29-1 - Monitoring the state of health of VRLA batteries

through ohmic measurements

Asa R Waters Sprint Florida, Box 16500, M/C 5344, Altamonte Springs, FL 32716

Kathryn R Bullock* and Chalasani S C Bose Power Systems, Lucent Technologies, 3000 Skyline Drive, Mesquite, TX 75 149

e-mail: chalasani@lucent.com * formerly with Lucent Technologies

Abstract: This paper is based on a joint study conducted by Lucent Technologies and Sprint Florida on monitoring the state of health of VRLA batteries. The purpose of the study was to see whether ohmic techniques (impedance, resistance and conductance) can accurately predict the state of charge and state of health of the batteries. This study consisted of field testing as well as accelerated laboratory testing. Comparison of the commercial meters and recommendations on minimizing the measurement errors have also been made.

Introduction

Monitoring the state of health of telecom batteries is very critical for providing uninterrupted service. The conventional discharge test is by far the most reliable method to monitor the state of health of a battery. However, discharge testing is a time consuming and expensive method. Moreover, it must be done off-line, which could potentially result in down time problems. This method is also detrimental to batteries, since routine discharges could reduce the life of the battery. There are various ohmic techniques such as impedance [ 11, resistance [2] and conductance [3] measurements that are being used in order to minimize some of the problems. Although these ohmic techniques are useful in getting an easy, economical and faster way of monitoring batteries, many precautions are needed in order to collect accurate and reliable data.

Ohmic techniques are based on the principle that the impedance and the resistance of a valve regulated lead-acid (VRLA) battery increases as the battery ages and loses capacity. The expected failure modes of VRLA batteries are dry-out and grid and strap corrosion, which results in an increase in battery irnpedance and resistance [3]. Impedance [I] and conductance [2] instruments reported in this paper basically involve measuring the impedance of a battery by applying a known ac frequency signal in 1he range 10 to 100 Hz [4]. On

the other hand, the resist,ance instrument [2] involves actual battery discharge at 72A for about 3 seconds. From the drop in the voltage as a result of the 72A load the resistance of the battery is determined. All these instruments [ 1 to 31 have been claimed to accurately measure the state of charge /state of health of batteries. The purpose of this study is to see whether ohmic instruments can accurately measure the state of charge and state of health of the batteries.

Procedure

Ohmic meters used: Six commercially available ohmic meters from three different manufacturers were used in this study. .4 milliohmeter (meter D) used for laboratory measurements to check the lose contacts was also used. For the sake of simplicity only the results obtained from four meters are presented in this study. These meters are (1) resistance meter (meter ,4), (2) conductance meter (meter B), (3) impedance meter (meter C) and (4) Milliohmeter (meter D).

In addition to field study, accelerated laboratory tests were also conducted for Lucent Technologies’ VR, 12IR125, and IR30 and IR40 batteries.

Field data: Field data was collected mostly on-line from batteries in cabinets located at different locations. Some data on field returned batteries was collected on-line in a central offices by connecting to an appropriate power supply.

On-site test: Most of the ohmic and discharge capacity data was collected on-line from the batteries inside cabinets. Age of the batteries in these cabinets ranged from 1 to 3 years. Before collecting the data we made sure that there had been no discharge for at least one week prior. The connections between the bus bars, cables and the battery terminals were retorqued prior to making the ohmic measurements in order to minimize the errors due to the poor connections. Duplicate ohmic measurements wlere taken to minimize the

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errors due to poor contacts between the leads of the ohmic meter and the battery posts. After completing the on-line measurements, the batteries were disconnected from the rectifier. String discharges at constant current rate ranging from one to three hours were performed to determine the capacity and thus the state of the health of the battery. Electronic load and resistance load banks were used for discharging the batteries. Multimeters were used to monitor the voltage of the batteries. Appropriate shunts were used to monitor the discharge currents. Discharges were continued until the cell voltage of all cells reached 1.9V/ cell ( 1 1.4V for battery). Using the discharge capacity data from the product manual of the battery, the percentage of the rated capacity was calculated.

