Are batteries a waste of time
and money ?
by Rick Lawrence and Marius Pitzer, Megger
Batteries are indeed, a waste of money. This means to say that if the power grid was 100% reliable, batteries would not be necessary!
The many worldwide power outages over the testing only the voltage and specific gravity Float voltage
past several years make batteries essential doesn’t work - it never has and never will. The
Taking them one at a time, voltage can be
as a backup source. Who actually gives reason for this is that the sum of all of the
one of those misleading tests. Voltage is
batteries a second thought? We simply cells’ voltages must equal the charger output.
important, absolutely, and if it is abnormal,
expect them to work when called upon. Voltage (and specific gravity) of lead- acid
then it indicates something about the
Experience has shown that this expectation batteries basically follows the sulphate. If a
condition of the battery. If it is normal, it
is pure fiction. battery is fully charged, the sulphate will be in
indicates nothing at all about a battery’s
Batteries are extremely important to the acid and its voltage and specific gravity
condition. This is because voltage is more of
provide electricity to support many assets will be normal (with few exceptions.) If it is in an indicator that the charger is functioning
and revenue streams during outages. a discharged state, the voltage will be low properly. The sum of the voltages of all of the
For example, in a generating station, if and since there is at least some sulphate on batteries in the bank must equal the charger
the turbine suffers an outage, without the the plates, the specific gravity will also be output voltage, resistive losses excluded.
back-up battery the turbine lube oil priming low. These tests will therefore reveal the state A normal voltage is not an indicator of battery
pumps would not continuously keep the of charge of the battery, but not its state of capacity, but an abnormal voltage needs
bearings lubricated causing major damage health. If the battery has a normal voltage, further investigation.
and lengthy outages. In hospitals, who wants there is no indication of the health of the
to be in the middle of an operation when battery. When the voltage is abnormal, it
an AC outage occurs without proper battery may be an indication of a potential problem, Specific gravity is similar to voltage as an
back-up? The applications for batteries but may also be that the battery is merely to indicator of battery health. The sulphate is
are innumerable and frequently unseen. In some extent discharged. part of the electrochemical reaction. If the
this world of dependency upon electricity, battery is discharged, some of the sulphate
Nickel cadmium batteries behave somewhat
it is impossible to survive without battery migrates to the plates and the acid is
differently than lead-acid batteries. In
back-up. reduced in specific gravity. If the battery is
lead-acid batteries the acid is actually part
fully charged, all of the sulphate is in the
Battery basics of the electrochemical process; it reacts
acid and the specific gravity is normal, say
So what makes a battery tick? All batteries, with the lead and lead oxide to make
1,215. There aren’t any studies to validate
whether rechargeable (secondar y) or electricity. The KOH electrolyte in NiCd any correlation between specific gravity
disposable (primar y), use chemical batteries is simply a carrier for the current and battery capacity. In fact, IEEE 450 has
reactions to make electricity. It is necessary and does not enter the chemical reaction. de-emphasised specific gravity to the point
to have two dissimilar metallic materials in a Therefore, measuring specific gravity of NiCd of checking only 10% of the batteries each
current-carr ying medium. In lead-acid batteries in service doesn’t indicate anything quarter and the full bank annually.
batteries, the two dissimilar metallic materials about the condition of the battery. The one
are lead and lead oxide in a sulphuric exception is carbonation of the electrolyte.
acid medium. Nickel Cadmium batteries This is caused, over time, by the absorption In order to keep a battery charged, there is a
use nickel and cadmium compounds in a of carbon dioxide from the air into the KOH battle of sorts going on in the battery between
potassium hydroxide electrolyte medium. and reduces the specific gravity of the its self-discharge and the charger. The battery
Nickel metal hydride batteries (NiMH) are electrolyte. If this happens, check with the is always in a state of self-discharge which
comprised of the same nickel compound as battery manufacturer. It may simply be a creates a differential in potential between the
in NiCd cells but the cadmium compound matter of replacing the electrolyte. battery bank and the charger. This differential
is replaced with a metallic hydride and the in potential causes a small current to flow to
liquid electrolyte is replaced with a paste to keep the battery fully charged. This DC current
carry the current. The two types of nickel cells The range of possible tests on a battery is called float current.
are virtually identical in performance. Even ranges from doing nothing (not a good In flooded lead-acid batteries there is no
their voltages are the same! Lithium batteries idea) to doing every test possible (still not possibility of thermal runaway because the
use a lithium-containing oxide or phosphate a good idea). The possible tests include liquid acid cools the battery through the
and carbon. For the purposes of stationary voltage, specific gravity, float current, process of evaporative cooling. However,
battery testing, this article focuses mainly on
ripple current, cell temperature, ambient VRLA batteries do not have extra acid, nor
temperature, discharge current and time, is it in a free liquid form. If the float current
With the importance of batteries established intercell connection resistance, capacity increases due to some impending failure
let us consider their chemistry and ways of (load test), impedance (an internal ohmic or overcharging condition, the temperature
assesing their health. The age-old habit of test), among others. increases . The increased temperature allows
energize - October 2006 - Page 14
for more current to flow and further increases discharging and recharging as a result of drop across a battery caused by the AC
the temperature of the battery. A runaway outages. The posts expand and contract current signal. Following Ohm’s law, Z = E/i,
chemical reaction ensues, which can lead and because lead is very malleable will impedance is calculated. Impedance
to the melting of the battery causing an open cold flow with each cycle. This is one of the is inversely proportional to capacity in
circuit. The time from when the float current reasons that battery manufacturers tend to that as capacity decreases, impedance
starts to increase and when thermal runaway recommend tightening bolts to the low end increases. This test is fast (about 30 minutes
might occur is between one and four months. of the torque range so as not to add further for a 60-cell substation battery bank) and is
Float current is an important parameter to stress during cycling. non-invasive.
measure in VRLA batteries.
