International Journal of Engineering Trends and Technology (IJETT) – Volume 25 Number 2- July 2015
Residual Capacity Estimation for Online UPS Lead Acid
Batteries
Jayakumar Vk#1, Suryateja Mudimi#2, Deepa Selvaraj#3, Kannan Sankar#4
#
Engineering and Industrial Services, TATA Consultancy Services
Siruseri Special Economic Zone, Plot No. 1/G1,, SIPCOT Information Technology Park, Navalur Post,,
Siruseri, Tamil Nadu 603103
Abstract — UPS failure is the primary cause for data
centre outages in commercial buildings. The chief
contributor for UPS failure is attributed to battery failure.
Though instantaneous battery failure can be determined
using typical battery monitoring systems, they do not
encompass to determine the residual life time of battery. The
aim of the study is to estimate the residual life of VRLA
battery used in online UPS for office buildings. Commercial
buildings employ online UPS for their loads, as the UPS
delivers power to the loads via battery, eliminating the need
for switching mechanism, as in the case of typical off line
UPS. In a typical Online UPS, the battery operated inverter
works continuously whether the mains supply is present or
not and hence transfer time has no role in power failure
situation. As the online UPS is battery operated
continuously, it is very important to estimate the residual life
of the battery, as minute changes in the battery parameters
viz., internal resistance, ambient temperature, Voltage and
current can indicate the potential performance degradation
of battery and hence affecting the battery life.
In the present work, the residual capacity was theoretically
estimated using developed analytical model in MATLAB
r2015a. For commercial buildings, battery for UPS system
plays an important role, as critical loads viz., computers and
lighting shall be connected to the UPS. With real time data,
under lab environment, it was observed that if the stress
factors are maintained at manufacturer prescribed range,
then the residual replacement life can be increased up to
33%
inside the cell during charging is recombined within
the cell to re-form water. Though technically it is
called maintenance free, other routine maintenance are
still required. Equation 1 shows the electrochemical
reaction of VRLA batteries
Pb + H2SO4 + -O2 = PbSO4 + H2O ---------- (1)
VRLA batteries provide many advantages over
conventional lead acid batteries the gases eliminated
from the batteries are eliminated and can be used in
scenarios where space is limited as in the case of
typical office environment [4].
Having said so, the VRLA life cycle of VRLA
batteries
decrease with deep discharge cycles. A
maximum of 10 year is achievable for VRLA batteries
under proper stress conditions.
A commercial establishment is bound to use battery
powered UPS. In case of commercial buildings, online
UPS is preferred as the switching time is eliminated.
Also the UPS is powered using VRLA battery banks,
because of reduced maintenance cost. Figure 1 shows
the typical block diagram of online UPS.
Keywords — Online UPS, Residual Life, VRLA Battery,
commercial buildings.
I. INTRODUCTION
Batteries are indispensable power sources. All
electronic appliances use various types of batteries
like lead acid, Li-Ion, NiMH etc. [1]. Among these,
lead–acid batteries offer the properties of low internal
resistance, low price, and large capacity. Also lead
acid batteries provide high power to weight ratio. The
major application to lead acid batteries include Engine
starting, motive power i.e. to propel an electric vehicle
and stand by power. As per the electrochemistry of
battery design, the conventional lead acid batteries
evolve hydrogen and oxygen when charged. This is
because of the electrolysis of water inside the battery.
Since electrolysis of water takes place, frequent
maintenance of the battery needs to be performed.
To overcome the need for frequent battery
maintenance, a new design of lead acid battery called
the recombination type or Valve Regulated Lead Acid
(VRLA) battery was designed [4]. The battery is fitted
with pressure-release valve to maintain oxygen
pressure inside the battery, so that oxygen generated
ISSN: 2231-5381
Figure 1: Online UPS block diagram
Out of the multitudinous of devices that use UPS, an
interruption free power supply is mandatory for
computers and data storage centres. Any interruptions
in power supply may cause variety of components to
fail, which in turn may cause corruption or loss in
memory(data), thus leading to overhead to building
owners[2]. Extreme power distortion from mains
requires frequent power usage from battery, thereby
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International Journal of Engineering Trends and Technology (IJETT) – Volume 25 Number 2- July 2015
increasing the discharge. Frequent discharge cycles
from battery will lead to reduced battery life [1].
Thus an approach is mandatory to estimate the
residual life of battery. General methods to estimate
the capacity or residual life include impedance method
resistance method [5, 6]. Coulometric method can
measure the charge or discharge current of battery.
Based on the depth of discharge, the life cycle of
battery has been estimated. There are various factors
that affect the age of VRLA batteries viz., ambient
temperature, that is, the temperature that at which the
battery is operated, battery temperature- temperature
measured at the terminal of the battery, internal
resistance, and ripple voltage.
Deep discharge also reduces the battery life. The life
time calculations are usually based on 1 to 2 deep
discharges per year [7]. Eddie Davis et.al has in their
technology evaluation has measured the internal
resistance of battery. Also an increase or decrease of
battery temperature shortens the cell life [8]. Kevin
White in their patent has provided the equation for
temperature compensated life (TCL) as:
TCL=EL/2[(BT−25° C.)/10]
Where EL refers to expected life as specified from the
manufacturer, which typically varies from 3 years to 5
years for VRLA batteries [9].
Also after for every 1oC increase, after 25oC, life
expectancy reduces by 50%. Hence the battery
temperature and ambient temperature plays a crucial
role in estimating the residual life of batteries.
Peukert’s law expresses the capacity of battery at the
rate at which the battery is discharged.
