Maintenance
philosophies batteries.
There are
different philosophies and ambition levels for maintaining and testing
batteries. Some examples:
1.
Just
replace batteries when they fail or die. Minimum or no maintenance and testing.
Obviously, not testing batteries at all is the least costly with considering
only maintenance costs but the risks are great. The consequences must be
considered when evaluating the cost-risk analysis since the risks are
associated with the equipment being supported. Batteries have a limited
lifetime and they can fail earlier than expected. Time between outages is
usually long and if outages are the only occasions the battery shows its
capability risk is high that reduced or no back-up is available when needed.
Having batteries as back-up of important installations without any idea of
their current health spoils the whole idea of a reliable system.
2.
Replace
after a certain time. Minimum or no maintenance and testing. This might also be
a risky approach. Batteries can fail earlier than expected. Also it is waste of
capital if the batteries are replaced earlier than needed. Properly maintained
batteries can live longer than the predetermined replacement time.
3.
A
serious maintenance and testing program in order to ensure the batteries are in
good condition, prolong their life and to find the optimal time for replacement
. A maintenance program including inspection, impedance and capacity testing is
the way to track the battery’s state of health. Degradation and faults will be
found before they become serious and surprises can be avoided. Maintenance
costs are higher but this is what you have to pay for to get the reliability
you want for your back-up system.
The best testing
scheme is the balance between maintenance costs and risks of losing the battery
and the supported equipment. For example, in some transmission substations, there
is upwards of $10 million per hour flowing through them. What is the cost of not
maintaining battery systems in those substations? A $3000 battery is fairly insignificant
compared to the millions of dollars in lost revenues. Each company is different
and must individually weigh the cost risk of battery maintenance.
How to maintain the battery
Standards
and common practices
There are a
number of standards and company practices for battery testing. Usually they
comprise inspections (observations, actions and measurements done under normal
float condition) and capacity tests. Most well-known are the
IEEE standards:
Ø IEEE 450 for
flooded lead-acid
Ø IEEE 1188 for
sealed lead-acid
Ø IEEE 1106 for
nickel-cadmium
IEEE
450
IEEE
450, “IEEE Recommended Practice for Maintenance,
Testing and
Replacement of Vented Lead-acid Batteries for Stationary Applications”
describes the frequency and type of measurements that need to be taken to
validate the condition of the battery. The standard covers Inspections,
Capacity test, Corrective actions, Battery replacement criteria etc.
Inspections
Ø Monthly
inspection include appearance and measurements of string voltage, ripple voltage,
ripple current, charger output current and voltage, ambient temperature,
voltage and electrolyte temperature at pilot cells, battery float charging
current or specific gravity at pilot cells, unintentional battery grounds etc.
Ø Quarterly
inspections include same measurements as monthly inspection and in addition
voltage of each cell, battery float charging current or specific gravity of 10%
of the cells or float current, and electrolyte temperature (≥10% of cells).
Ø Once a year a
quarterly inspection should be extended with measurement of float charging
current or specific gravity of all cells, temperature of each cell,
cell-to-cell and terminal connection resistance are performed on the entire
string.
Capacity
test (discharge test) should be done
Ø At the
installation (acceptance test)
Ø Within the first
two years of service
Ø Periodically.
Intervals should not be greater than 25% of the expected service life.
Ø Annually when
the battery shows signs of degradation or has reached 85% of the expected
service life. Degradation is indicated when the battery capacity drops more
than 10% from its capacity on the previous capacity test or is below 90% of
manufacturers rating. If the battery has reached 85% of service life, delivers
100% or greater of the manufacturer's rated capacity and has no signs of degradation
it can be tested at two-year Intervals until it shows signs of degradations.
IEEE
1188
IEEE 1188, “IEEE Recommended
Practice for Maintenance, Testing and Replacement of Valve-Regulated Lead-Acid
Batteries for Stationary Applications” defines the recommended tests and
frequency.
Inspections
Ø Monthly
inspection include battery terminal float voltage, charger output current and
voltage, ambient temperature, visual inspection and DC float current per string.
Ø Quarterly same
measurements as for monthly inspection shall be done and additionally cell/unit
impedance value, temperature of negative terminal of each cell and voltage of
each cell. For applications with a discharge rate of one hour or less,
resistance of 10% of the intercell connections shall be measured.
Ø Annually above
measurements should be taken and in addition Cell-to-cell and terminal
connection resistance of entire battery and AC ripple current and/or voltage imposed
on the battery.
Capacity
test (capacity test) should be done
Ø At the
installation (acceptance test)
Ø Periodically.
Intervals should not be greater than 25% of the expected service life or two
years, whichever is less.
Ø Where impedance
values has changed significantly between readings or physically changes has
occurred
Ø Annually when
the battery shows signs of degradation or has reached 85% of the expected
service life. Degradation is indicated when the battery capacity drops more
than 10% from its capacity on the previous capacity test or is below 90% of
manufacturers rating.
Battery
replacement criteria
Both IEEE 450 and IEEE 1188
recommend replacing the battery if its capacity is below 80% of manufacturer’s
rating. Maximum time for replacement is one year. Physical characteristics such
as plate condition or abnormally high cell temperatures are often determinants
for complete b
IEEE
1106
IEEE 1106, “IEEE Recommended
Practice for Installation, Maintenance, Testing and Replacement of Vented
Nickel-Cadmium Batteries for Stationary Applications”.
Inspections
Ø Inspection at
least once per quarter include battery terminal float voltage, appearance,
charger output current and voltage, pilot-cell electrolyte temperature.
Ø Semi-annually
general inspection and measurement of voltage of each cell shall be done.
Capacity
test (discharge test) should be done
Ø Within the first
two years of service
Ø At 5-year
intervals until the battery shows signs of excessive capacity loss.
Ø Annually at
excessive capacity loss
Summary
best way to test and evaluate your battery
Test
intervals
1.
Make
a capacity test when the battery is new as part of the acceptance test.
2.
Make
an impedance test at the same time to establish baseline values for the
battery.
3.
Repeat
the above within 2 years for warranty purpose.
4.
Make
an impedance test every year on flooded cells and quarterly on VRLA cells.
5.
Make
capacity tests at least for every 25% of expected service life.
6.
Make
capacity test annually when the battery has reached 85% of expected service
life or if the capacity has dropped more than 10% since the previous test or is
below 90% of the manufacturers rating.
7.
Make
a capacity test if the Impedance value has changed significantly.
8.
Follow
a given practice (preferably from the IEEE standard) for all temperature,
voltage, gravity measurements etc. and fill in a report. This will be a great
help for trending and for fault tracing.
Evaluation
1.
Replace
cell if the impedance is more than 50% above baseline. Make a capacity test if
20-50% of baseline.
2.
Replace
battery if capacity test shows less than 80% of rated capacity.
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