Central office test: Central office tests were performed on batteries which were removed from service because of problems such as low capacity, acid leaks, and so on. These batteries were fully charged before starting the test. On-line ohmic data was collected by connecting the battery string to an appropriate power supply. The remaining test procedure was the same as in the field testing.

Laboratory data: Accelerated float tests at 5OoC were conducted on different batteries in order to collect the data as the batteries age. This test involved float charging the batteries at 5OoC to age the batteries faster. After every one week of floating at 2.27V/ cell and at 5OoC, the batteries were cooled to room temperature and ohmic and discharge tests were conducted. Discharges were performed at the Ci8 rate to check the state of the health of the batteries. Discharges were conducted on strings containing four batteries each until the string voltage dropped below 45.6 V. Appropriate capacity corrections were made to the batteries for which the end of discharge voltage was higher than 1.9V /cell. This test was continued until the capacity of the battery reached below 40% of the rated capacity. Two strings of four batteries each of 12IR125, IR30 and IR40 batteries were used in this test. Twelve cells of VR were used to make a 48V string for this testing. In this paper only the results on IR30 and IR40 batteries were presented.

Fig. la: Impedance (milliohmeter)

0 20 40 60 80 100 120 % of Rated Capacity

Fig. 1 b: iR30 Conductance 1

100 c-*- 0 4 I I I 1

0 20 40 60 80. 100 120 % of Rated Capacity

Fig. IC: IR30 Resistance 60000 I S , I

$ 50000

E ,,40000

$ 530000

.-

3 020000

U) 10000 2 .-

0 0 20 40 60 80 100 120

% of Rated Capacity

Fig. Id : IR30 Impedance

- i-- _-. 0 30 60 90 120

% of Rated Capacity

Figure 1. Field data for IR30 batteries (a) meter (D), (b) meter (B), (c) meter (A) and (d) meter (C) as a function of the percentage of the rated capacity.

Results and Discussion

Field data for IR30 batteries is summarized in figures (1 a) to (1 d), where measured ohmic values are plotted as a function of the percentage of rated capacity. Each data point corresponds to a battery. The discharge capacities were normalized based on the rated capacity of the battery. As expected, the impedance value of meter (D) and the resistance value of meter (A) is increasing and the conductance decreasing as the capacity of the battery is decreasing, reasonably a linear relation exists between 1 10% and 10% of the capacity. At low capacities the ohmic value changes very quickly. Low capacity data was not available to see such trend in the case of impedance measurements for the meter (C).

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Fig 2a: IR30 Impedance(milliohmeter) I

In E 12 0' d 4 4 % g o

E 8

c

0 20 40 60 80 100 120 % of rated capacity

Fig 2b: llR30 Conductance

E 0.6 -+ 6 E In 0.4 - ^ _ i - _ l - . . ~ - - d - ~ - - - 0

* 2 I

3 E 0.2 --7--t----7- z 0 0 - I I 1 I

0 20 40 60 80 100 120 %, of rated capacity I

0

Fig. 2c: IR30 Resistance

0 1 ' I I

0 20 40 60 80 100 120 %, of rated capacity

Figure 2. Laboratory data for IR30 batteries (a) impedance (meter D), (b) conductance (meter B) and (c) resistance (meter A) as a function of the percentage of the rated capacity. The data points are for eight batteries,. The least square curve fitted lines are shown as solid lines.

In figures (2a) to (2c) laboratory data for eight IR30 batteries are suimmarized, where the ohmic value is plotted as a function of the corresponding percentage of rated capacity. As the battery ages at 5OoC with time, it loses capacity, the corresponding ohmic value changes,. The impedance and resistance values are increasing whereas conductance is decreasing. The solid lines are least square curve fitted lines.

