Ohmic Battery Testing in Indian Telecommunication Networks

T. Venu Gopal – Vice President R & D, HBL Power Systems Limited, India

Todd Stukenberg, Executive Director – Marketing ,Midtronics, Inc., USA

Abstract -- Phenomenal growth of the telecom network in India has resulted in an increased usage of Lead Acid Batteries. The telecom network in India is vast and spread all over the country with different geographical conditions and having extreme operating environments. Monitoring of batteries, which is one of the critical elements in the network, has become a challenge for the operators. To ensure system reliability, operators are depending on conventional methods for battery health checking which are time consuming and expensive. In today’s cutthroat competition era operators are forced to reduce their CAPEX and OPEX and hence they are working on all possible options to achieve the same. As a result, usage patterns have changed and battery has become primary source in the events of power failure. Battery life is affected significantly under these operating conditions and in some cases batteries are even suffering from premature failure modes. At present 90% of Indian telecom network is equipped with AGM VRLA batteries. Tubular Gel VRLA batteries are gaining popularity due to their superior features suitable for cyclic applications & tropical environments and hence the market share of Tubular Gel VRLA batteries is expected to increase in future. Whether it is AGM VRLA or Tubular Gel VRLA, challenge of battery monitoring remained same and hence all Telecom operators are looking for a reliable battery monitoring solution, which can be used as a tool for deciding the battery’s State of Health (SOH) and take timely actions to avoid catastrophic failures there by ensure system reliability. To support its customers, HBL being the major supplier of Lead Acid batteries to the Telecom segment in India, has conducted extensive study for more than 3 years to evaluate the accuracy of SOH prediction and reliability of various types of health monitoring devices available in the market. Validation includes testing of the devices both in laboratory and in field. During the validation, HBL found that, the Conductance testers of Midtronics make are demonstrating promising correlation accuracy both in SOC and SOH predictions. A correlation accuracy of 90% and 80 % was observed in predicting battery SOC and SOH respectively. HBL has tested the stand-alone testers & data logging units of Midtronics, during the validation process and both the devices were observed accurate and reliable. This paper outlines various types of failure modes those Lead acid batteries generally come across in different types of applications, findings of validation, recommendations to the users so that they can achieve maximum benefit out of battery health monitoring devices. Paper also, describes about the shortfalls of these devices resolving of which can improve the diagnosing accuracy and reliability.

1. INTRODUCTION HBL Power Systems Limited is one among the leading battery manufacturers in India, and is a 33-year-old organisation. HBL manufactures different types of lead acid batteries such as Conventional lead acid batteries, AGM VRLA batteries, Tubular GEL VRLA batteries and Pure Lead Tin AGM VRLA Monobloc batteries. It also manufactures batteries with other chemistries like Nickel Cadmium (Pocket plate, Sintered plate, Fibre plate), Silver Zinc, Lithium-Ion, Thermal batteries etc. HBL is the major supplier of lead acid batteries to Indian telecom segment, holding a 70 % market share. The Indian Telecom network is one among the largest telecom networks in the world and 90% of it is equipped with AGM VRLA batteries. Usage of conventional lead acid batteries in the Indian Telecom network was initially started during 1910’s. AGM VRLA batteries have replaced the conventional lead acid batteries from the time of their inception during 1990’s due to their low maintenance features. Up to the year 2005, AGM VRLA battery was used as a secondary power source for the telecom equipment in the events of power failure, Diesel Generator (DG) being the primary source. Most of the time, battery use to be on float charge and hence the applications were pure Float. Service life of the battery under these conditions was ~ 8 to 10 years. After the year 2005, while Cellular phones started gaining popularity, many telecom operators entered in to the market leading to increased competition. Subsequently, telecom charges have been reduced to as low as ~ 2 % of the charges in comparison with the charges that were present at the time of introduction of cellular phones while cost of all other materials are increasing. To provide the telecom services at such a lower price, telecom operators are forced to work on all possible options to lower their CAPEX and OPEX. Similar to all other countries even in India, rate of power consumption has increased drastically in last 20 years, but the infrastructure for power generation did not grow at the same pace. Because of this, frequent and prolonged power outages have become routine and are further increasing day-by-day.

