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Necessity and recommendations for UPS battery maintenance

Oct-23, 2019 News

With the market demand, UPS is widely used in various industries, and the battery has become the power guarantee for the uninterruptible power supply system. The UPS without the battery can only be called the stabilized frequency stabilized power supply. When the utility power is abnormal, the battery turns the chemical energy into electric energy, and the inverter converts the AC power into the load through the inverter, so as to ensure the continuous operation of the load without interruption.

Valve-controlled maintenance-free sealed lead-acid batteries have been widely used in large, medium and small UPS, accounting for 1/4 to 1/2 of the total cost of the UPS. According to the survey, the life of a normal-use battery is generally about 5 years, and about 50% of the UPS failures at the end of the use are related to the battery. The failure of the battery mainly manifests that the individual battery is backward or the battery float voltage is low, the backup time is short (the capacity is insufficient), and the UPS that needs the battery start cannot be started after the abnormality of the mains. In order to ensure the normal operation of the UPS system, it is necessary to establish a reasonable maintenance plan especially for the condition of the battery.

1 battery float voltage monitoring
(1) In the medium and large UPS, the monitor is generally equipped. By monitoring the upper and lower limits of the float voltage, the battery health status can be monitored at any time, and abnormalities are detected in time.
(2) Measure the float voltage of the battery with a multimeter.

Through the above method, refer to YD/T799-2002 “Valve-Controlled Sealed Lead-Acid Battery for Communication” standard, the floating charge voltage deviation of the battery in the floating state is not more than 480mV (12V battery), if the test voltage deviation is large, it needs to be considered After the average charging mode is observed and tested, if it has not changed after being converted into floating charge, it is necessary to consider the following methods for testing and verification.

2 battery capacity test
In general, when performing periodic capacity tests on batteries, the following capacity test methods can be selected.
2.1 Off-line measurement (if conditions permit)

(1) After the battery pack is fully charged, it is left out of the UPS for 1 h, and the external (smart) dummy load is used under the condition of an ambient temperature of 25 ° C ± 5 ° C, and a discharge test is performed using a discharge rate of 10 hours.

(2) The terminal voltage, ambient temperature and time of the battery should be measured before the discharge begins.
(3) Record the terminal voltage, discharge current and indoor temperature of the battery during discharge. The measurement time interval is 1h, and the discharge current fluctuation shall not exceed 1% of the specified value.

(4) Record the terminal voltage and room temperature of the battery during discharge, and the measurement interval is 1h. It should be measured at the end of the discharge period to accurately determine the time to reach the discharge termination voltage.

(5) The discharge current multiplied by the discharge time is the capacity of the battery pack. When the battery is discharged at a rate of 10 hours, if the temperature is not 25 ° C, the actual measured capacity should be converted into the capacity Ce at 25 ° C according to the following formula:
Ce=Cr/[1+K(t-25°C)] (1)

Where: t—the ambient temperature at the time of discharge
K—temperature coefficient (when discharged at 10 hours rate, K=0.006/°C;
When the 3-hour rate is discharged, K=0.008/°C; when discharging at 1 hour rate, K=0.01/°C)

(6)After the discharge is completed, the battery pack should be charged, and the charge amount is 1.2 times or more of the discharge amount.

2.2 Online measurement
(1) In the DC power supply system, adjust the UPS output voltage to the protection voltage, and the battery supplies power to the actual load. In the discharge, find a battery with the lowest voltage and the worst capacity in the battery pack as the capacity test object.

(2)Turn on the UPS to charge the battery pack, and stabilize the battery pack for more than 1h after it is fully charged.

Perform a 10-hour rate discharge test on the battery that was found to be the worst when discharging in (1). The terminal voltage, temperature, discharge time and room temperature of the battery were measured and recorded before and after discharge. Recording is measured every 1 hour (3)thereafter. When the discharge is fast to the end voltage, the record should be measured at any time to accurately record the discharge time.

(4)The discharge current multiplied by the discharge time is the capacity of the battery pack. If the room temperature is not 25 ° C, it should be converted to the capacity at 25 ° C according to the formula (1).

(5)After the discharge test is completed, the battery is recharged with a charger to restore its capacity.

(6) Draw a discharge curve based on the measured record data.

2.3 Checking discharge test method
In order to be able to grasp the approximate capacity of the battery pack at any time, it is necessary to conduct a verification discharge test by:

(1)In the DC power supply system, adjust the UPS output voltage to the protection voltage, and the battery supplies power to the actual load. Before and after the battery pack is discharged, it is necessary to measure and record the terminal voltage, temperature, room temperature and discharge time of each battery. Release 30% to 40% of the rated capacity.

(2) After the discharge is completed, the battery should be charged, and the charge amount is 1.2 times or more of the discharge amount.

(3) Draw a discharge curve based on the data recorded by the measurement, and keep it for comparison when it is measured again.

(1) For the battery pack of the UPS power supply system, offline measurement is not recommended for capacity testing.
(2) When performing the online measurement method and the verification capacity test, for the UPS equipment that has the battery discharge test function, it is necessary to turn on the battery discharge detection function to perform the discharge test on the battery. For UPS without this function, it is necessary to turn off the AC input power and perform the discharge test.

2.4 Precautions
(1) It is very important to ensure the operation of the system during the capacity test. Therefore, when doing capacity test, you should know in advance whether there is a planned power outage of the mains, and the standby generator set should be in good condition.

(2)Before performing the battery capacity discharge test, a multimeter, an internal resistance meter, and a conductivity meter are used to perform a preventive test on the performance of the battery.

(3) In order to ensure the accuracy of the capacity test, the professional battery capacity online test instrument and dummy load should be used for testing.

3 battery start instant output high current test

In the actual use process, the switching time requirement of the backup UPS from the mains supply to the inverter is less than 7ms, and the general design is 4~5ms. When the mains supply is abnormal, the battery must output the current required by the load in less than 4 to 5 ms. If there is a battery in the battery pack that fails, it may meet the requirements of the above terminal voltage and capacity, but it is unqualified when discharging at a high current. This situation is a hidden danger and the battery is in an unqualified state. Since the characteristics of the battery outputting a large current instantaneously can only be tested when the utility power is turned off, it is risky to test without knowing the performance of the battery, and such detection is generally not recommended.

4 Conclusion
It is recommended that the UPS battery be developed during the use of the quarterly and annual maintenance plan. When the battery usage time exceeds the manufacturer’s specified warranty period, it is necessary to pay attention to and increase the frequency of maintenance to ensure safe operation.

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