Fig. 3a: IR40 Impedlance(mil1iohmeter)

8

0 20 40 60 80 100 120 % of rated capacity

I I Fig. 3b: IR40 Conductance I . 1 2

1 0 20 40 60 80 100 120 %of rated capacity I

Fia. 3c: IR40 Resistance - 14000 - ,

0 20 40 60 80 100 120 % of rated capacity

Figure 3. Laboratory data for IR40 batteries (a) impedance (meter D), (b) conductance (meter B) and (c) resistance (meter A) as a function of the percentage of the rated capacity. The data points are for eight batteries.

Laboratory data for IR40 batteries are summarized in figures (3a) to (3c). As seen in these figures, the ohmic values are independent of the discharge capacity of the battery. This finding is in contrast with another battery of similar construction: IR30. These results suggest that ohmic value of the battery strongly depends upon the design of the battery. Field data for these batteries is not available at this stage.

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Fig. 4a: iR30 Impedance(mi1liohmeter)

- - - - .

~-

0 20 40 60 80 100 120 % of Rated Capacity

Fig. 4b: IR30 Conductance 120 T I

ohmic measurement. This may explain why conductance pickup differences between the laboratory and the field data, whereas the impedance and resistive techniques do not.

Another striking feature of the data in the figures (4a) to (4c) is that the ohmic value is almost independent of the capacity between 85% and 1 15%. This suggests that the ohmic measurements may not make accurate predictions in this region. By comparing the data obtained for IR30 and IR40 batteries, it is evident that ohmic techniques may

I + Field data

--Labdata I I ?

0 20 40 60 80 100 120 % of Rated Capacity

Fig. 4c: IR30 Resistance 500 -t I

400 s 2 300 0

m U

.; 200 B

100

I I ,

48 68 8& 100 120 1 *'Yo of ated apaci

Figure 4. Field (data points) and laboratory (solid line) data for IR30 batteries (a) impedance (meter D), (b) conductance (meter B) and (c) resistance (meter A) as a function of the percentage of the rated capacity.

In the figures (4a) to (4c), both field and laboratory data on IR 30 batteries are compared. The data points are the normalized field data and are the same as in the figures (1 a) to (1 c). The solid lines are the least square fitted lines of the laboratory data shown in figures (2a) to (2c).

Laboratory data and the field data agree well for impedance and resistance data. However, a small discrepancy appears in the conductance data. This suggests that there are differences in the methodologies employed in various ohmic instruments. Different ohmic instruments employ different frequencies of ac signal or dc pulse to measure the impedance/ conductance response of the battery. Depending on the signal, the contributions from resistive, capacitive, and inductive components vary and affect the resultant

battery type cannot be generalized for the rest of the batteries, although this is recommended by the manufacturers of ohmic meters. After verifLing the validity of the technique by generating curves similar to those in figures (la) to (1 d), it may be possible to use these instruments for monitoring the state of battery health. Otherwise the use of the instrument is limited to screening the battery condition. Curves similar to figure 1 should be used for setting the ohmic limits for a good and bad bdttery.

While making the ohmic measurements, many precautions must be taken to minimize the measurement errors. The following section presents some of the details.

I. Measurements

1. Power outages before the ohmic measurements: Ohmic measurements should be taken only on batteries which have not undergone discharge at least one week prior to actual measurements. These discharges can be caused by power outages or because of routine automated discharge tests. In either case batteries would not be charged fully if enough charging time is not allowed. Incompletely charged batteries cause inaccurate capacity results from ohmic measurements. Depending on the depth of discharge, it can take up to one week of charging at the recommended float voltage in order to charge the batteries to full capacity.

2. Temperature corrections for battery capacity:

Temperature has a large influence on the capacity of the battery. Avoid taking measurements at temperatures higher than 45 OC (M 1 10 OF) or lower than 10 OC (x 50 OF). Battery capacity should be corrected appropriately for battery temperatures other than 25 OC (= 77 OF). Guide lines specified in the meter manual may be used for this correction.

