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| UPSonNet NewsLetter August 2011 |
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UPSonNet
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UPS Upgrading: Costs & Gains
When UPS Turns Power Down
| Spread Your Word On the Globe |
How much to Pay for Best UPS
Backup Battery & UPS Reliability
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Best UPS Battery Technology
UPS Waveform & New Computers
UPSonNet
Specifying the Right Off Line UPS
How to Drop a Protected Computer
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5 Nines Three Phase Scalable UPS
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Redesign UPS & Boost Efficiency
Life Expectancy of UPS Batteries
| Charging UPS Batteries – A Guide for Perplexed Users. |
script type="text/javascript">
Editorial Topic by Meir Portnoy: Charging UPS Batteries - A Guide for The Perplexed
users.
I was recently exposed to engineers debate concerned with the best way to charge and maintain
Valve Regulated Lead Acid (VRLA) UPS batteries. While browsing through literature, including
comments in discussions groups, it seems that the topic is of general concern and desires some
basic guiding for the perplexed users.
Most VRLA batteries in UPS application today are required to provide short backup time (5 to 15
minutes) to ride through power outage or to provide backup until a generator kicks in. This
requires designing the battery with thin plates in parallel, thus increasing plate's surface in order to
withstand the high rate currents involved. Unfortunately, the thin plate structure is responsible for
the short longevity of UPS batteries.
In a previous UPSonNet's newsletter dealing with Life expectancy of UPS batteries, we indicated
that 60% of VRLA battery failures are caused by grid corrosion, which gradually reduces the
amount of active material in the battery, causing eventually total battery failure. As explained in the
document, the corrosion process depends on the ratio of plate's surface area to weight. A thin,
plate with larger surface area to grid weight, compared to thick plates containing a few thin wires,
has faster rate of corrosion per AH capacity. Therefore, UPS batteries, for short back up times,
which need large area to provide the high currents, have short life expectancy.
In addition UPS battery banks in medium and high power systems are connected in parallel strings,
each comprising about 20 to 40 batteries in series, where a single battery failure may have a crucial
effect on system operation. Additional complexity arises also due to the need to mix new batteries
with old ones, in order to replace a failed battery in a string, or to add battery strings for increased
capacity.
The type of batteries as well as charging and maintenance of such battery banks requires special
consideration, which takes in account also additional factors, such as ambient temperature, battery
life expectancy, testing, isolation of failed units and more.
Charging UPS Batteries
The Constant voltage charger used throughout the charging stage and thereafter in float regime, is
the classical and generally recommended method by battery manufacturers to charge UPS batteries.
Such charger is designed with Constant Voltage, set to battery float voltage, with output current
limited by charger capacity, if not set to the desired battery charging current limit. Initial battery
current, if not limited, when a discharged battery is connected to the charger, can reach 3C Amps
(30Amperes for a 10AH battery). After a short time (several minutes for small batteries) the current
tapers off until it reaches a stable value of float current.
VRLA Float voltage is about 2.24V to 2.27V per cell at 25ºC, calculated by adding electrodes
polarization voltages of about 90mv to120mV to cell's Open Circuit Voltage (OCV) of a typical
charged VRLA cell, which at 25ºC is 2.15 V (derived by adding a constant value of 0.845 to
VRLA's specific gravity which is 1.300). This voltage should be compensated when battery
ambient temperature changes by -3mv/ ºC, to 2.33-2.345V at 0ºC or 2.21 to 2.24 at 35ºC.
Certain UPS manufacturers suggest intermitted charging to achieve longer battery service life.
Eaton for instance, offers UPS systems with an ability to select an implementation of its unique
charging method called ABM, which after initial charging with high voltage and current limit,
transfers to float charging for up to 48hours, disconnecting thereafter the batteries from charger
for 28 days. The charging is resumed if cell voltage drops below 2.1V/cell. This technique
according to Eaton, when associated with automatic diagnostics of battery health, extends service
life of 6 minutes backup batteries from 32 to about 70 month.
Battery overcharging – The main cause of failures during service lifetime.