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978-1-4244-3384-1/10/$25.00 ©2010 IEEE

Similar to all other business segments, Telecom segment is also incurring huge amounts towards power generation through alternative sources increasing their OPEX. To minimise OPEX, telecom operators have started using the battery as primary source in the events of power failure, which is affecting the service life of the batteries significantly and in some cases, batteries are even suffering from premature failure modes. Telecom operators cannot afford their network going down which will result in huge revenue loss in addition to customer dissatisfaction. So, by any means their network shall run reliably for which they need reliable batteries and reliable maintenance tools. AGM VRLA batteries being sensitive for the existing operating conditions of Telecom application, Tubular Gel VRLA batteries are evolving as an alternative solution. Development works are under progress in different fronts to make even AGM VRLA batteries robust enough to optimise their service life under existing operating conditions. Whatever may be the type of battery used, it is bound to fail after completion of its service life and/or may undergo premature failure modes due to either manufacturing problems or abusive operating conditions. In any case, there is a need of accurate and reliable test method with which the user can know the SOH of the battery and take timely corrective / preventive actions. It is a known fact that, timed discharge testing is the accurate method for finding the battery health at any given point of time. But, even discharge testing cannot reveal the information on remaining life of the battery. For that matter, no parameter / test method can give accurate information on left out life at the battery, however approximate life can be estimated through trending the discharge test data recorded at regular intervals. But, it is practically difficult to conduct timed discharge testing of such huge number of batteries spread all over the country at regular intervals as it consumes time and is also expensive. So, there is a need for simple & fast, low cost solution for finding the battery SOH. As an alterative solution for timed discharge testing, different types of testers are available in the market, measuring different parameters of the battery viz. Conductance, Impedance & Internal resistance through different measuring techniques there by diagnosing the battery SOH. Battery users are in need of a simple tester which can show a weak battery / potential weak battery as weaker.

HBL being the major lead acid battery supplier in India, wanted to help out their customers and hence conducted extensive study on various types of testers available in the market to evaluate their efficacy in finding weak / potential weak cells which is the prime concern of the user. From the validation conducted, HBL found that, the Conductance testers of Midtronics make are demonstrating promising correlation accuracy both in SOC and SOH predictions. 2. TYPES OF BATETRIES USED IN INDIAN

TELECOM NETWORKS As detailed above, Conventional lead acid batteries were used in Indian Telecom networks in the initial days. Since these batteries emit hydrogen gas and corrosive acid fumes, they use to be installed in separate rooms with appropriate ventilation arrangements. Routine maintenance of conventional batteries viz. adjusting of electrolyte specific gravity and electrolyte levels, cleaning of electrolyte leached out from the batteries has made the telecom operators to opt for low maintenance AGM VRLA batteries in place of the former. AGM VRLA batteries were operated in controlled temperature environments along with the telecom equipments. As long as applications were pure float, there were no issues with AGM VRLA batteries both in terms of performance and durability. But, reduced life of AGM VRLA batteries under existing operating conditions viz. frequent power outages there by frequent cycling of batteries, operating the batteries at ambient temperatures and in outdoor cabinets etc. made the operator to think of an alternative option which can last for longer periods under same conditions. Now the alterative option for the telecom operator appears to be Tubular Gel VRLA battery, which has got relatively higher cyclic life and lesser reduction in life while it is operated at higher temperatures. Typically batteries are operated at an average ambient temperature of 35 deg. C wherein the temperature varies between 5 to 50 deg. C throughout the year. Batteries are subjected from 1 to 4 charge / discharge cycles in a span of 24 hrs. Now the challenge for all types of batteries is, they should get charged faster and last for longer periods while they are subjected to frequent cycling and being operated at higher temperatures than the specification (which is 27 deg. C in India).

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3. VARIOUS TYPES OF FAILURE MODES IN LEAD ACID BATTERIES

Lead acid batteries suffer from different types of failure modes, which depends on type of the application in which they are used. Applications can be broadly categorized in to two types Float and Cyclic. Application is categorized as Float, if batteries are subjected to 3 to 5 discharges in a span of 1 year and/or batteries are on continuous charge for a minimum period of 15 days between two successive discharges otherwise such application is categorized as Cyclic. In cyclic applications, batteries are subjected to as low as 2 charge / discharge cycles in a span of 15 days and as high as 4 charge / discharge cycles in a span of 24 hrs i.e. within a day. In general, float life of the batteries is declared in calendar years where as cyclic life of the batteries is declared in number of charge / discharge cycles. Cyclic life of the batteries can also be declared in calendar years if, duration of typical charge / discharge cycle is known and it will be generally in the order of 2 to 3 years. Operating temperature will affect both float and cyclic life of the battery. It is well known that, with an increase of 9 to 10 deg. C rise in operating temperature float life of the battery reduces by 50 %. As observed by HBL in lab testing, cyclic life of AGM VRLA batteries and Tubular Gel VRLA batteries reduces by 35 % and 20 % respectively for the same rise in operating temperature. Failure modes in AGM VRLA & Tubular Gel VRLA batteries specific to the application are as follows: • Float Application

o Water loss & Dry out o Grid corrosion o Grid growth o PCL 1 – Loss of grid to active material contact (Applicable for AGM VRLA batteries) o Sulphation of negative plate o Strap corrosion o Failure of safety valve