3. Retorquing the bus bar/ battery post contacts:

It is recommended to retorque the cable (bus bars), battery post connections before making the ohmic measurement. By Retorquing, poor contacts/

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connections will be eliminated or at least minimized. Poor cont,acts/ connections result in poor capacity readings.

4. Duplicate readings: In order to minimize errors, duplicate sets of readings should be taken. The lower impedance and resistance or higher conductance value of the two readings should be chosen for the final calculation of the state of health/ state of charge. This way the measurement errors will be minimi:zed.

5. Terminal contacts for readings: To get a better contact between the leads of ohmic meter and battery terminal posts, the recommendations from the ohmic meter manufacturer should be followed.

6. Faulty rectifiers: If a rectifier generates a lot of noise, it may not be possible to make on-line ohmic measurements.

11. Baseline measurements: In order to find out how good a battery is at any given time, the measured ohmic value must be compared to the baseline ohmic value of the battery that was established when the battery was in 100% state of health. Establishing the baseline measurements is critical for determining the health condition of the battery. These measurements have to be made when the batteries are in good health and fresh with 100% capacity undeir the actual operating conditions by connecting to actual electronics (on- line conditions). The base line values provided by the manufacturer of the ohmic meter may not be valid since these are not measured under the actual operating conditions. The ideal time to make base line measurements i:; after two weeks of floating at recommended float voltage, following installation of the batteries in the cabinet.

Measurement Schedule: It is difficult to find out whether a battery is good or bad simply by using a single set of ohmic measurements. However, by measuring the values at regular intervals and comparing the data ,with the base line values, one can monitor the trends in the state of health of battery. A recommended measurement schedule is every six months.

Comparison of various instruments: In choosing a suitable ohmic meter for monitoring batteries, the end user should use his own judgment depending upon the requirement. As far as accuracy of the meter is concerned all the meters used in this study fare similarly. However, the differences are in the ease of use. Table I summarizes the information on the comparison.

Summary and conclusions

Various ohmic meters have been tested for their usefulness in monitoring the state of the health of the VRLA batteries. The predicted AH capacities by these meters were compared with the actual discharge capacities of the batteries tested in the field as well as in the laboratory.

From the comparison it is apparent that all these instruments yielded similar results. Each instrument has its own merits and demerits. The end users should use their own judgment in selecting a proper meter for their applications.

Results indicate that the ohmic value of the VRLA battery depends strongly on the design of the battery. As seen in the case of IR40 batteries, these instruments may not be useful for state of health monitoring for all VRLA batteries.

In order to use these instruments for accurate monitoring of the state of health of a VRLA battery, one should thoroughly check the prediction vs actual capacity of the battery. Simply checking on one type of battery and generalizing the trend to other batteries, as is normally recommended by the manufacturers of meters, might lead to erroneous results and conclusions. In the absence of this type of detailed study, the use of these instruments is useful only to screen VPLA batteries to detect potential problems. Those batteries will need to be further investigated using conventional load tests.

Acknowledgments

The authors gratefully acknowledge R. R. Arp who performed some of the experiments. The authors also would like to thank G. W. Mathesien and S. W. McCluer for lielpful discussions.

References

1. G. J. Markle, Proc. lntelec 1993, Paris, France, page 23

2. G. Alber, Proc. Intelec 1994, Vancouver, Canada page 245.

3. D. 0. Feder, J. Power Sources, 48(1994)135.

4. M. Hawkins and L. 0. Barling, Proc. Intelec 1995, The Hague, The Netherlands, page 27 1.

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Table I: comparison of all the instruments used.

End contacts clips large alligator large alligator Data Read out digital read out can store in

memory hard to read in day

small small analogue needle digital can be

printed hard to read in day

light na

ac only ac only

quite useful for strings with high laboratory testing impedance should

be made shorter

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