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Capacity Testing of Longway VRLA Batteries
Click:927 Date:2008-12-27 00:00:00 Information Source:

The Longway VRLA battery is rated in ampere-hours or watts per cell (w/c) at specific discharge time duration's. For example, a GB series battery may be rated as a 100 Ah battery capable of providing 10 amperes for 10 hours to 1.75 v/c. A capacity test is simply a test designed to determine the actual capability of the battery as compared to the rated capability. If this capacity test is performed at the factory prior to delivery or on site when the battery is first installed it is referred to as an "acceptance test". If the test is performed with the battery in an "as found" condition with no special preparation, it is referred to as a "service test". When a capacity test is being performed to determine the batteries percent of rated capacity for service life determination or warranty purposes it is referred to as a "performance test" and is conducted exactly as is the "acceptance test".

BATTERY PREPARATION FOR ACCEPTANCE TESTING 
The battery system should be assembled at the manufacturer's site, simulating the user's installation, or completely installed at the user's location following relevant installation instructions.
1. Measure and record all cells1 units open circuit voltage to assure minimum acceptable voltage prior to interconnecting. 
2. The individual cells1 units should be interconnected using the intercell/unit cables or bus bars specified for the application and with which the battery's performance is rated.
It is important that all cell1 unit terminals and contact surfaces of the intercell/unit connecting cables and bus bars be properly cleaned and greased prior to installation and the bolted connections be properly torqued. Improper connections can result in low measured capacity due to voltage drop at the connections. 
3. Equalize the battery for 24 hours at the recommended voltage (e.g., 2.4 volts/cell) to assure the battery is fully charged. 
4. Following equalization, the battery should be place on float charge at the recommended voltage (e.g., 2.30 volts/cell) for 3 to 7 days prior to the acceptance test. 
5. Just prior to initiating the acceptance capacity test, measure and record the individual cell/unit float voltages.

ACCEPTANCE TEST TIME AND DISCHARGE RATE CALCULATION
1. The discharge time and end point voltage selected should be one at which the battery has a published rating and is approximately the same as that of the intended application.
2. The discharge rate (amperes or wats/cell) to a specific end point voltage for the selected time, as taken from the published ratings for the battery must be adjusted for battery temperature if outside the range of 75' to 80°F. For elevated temperature, the rate will be increase while for cooler temperatures, the rate is reduced. The temperature adjustment factors are rate dependent and are noted in Figure 1.
For example, if a cell having a one hour rating of 61.5 amperes to 1.75 V/cell at 77° were tested at 60°F, the discharge rate used for a 1 hour discharge would be:
61.5 amperes * 0.93 = 57.2 ampere
For accuracy, capacity tests should only be performed between 60°F and 90°F, and as close to 77°F as possible.

ACCEPTANCE CAPACITY TEST PROCEDURES
1. Equipment Requirements 
a. Load bank capable of providing the appropriate discharge current and kilowatt load.
b. Digital voltmeters to monitor full battery discharge voltage. 
c. Amp meter to monitor battery discharge current.
d. Digital voltmeter to monitor individual cell/unit voltages during the discharge. 
e. Stop watch to monitor time of the discharge.

2. Performance
a. Assure that the instrumentation is operational and properly connected to the battery to continuously monitor battery discharge voltage and current. If parallel strings are being tested, the individual string current and total current must both be monitored.
b. Measure and record the float voltage of each cell/unit and assure all cells/units are floating properly.
c. Remove the charging current from the battery.
d. With the Load Bank OFF, connect it to the battery.
e. Start the timer and turn the load bank ON, adjusting and maintaining it for the appropriate discharge rate (amperes or watts).
f. Record the battery discharge voltage and current with the battery terminals and time at the start and end of the test and periodically throughout the test as many times as practical. 
The individual cell/unit voltages shall also be measured and recorded as often as is practical during the discharge. The number of sets of discharge readings must be 3 or more. The longer the test duration, the more readings should be taken so the capacity of individual cells can be analyzed. 
Continue the discharge beyond the required battery end point voltage (e.g., 1.85 VIC) to a lower rated voltage (e.g., 1.75) when possible to assure most cells actually discharge to the required end point.
Terminate the capacity test when the battery is discharged to the predetermined system end point voltage, a cell or unit is going into reversal, or a safety hazard is noted.

CALCULATING BATTERY CAPACITY
The ampere-hour rating is the product of the number of amperes of current the battery can supply multiplied by the number of hours (or fraction thereof) over which the current is supplied to a specified end point voltage. For example, a GB 6FM100G is rated as a 10 Ah battery at the 10 hour discharge rate of 10 amperes (10 Amperes * 10 hours = 100 Ah) to 1.75 v/c when new and at 100% of rating. It is also rated capable of providing 17 amperes for 5 hours (85 Ah) and 25 amperes for 3 hours (75 Ah) to 1.75 v/c. Notice that as the load current increases, the battery becomes less efficient. 
The percent of rated capacity would be the ratio of the Ah provided at a given discharge rate for the actual operating time to the rated Ah capacity for the same time period and operating conditions. For example, if the GB 6FM7 were discharged at the 5 hour rate of 1.19 amperes and it reached the end point voltage of 1.75 v/c at the 4 hour mark, the resulting Ah delivered would be 4.76 (1.19 amperes * 4 hours = 4.76 Ah).
Since the ratios of Ah’s are both at the same time duration, the % rated capacity can also be considered the ratio of the actual load current for the actual test duration to the rated ampere load for that same actual test time. 
This is a very important concept in that the greater the discharge rate the greater will be the difference between the ratio of the Ah’s (or currents) obtained verses the operating times. This situation is illustrated more dramatically when testing UPS batteries at maximum discharge rates for rating times of less than one hour. 
It is this ratio of the batteries actual to rated amperes or wattage for a test duration that reflects the true percent of rated capacity and condition of the internal components of the battery. It is on this basis that the aging factor used in sizing a battery is used as a multiplier with respect to the load current rather than the operating time. The percent rated capacity is then calculated as;
% rated Ah capacity = 100% * Ampere test load for the test time duration / Rated Ampere load for the test time duration OR
% rated w/c capacity = 100% * w/c test load for the test time duration / Rated w/c load for the test time duration Note that the Longway VRLA batteries are guaranteed to provide 90% of their rated Ah capacity and 100% of their rated watts/cell capacity when new and properly installed. When the battery tests at 80% of the rated capacity, even though it may still meet the users operating time requirements, it should be replaced. This is because the loss of capacity reflects the actual deterioration of the internal components of the battery. If the low capacity battery is not replaced, the eventual result could be shorted or open cells, which could result in system shutdown during a commercial power loss or other hazards.


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