Capacity Data analysis
Capacity tests are a necessar y evil. If What is done with the data collected? How
Ripple current is a product of the charger performed properly, they are expensive, time- is the data interpreted to ensure that the
which converts AC into DC. No charger consuming and have limited predictive value battery bank will meet the required duty
has a 100% ripple-free conversion process, depending upon their frequency. Consider a cycle? With the advent of better testing
which is why filters are frequently added in battery bank that is designed to provide eight methods such as impedance, more useful
certain applications. Ripple current generally hours of back up time. A proper capacity test data (rather than only voltage and specific
increases slowly over time as electronic incorporates a second battery in case of a gravity) can now be obtained. With such
components degrade. If, however, a diode power outage during the discharge test. This data come the data-handling problems
in the rectifier blows, the ripple current can second battery must be at least the same size and the analysis to paralysis problems.
double. As with float current, an increase in or bigger than the main battery being tested. The recommended method is to use a
ripple current to a point greater than about The resistive load bank must be connected database to track and trend all battery data
5 amps rms for every 100Ah of battery to the main battery bank and voltage leads over time and dispense with paper forms
capacity (5%), leads to increased temperature are connected to each battery in the bank. that cannot compare today ’s data with
and shortened battery life. Ripple current is This usually takes a full day. yesterday’s. A specialised database, one
another parameter that should be measured with space for all measured parameters, is
On day 2 the eight hour test begins. A test
periodically. important to aid in determining the condition
for intercell connection resistance is often
Temperature of batteries and banks.
performed before the start of the capacity
Battery and ambient temperature, although test. There are two schools of thought Entering limits, with which the user is
they don’t dictate immediate doom for a about performing the intercell connection comfortable, in order to gain the most life
battery, can lead to premature failure. For resistance test: from a battery without increasing risk, aids
every increase of 10°C in battery temperature tremendously in extracting the most from
1) It is not representative of a true “as found”
above 20°C, the battery life is halved. This a battery. The limits should be set for each
means, for example, that a 20 year battery parameter measured. For example, float
2) Certain precautions need to be taken to
maintained at 35°C instead of the specified voltage limits should follow manufacturers’
ensure that no major malfunctions occur
25°C will only last about ten years. In Europe guidelines. Internal ohmic test limits are more
that could have been avoided.
the standard temperature is 20°C and 15 year debatable. In some cases, users will set a
If there are major malfunctions, then the bank “failure limit” of 50% impedance increase
design life for flooded batteries.
or at least some of the batteries will need to for VRLA batteries from a predetermined
Discharge current and time be replaced in an emergency situation. This baseline value. Float current limits tend to
Discharge current and time is now used decision is for those at each company who be less precise depending upon the size,
more frequently in on-line monitors to aid write procedures and maintain batteries. age and alloy of the battery.
in determining amp-hours removed and
Day 3 is the continuation of the recharge Conclusion
replaced. The value of measuring current
of the main bank. The voltage leads are
and time and calculating Ah removed There are many failure modes for batteries.
removed and the resistive load bank is
and replaced is that battery capacity can With care and measurement these can be
disconnected. The main battery recharge dramatically reduced, especially if little to
presumably be calculated. This author
believes that there is value in this calculation. may continue on day 4 if the battery is not yet no testing is presently being performed. The
The caveat is discussed below under capacity fully charged and ready for service. A properly battery is installed, not to add to work load,
(load) tests. run capacity test is the only true method of but to support critical electrical equipment
determining the bank’s actual capacity. or revenue streams. Proper testing and
Intercell connection resistance
data analysis can help determine when
Impedance and resistance
Intercell connection resistance is one of the a batter y should be replaced. Testing
tests that needs to be performed, especially Impedance, an internal ohmic test, indicates also helps reduce emergency batter y
if frequent outages occur. It has been said the capability of a cell to deliver current. It is replacements and assists in budgetar y
that more than 50% of battery bank failures correlated to capacity. Although correlation planning, thus reducing cost. A properly
are due to loose intercell connectors. This is a to capacity is not 100%, it is an excellent implemented battery testing regime does
straightforward test to perform and it can be way of finding weak batteries in the bank. not necessarily reduce the work load but
done in conjunction with impedance testing The EPRI study reveals how well impedance it will, most likely, increase reliability of the
(discussed below) or as a stand-alone test and other internal ohmic tests work in finding entire DC network.
using a low resistance ohmmeter.
weak cells. The impedance test applies Contact Marius Pitzer, Megger,
Intercell connections come loose due to and measures an AC current signal and Tel (011) 452-6287,
heating and cooling cycles caused by measures simultaneously the AC voltage email@example.com
energize - October 2006 - Page 16