• Cyclic Application o Active material softening and shedding o Formation of soft-shorts due to active material

shedding / dendrite growth resulted from deep discharges

o Sulphation of positive & negative plates o PCL 2 – Loss of contact within the active material

particles o PCL 3 – Decay of expanders in negative plate o Grid corrosion o Bursting of gauntlets

(Applicable for Tubular Gel VRLA batteries)

4. ELECTRICAL EQUIVALENT CIRCUIT OF THE BATTERY’S INTERNAL RESISTANCE (IR)

Since battery is made up of various conductive materials, it possesses some resistance, which will be in the order of milli-Ohms. Battery’s internal resistance can be represented with the following typical Randles equivalent circuit: Rct Wi Rm Re Ragi Cdl Where Rm = Metallic Resistance (Resistance of grids, strap, terminals & active material to certain extent)

Re = Electrolyte resistance

Ragi = Active material to grid interface resistance

Rct = Charge transfer resistance (This is an equivalent electrical Resistance offered by chemical reactions for conversion of electrical energy to chemical energy and vice versa, which depends upon the rate of chemical reaction).

Wi = Warburg impedance (is a function of Specific gravity of electrolyte)

Cdl = Capacitance across the plates (indicates the Energy stored across the plates and accounts the SOC of the battery) Due to any or combination of the failure modes except formation of soft shorts stated in Section 3, battery’s internal resistance will increase. Battery’s IR was evaluated through high rate discharge test as per IEC 896-2,1995 standard both in fully charged & fully discharged condition and it was observed that, the IR value in fully discharged condition increased by ~ 250 % w.r.t the value observed in fully charged condition. From this, it is understood that, the battery IR value changes both with State of Charge (SOC) and State of Health (SOH), which is the function of conductivity & integrity of internal components. Hence, battery IR value measured at any given point of time can be used for predicting and trending the SOC and SOH of the battery respectively. So, any instrument that measures the true internal resistance of the battery can reveal the charge / health condition of it.

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While testing various types of batteries having single terminal per polarity i.e. +ve or –ve, HBL observed that the, conductance value measured with the Conductance tester of Midtronics make (Model: CTE 1000 / CTA 2000 / TEC 4500 / TEC 6500) and its inverted value is comparable (difference is within +/- 10 %) with the IR value obtained through high rate discharge testing as per IEC 896-2,1995 standard. This shows that, the conductance testers of Midtronics make are giving results equivalent to high rate discharge test results. In all the testers of Midtronics make stated above, repeatability accuracy observed was +/- 2 % of the value measured. Repeatability accuracy of conductance measurements was observed relatively better in comparison with the repeatability accuracy of IR measurements made through high rate discharge testing as per IEC 896-2, 1995 standard. 5. FINDINGS IN VALIDATION TESTING CONDUCTED TO

EVALAUTE THE EFFICACY OF THE CONDUCTANCE TESTERS

5.1 Evaluation of Correlation Accuracy between Measured

Conductance and Battery’s Absolute Capacity (w.r.t rated 10 hr capacity)

20 Battery banks consisting 24 Nos. of AGM VRLA Cells and 24 Nos. of Tubular Gel VRLA cells each were subjected to discharge test at 10 hr rate after measuring the conductance values in fully charged condition. Correlation accuracy between measured Conductance and Absolute capacity w.r.t rated capacity (average value of respective battery bank) was observed > 90 %. Typical plots showing the relation between measured Conductance, Absolute capacity & Correlation accuracy for both AGM VRLA battery and Tubular Gel VRLA battery are given below: Graph 5.1(a): AGM VRLA Batteries (200 Ah)

Graph 5.1(b): Tubular Gel VRLA Batteries (300 Ah)

5.2 Evaluation of Correlation Accuracy between

Measured Conductance and Battery’s State of Charge (SOC)

5 Battery banks consisting 24 Nos. of AGM VRLA Cells and

24 Nos. of Tubular Gel VRLA cells each were subjected to discharge test at 10 hr rate after measuring the conductance values in fully charged condition. Conductance values were measured at different stages during discharge. Correlation accuracy between measured Conductance and State of charge w.r.t the actual capacity was observed ~ 65 %. Typical plots showing the relation between measured Conductance, State of charge & Correlation accuracy for both AGM VRLA battery and Tubular Gel VRLA battery are given below: Graph 5.2(a): AGM VRLA Batteries (200 Ah)