The above mentioned document also points out that additional 30% of battery failures are caused
by overcharge, resulting in battery dry out. Even a small float over voltage, or high rate of
charging current, can produce oxygen and hydrogen at a higher rate compared to the rate of
recombination reaction, thus, increasing the gas pressure in the battery, until the pressure relief
valves open, and electrolyte, which cannot be replaced in sealed batteries, is permanently lost.
Battery dry out causes eventually shrinking or disintegration of separator away from the electrodes
associated with large increase in internal battery resistance. The sudden resistance increase
phenomena, termed by the industry as Sudden Impedance/Resistance Rise (SIRR), can happen
anytime, increasing exponentially almost tenfold within a week or two. Growth of internal battery
resistance, if not revealed in time by a systematic and frequent monitoring of internal resistance
value trend, will cause eventually opening of the battery jar or thermal runaway, resulting in failure
of numerous, and sometimes all batteries in UPS battery string.
Practically, taking in account that corrosion affects mainly battery longevity, during batteries'
lifetime overvoltage becomes not the second but the main reason for battery failure, responsible
for about 75% of failures. The effect of battery failures on UPS reliability is considered in our
Newsletter Impact of Backup Batteries on UPS Reliability.
Unfortunately, selecting UPS battery floating voltages or operating regimes is frequently in the
hands of UPS operators, who may lack experience, knowledge or awareness to the need of taking
every possible measure to eliminate any reason that may cause battery dry out. I have personally
been involved in a traumatic event of SIRR failure of a total UPS battery bank with VRLA batteries,
ten days after a thorough inspection of the batteries, which included a detailed record of battery
voltages, jar temperature and load discharge test. The batteries and the UPS failed, when batteries
were called to action at power outage. Our customer suffered a failure of its main production line,
lost money and lost confidence in our company. We have lost a major customer.
Charging Parallel strings.
Most high power UPS systems use multiple battery strings for redundancy, modularity, or growth.
All strings are generally connected in parallel to UPS charger, which is set to provide constantly
float voltage at 25°C of at least 2.24V per cell, above Open Circuit Voltage which is about 2.15V
for a charged VRLA cell. Thus the lowest floating voltage of 20 battery string at 25°C will be
269V whereas its Open Circuit Voltage is 258V.
All battery strings are connected to the same voltage source, which is always above the total open
circuit string voltage. Therefore no circulating current or other influence exists between the battery
strings, and old or new battery strings even with different capacities can be connected in parallel.
In discharge state, each string contributes to total current according to its capacity and internal
resistance.
Most VRLA manufacturers do not require limiting the battery current when connected to float
voltage charger. However, studies indicate that high gassing is associated at the transition point
when the charging regime changes from constant current to constant voltage, recommending to
limit battery string current to about 0.2C ( 2 Amps for a 10AH battery). In addition, limiting of
battery string charging current, including means to isolate a failed string with a fuse or other
disconnect device from the healthy strings is needed in order to protect the healthy batteries and
preserve UPS's backup ability.
The situation is more complicated when parallel strings are connected to an intermittent type of
charging, such as Eaton's ABM method. The batteries here are most of the time in open circuit
mode, and need disconnecting or other devices which eliminate the possibility of destroying
healthy string by circulating current into a failed string.
Consideration should also be given to another aspect of parallel string in Intermittent charging
environment. Enersys, a leading world battery manufacturer published a paper titled Intermittent
Charging , which without mentioning specific manufacturers, refers to a charging method that
"allows the batteries to go into open circuit for a period of time until they are intermittently
charged at a voltage generally greater than the normal float voltage of the system."
The paper indicates that that: "When batteries are installed in strings, the charging phase might not
be sufficient to fully recharge and equalize or harmonize all blocs to the same charge level, which
could lead to progressively unbalanced strings, wide voltage dispersion and eventually premature
failure by sulfation for the blocs staying at lower charge level."; eventually recommending "that
batteries be operated in a float environment for longest battery life."
Latest studies also show that the ability to equalize cells by applying voltages above the float string
voltage is not justified in UPS application, considering the possible damage which may be caused
due to excess gassing. Thus, most battery manufacturers do not recommend to equalize UPS
batteries, except for the purpose of re-freshening stored batteries.
