F above this). This
factor alone is probably responsible for
a significant portion of the disappoint-
ments in the life of lead-acid batteries.
It is critically important to a batterys
life to regulate the temperature in its en-
vironment.
On the other hand, low temperatures
are also important to consider, since
batteries will experience reduced capac-
Figure 2. Relative correction factors used for determining the proper
cell size based on discharge temperature.
Figure 1. The effect of glass mat retainer on cycle life: Flooded UPS
battery at 15 minute rate of discharge.
shedding (loose particles of plate mate-
rial) of the positive, and collection of
this material in the sediment space at
the bottom of the cells. As this material
accumulates, it will eventually cause an
internal short when its level reaches the
bottom of the plates. The effect of the
glass retention mat on the cycle life of a
quires new equipment, it is not unusual
for a new boiler, compressor or a pump
to be installed in the same room as the
battery. All of this equipment generates
significant heat, and will cause the
rooms ambient temperature to rise. The
good news is that one can expect higher
capacity from the battery with an in- the acid-starved element and the re-
duced heat transfer (as compared to a
flooded battery container). Should the
temperature rise without the voltage
being lowered to compensate, the bat-
tery will be forced to generate additional
gas, which will recombine and add more
heat to an already warm battery. Should
the battery reach the point where more
heat is generated internally than can be
dissipated to the surrounding air, the
batteries will reach a point of thermal
runaway. At this point, the batteries will
almost certainly be damaged beyond
repair, and may generate enough heat
to damage surrounding equipment. This
is an extreme example, but it is impor-
tant to consider the ramifications of al-
lowing a battery rooms temperature to
climb, while not compensating the float
voltage.
Charge Compensation
Charge compensation is another criti-
cal factor if the battery temperature
drops after its initial installation. In this
case, the uncompensated float voltage
is too low to maintain the full state-of-
charge of the battery. If the cell is al-
lowed to remain in this sulfated state for
an extended period of time, even an
equalization charge may not be suffi-
cient to recover the capacity of the cell.
Again, the same rule-of-thumb of 2.8
original temperature. The battery will
also be affected if there are changes in
the winter heating conditions, or even
if the building heat is turned down on
weekends for energy conservation rea-
sons.
Voltage Regulation
While were on the subject of tem-
perature, it is important to note voltage
regulation. If the temperature of the bat-
tery room is allowed to drift either
higher or lower after installation, it is
critical to compensate the charge volt-
age. A flooded, lead-calcium battery
that was originally sized for 77°F and
is later kept at 87°F, should have its
charge voltage reduced by 0.028 volts
per cell, or 2.8 millivolts per degree F
per cell (or 5mV/°C per cell). A VRLA
cell should be compensated in the same
manner by 0.002 volts per degree F per
cell (or 3.6mV/°C per cell). For a total
battery, this actual voltage change can
be significant, as shown in Table 3.
Thermal Runaway
The result of not compensating the
float voltage on flooded batteries that
operate at high temperatures is exces-
sive gas evolution, greatly increased
water loss, excessive shedding of posi-
tive active material and increased posi-
tive grid corrosion all factors in low
millivolts per degree F per flooded cell
applies. For example, a flooded battery
on fl