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Graph 5.2(b): Tubular Gel VRLA Batteries (300 Ah)

5.3 Evaluation of Correlation Accuracy between Measured

Conductance with Internal Resistance measured with IEC 896-2, 1995 standard

1 Batch consisting 24 Nos. of AGM VRLA Cells, 24 Nos. of Tubular Gel VRLA cells and 12 Nos. of Pure Lead Tin (PLT) Monobloc batteries were tested for Internal Resistance (IR) as per IEC 896-2,1995 [1] standard both in fully charged and fully discharged condition. High rate discharge testing was repeated for 3 times @ 100 % SOC to check the repeatability in IR values. Prior to every high rate discharge testing, Conductance values were measured which were comparable with the IR values. Brief summary of the results is summarized in the table given below:

Table 5.3

Battery capacity

State of Charge

(%)

Conductance (Siemens)

IR value estimated from Conductance

(m-Ohms)

IR value obtained with IEC test

method (m-Ohms)

AGM VRLA Batteries 2 V, 200 Ah 100 – 1 1268 0.789 0.813

100 – 2 1262 0.792 0.800 100 – 3 1261 0.793 0.826

0 465 2.150 2.060 TGEL VRLA Batteries

2 V, 300 Ah 100 – 1 1323 0.756 0.739 100 – 2 1329 0.753 0.754 100 – 3 1317 0.759 0.774

0 556 1.799 1.685 PLT Monobloc Batteries

12 V, 32 Ah 100 – 1 151 6.623 6.290 100 – 2 151 6.623 6.313 100 – 3 152 6.579 6.347

0 58 17.241 16.879

Note: Conductance values of Monobloc batteries were measured

with 2 V setting in Conductance tester of Midtronics make.

5.4 Evaluation of Correlation Accuracy between Measured Conductance and Battery’s State of Health (SOH)

1 Batch consisting 12 Nos. of AGM VRLA Cells and 12 Nos. of Tubular Gel VRLA cells each were subjected to Cyclic test at 20 deg. C as per IEC 896-2,1995. After every 50 charge / discharge cycles, cells were subjected to capacity test @ 10 hr rate and prior to every capacity test, conductance values were measured in fully charged condition.

1 Batch consisting 12 Nos. of AGM VRLA Cells and 12 Nos. of Tubular Gel VRLA cells each were subjected to Accelerated float life test @ 70 deg. C as per ANSI T1.330,1997 [2] standard. After completion of each ageing unit (30 days float charge @ 70 deg. C), cells were subjected to capacity test @ 10 hr rate and prior to every capacity test, conductance values were measured in fully charged condition.

In both cyclic and float life tests, conductance values measured at various stages were observed following the same trend as that of capacity. Correlation accuracy of measured conductance with battery’s State of Health (SOH) was observed good. Most of the cells whose capacities reached below 80 % of rated value were identifiable through conductance values. Curves of Conductance & Capacity values measured at different stages of above-stated life tests are given in Graphs 5.4(a), 5.4(b), 5.4(c) and 5.4(d). Graph 5.4(a): AGM VRLA Batteries (200 Ah)

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Graph 5.4(b): AGM VRLA Batteries (2 V/200 Ah)

Graph 5.4(c): Tubular Gel VRLA Batteries (300 Ah)

Graph 5.4(d): For TGEL VRLA Batteries (300 Ah)

5.5 Evaluation of the efficacy of Conductance testers in identifying weak cells (SOH prediction) installed in Telecom network

100 telecom sites connected with AGM VRLA and 20 telecom sites connected with Tubular Gel VRLA batteries (consisting 24 cells each) that have completed a service life of 1.5 to 3 years and reported as giving low back up because of some cells growing weaker were tested for both Conductance and Capacity. After charging the batteries fully, conductance values were measured (in on-line condition) and thereafter, batteries were subjected to discharge test with site load. For none of the battery sets, conductance values measured at the time of installation were available and hence the SOH diagnosis was done using typical values (declared by the battery manufacturer) and battery bank’s average conductance value. This may have influenced the following results as a proper individual trend, the most effective means for using conductance measurements, could not be developed in this case. Out of 2400 AGM VRLA cells tested, 256 weak cells were observed in discharge testing and 137 cells were identified through conductance measurements. Accuracy of SOH prediction observed was ~ 53 %. Out of 480 Tubular Gel VRLA cells tested, 36 weak cells were observed in discharge testing and 15 cells were identified through conductance measurements. Accuracy of SOH prediction observed was ~ 41 %. During the analysis the failed cells, it was noticed that, the batteries have suffered from premature failure modes viz. sulphation, water loss, active material softening etc. Also, there were some cells those suffered from manufacturing problems like damage to the end –ve plate, inadequate electrolyte filling. In some cases there were no apparent reasons for battery failure. Conductance measurements could identify weak cells but the predicting accuracy is most effective when initial baseline data is available for the batteries under test for the most effective use as a service tool. 5.6 Evaluation of the Battery Monitoring Systems