Additional Notes
The main lesson to learn when charging UPS Lead Acid batteries is to remember that while
charging electrolysis of water produces oxygen and hydrogen as by products. In Flooded b
atteries the oxygen and hydrogen gases generated at the plates, are vented to the air through the
top of the battery. Water should therefore be added during maintenance in order to replace the
missing quantity, and sustain the charge storage ability.
In Valve Regulated Lead Acid (VRLA) Batteries, pressure relief valves do not allow gases to leave
the battery. The oxygen generated at the positive plate diffuses to the negative plate, where it is
recombined with hydrogen to re-form water. Thus, water refilling is not required and no
maintenance is needed.
The valves will however allow gas relief, when internal pressure exceeds allowed value, as may be
the case at battery overcharging. As long as the battery charging voltage doesn't exceed battery
floating voltage the rate of gassing doesn't exceed the rate of recombination. Otherwise the
pressure of accumulated gasses will open the valves venting the electrolyte constituents into the air.
The lost electrolyte can never be replenished in a sealed battery, degrading battery performance,
and eventually leading to total battery failure.
Floating voltage depends on Electrolyte's specific gravity, which depends on temperature.Charging
voltage should therefore be compensated accordingly. Still, battery lifetime is halved when
temperature increases by about 10 degrees. Lowering battery temperature reduces its capacity.
The best way to keep the batteries at top performance and long life is to install batteries in air
conditioned environment of 25°C.
Unfortunately we cannot predict battery behavior by simple logic. There is no precise way yet to
foretell batterys' long term behavior by a simulation program, and most information is based on
field behavior and manufacturers' and users' experience. Thus, where high power UPS systems a
re involved, in which a failure of a single battery may affect UPS ability to fulfill its mission, and
potentially also cause damage to other batteries, strict compliance to manufacturer's instructions
should be adopted. It is particularly important to implement manufacturer's recommendation with
regard charging and discharging methods, voltage and currents, including floating current as well
as allowed voltage and current ripple ratings.
Thus, the conclusion of the matter is: Read Manufacturers instruction and act accordingly.
To avoid confusion of a perplexed user, it is advised to select a battery, designed specifically for
UPS applications such as Enersys HX line.
I welcome remarks. Please send to: meirp@upsonnet.com
Editor's Note:
" The Guide for the Perplexed " is the cited title of a book written by Mimonides, a 12th century
Jewish philosopher who questions with reference to the bible theological and reality issues,
many of which are still controversial today, such as: Did the universe have a beginning? Will it
ever end? What is the nature of evil? Does the complexity of organic life imply some kind of
rational design?
UPSonNet Website News
Our July newsletter concerned Pros and Cons of Lithium Ion versus Lead Acid UPS Battery,
aroused much interest from industry professionals and UPS users. To enable users to find basic
information and contacts of Li Ion battery manufacturers and vendors, we intend to publish on
UPSonNet website a LI -Ion battery suppliers' guide, which details main type of batteries and
contact details of potential buyers. (for example, see : Professional N.A UPS suppliers Guide.)
The inclusion in the Li Ion guide is currently free of charge for relevant companies.
Interested parties, please write to denis@upsonnet.com, including full name, few sentences
describing your company and relevant products, e-mail phone and task of person to contact,
country (state in USA), and link to relevant page on company's website.
Please include in subject line: Li Ion Suppliers Guide.
UPS Industry News
On 8 August, we have published on UPSonNet News a brief of Vycon's new hybrid UPS system,
which presents an interesting and worth noting solution of combined flywheel UPS with battery
backup. The company claims, based on EPRI study that 80% of all utility power disturbances last
less than two seconds and 98% last less than 10 seconds. These are taken care of by the flywheel
system, whereas longer times are treated by the battery backup, until utility is restored or generator
turns on. The system thus provides 99% efficiency with extended battery life time.
We welcome remarks on any Newsletter issue. Please e-mail to: denis@upsonnet.com
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