The
mathematical expression for Peukert’s law is shown in
equation 2
T=C/In ------------ (2)
Where T refers to time in hours i.e. how long the
battery will last, C is the theoretical capacity (in
Ampere-Hours) determined by manufacturer, n refers
to Peukert number typically varying in the range of
1.1 to 1.3, showing how the battery holds the charge
under high discharge rate. The Peukert number is
determined empirically [12].
The limitation of
Peukert law is that it does not include the temperature
effect of batteries and the self-discharge rate is not
included [13]. Hence the above model was not
utilized for current analysis.
Lucia cloth et.al, developed inhomogeneous Markov
Reward models to evaluate battery lifetimes [10].
Xiaolin Wang et.al, developed a life time model
customized for wind turbine generators [11]. By
statistically predicting the discharge profile, the
remaining life of battery was predicted. However it
was found that the technique could not be applied to
batteries that supply power to online UPS.
From the above literature survey, it was inferred that
the residual battery life estimation can be classified
into performance based models and weighted Ah
through put models. With the battery monitoring
system installed in commercial buildings, analytical
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model with empirical data fitting will be more
appropriate to estimate the residual life of battery.
II. METHODOLOGY
Battery Monitoring System was installed to monitor
the stress parameters leading to reduction in battery
life. The parameters that were monitored include
Voltage, Current, Internal Resistance, Battery
Temperature and Ambient Temperature.
For pilot study, 34 batteries for one UPS bank was
monitored. The sampling interval of data logging was
set to 20 seconds. The battery capacity based on the
depth of discharge as obtained from the manufacturer
is shown in Figure 2. The data logging was done using
Microsoft SQL data base.
Figure 2: Battery Capacity vs Depth of Discharge
Since the UPS under study is online, the depth of
discharge may be correlated to the situations where
there is power shut down and the corresponding
variations in internal resistance of battery. As the
sampling rate was low, the current (measured in
Ampere) transient during power shut down scenarios
was also measured
III. RESULTS AND DISCUSSION
MATLAB R2015a was used for estimating the
residual life of the battery. Internal resistance, input
voltage (Volt), Current (Ampere), Ambient
Temperature (0C), Battery Temperature 0C), – the
temperature measured at the terminals of the battery,
were the parameters that were measured for the
battery string of 34 batteries.
A single variation in one of the battery in the string,
can have extreme effect on the entire battery string.
Figure 3 shows a typical GUI built using MATLAB
GUIDE. The tool can be used by admin owners to get
an insight to understand the different effects and the
corresponding residual life of batteries.
The tool provides an option to monitor individual
battery in the string and the individual effect.
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International Journal of Engineering Trends and Technology (IJETT) – Volume 25 Number 2- July 2015
Figure 4 : Battery Temperature vs Float Life
Figure 3: GUI Developed Using MATLAB GUIDE
The built GUI fetches the stress parameters from the
database for the specified battery number. The
developed GUI has the feature to input the purchase
date, the year under analysis and the idle time i.e. the
time for which the battery has been unused. A
composite equation was conceived to achieve the
cumulative effect of stress parameters for estimating
the residual life of battery under study.
The temperature compensated life is given as a
function of expected life. The standard expected life,
as per the manufacturer warranty period would be 3
years or 200 discharge cycles (refer Figure 2). Since
the UPS is online in nature for pilot study, 3 years was
considered as expected life, as per manufacturer
warranty specification.
Similarly, the temperature controlled float life was
also provided as an option for the end user.
The variation of float life as a function of operating
temperature at which the battery is placed was
observed. Figure 4 shows the typical variation of float
life as a function of temperature.
It was inferred that by varying the temperature within
the manufacturer specified limits, there is a linear
variation. Hence the operating the battery at
appropriate conditions, the life of the battery can be
increased by approximately one year form the
warranty provided by manufacturer.
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The effect of internal resistance was alone observed
and it was found that there was no observable change
in the internal resistance. Thus it was inferred that the
battery was found to be healthy. Figure 5 shows the
variation of internal resistance.
Figure 5: Internal Resistance Variation
Also, it was observed from the battery monitoring
system data that as the battery temperature increases,
the temperature compensated life decreases. This is
may be attributed to the increased rate of oxygen
recombination of the lead acid batteries. Figure 6
shows the typical variation of temperature
compensated life.
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International Journal of Engineering Trends and Technology (IJETT) – Volume 25 Number 2- July 2015
work may include to extend the models for other
battery models.
ACKNOWLEDGMENT
The authors would like to thank Mr.Thanga
Jawahar Kalidoss, Mr. Sandip K. Ghosh of the
Engineering and Industrial Services, Rahul Kapoor,
Cdr. Sairam Srinivasan, Ajay Senguttavan of
administration division of TATA Consultancy
Services for their consistent support and
encouragement to carry out this work.
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Figure 6 : Temperature Compensated Life
Variation
IV. CONCLUSIONS(SIZE 10 & BOLD)
VRLA batteries plays a crucial role for online UPS.
The performance of UPS heavily depends on the
performance of the battery string. The residual life
estimation of batteries plays an important role in
identifying the variation of performance of the
batteries. Also this would help to plan for battery
replacement, providing sufficient cushion time for
commercial building owners – a concept of just in
time for battery procurement. Battery Monitoring
System was installed to monitor the performance of
VRLA battery. Various effects on the battery
performance was studied and with real time data it
was observed and validated that an increase in 33% of
residual life can be achieved if the stress factors –
including the deep discharge cycles was maintained
under prescribed manufacturer conditions. Future
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