measuring conductance and voltage

Monitoring of the batteries deployed in telecom network that has spread to PAN India levels has become practically difficult and hence telecom operators are looking for Battery Monitoring Systems (BMS) that can measure & record the battery parameters and transmit the data remotely to a location equipped with a central server.

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Whenever BMS flags an alarm from any of the telecom sites, telecom operator / the agency maintaining the batteries can rush to the sites simply to replace the faulty cells after minimal checks. Midtronics has developed BMS suitable for Telecom systems, which can measure and record the conductance value of each cell in the battery bank along with its voltage and temperature at the intervals set by the user. Data recorded by the BMS will be uploaded to a central server either through internet or remotely. Using this BMS, on-line status of a battery can be monitored at any given point of time. BMS will flag the weak cells based on the analysis of conductance and other data logged and user can take decisions based on the flags raised by the BMS. At present, HBL is validating the BMS units of Midtronics in test lab and later they will be shifted to field for real time validation by selecting some typical sites from which repeated problems are reported (due to uncontrollable operating conditions). Findings of the validation will be presented in future. 6. CUSTOMER’S PERSPECTIVE ON BATTERY

MONITORING Bharath Sanchar Nigam Limited (BSNL) is the biggest telecom operator in India who is holding maximum telecom’s market share (landline and wireless) and is operated by Indian Govt. BSNL is supported by core technical team (Telecommunication Engineering Centre – TEC) who formulates the standards for all the products used in its operations taking references from various international standards and considering the operating conditions of the field. TEC has made the standards for various types of the lead acid batteries (VRLA & TGEL) used in its Telecom network. In latest editions of battery standards released from the year 2005, TEC has added a new requirement called “ Conductance Matching ” in addition to Voltage and Capacity Matching. To comply with the requirement of conductance matching, 48 V systems shall be formed using the cells having conductance values spread in the range of +/- 15 % from string’s average value. Band specified is applicable for both fully charged and fully discharged conditions. Objective behind specifying this requirement is to form the battery systems using cells having closer performance features so that system’s reliability and longevity are improved. Most of the requirements specified in TEC specification are in line with the one stated in SR 4228[3] standard especially the requirement of cell matching which is a key factor for battery’s reliable performance.

TEC has also prepared guidelines for monitoring of lead acid batteries used in telecom network in which it recommends using of battery health monitoring devices as service tool to weed-out weak & potential weak cells. All other major telecom operators in India are looking for similar kind of alternative solution to conventional test methods and some among them have started using conductance testers as service tools. China Mobile [4] who is the major telecom operator in China has used the conductance testers of Midtronics make for weed-out the failed cells from the battery systems installed in thousands of base stations those suffered from snow storm & earthquake calamities happened in China during the year 2008. 7. CONCLUSION To optimise the operational / maintenance costs in Telecom network (in connection with the batteries), an alternative testing methodology to conventional timed discharge testing needs to be adopted for diagnosing the battery SOH and take necessary corrective / preventive measures on timely basis. Based on the findings of the validation conducted in various forms, HBL suggests the telecom operators using of stand-alone testers of Midtronics make as one of their service tool. This is a very good trending tool. However present SOH diagnosing accuracy of the tester has a scope for further improvements, as it is most effective when trending from installation of the battery system. IEEE 1188,1996 [5] standard suggests ohmic measurements for diagnosing the battery’s State of health (SOH). 8. REFERENCES [1] IEC 896-2,1995 “Stationary Lead Acid Batteries – General

requirements and Methods of test – for Valve Regulated Types” [2] ANSI T1.330,1997

“American National Standard for VRLA batteries

used in Telecommunications” [3] SR-4228,1996 “VRLA Battery String Certification Levels Based on

Requirements for Safety and Performance” [4] Jian Gao, Longyun Yu “Applied Ohmic Battery Testing in a Global

Mobile Telecommunications Operator: A Multi-year Study” [5] IEEE 1188,1996 "IEEE Recommended Practice for Maintenance,

Testing and Replacement of Valve-Regulated Lead-Acid Batteries for Stationary Applications”

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