First, the main technical indicators of UPS batteries
Among the indicators for measuring UPS batteries, the rated voltage and rated capacity of the battery are the two most commonly used technical indicators. For example, the Japanese Yuasa NP6-12 battery has a rated voltage of 12V and a rated capacity of 6Ah/20h; the German Sunshine A406/165 battery has a rated voltage of 6V and a rated capacity of 165Ah/20h.
The capacity of the battery refers to the amount of power that is output when the fully charged battery is discharged to the end voltage. In the case of constant current discharge, the capacity Q = It? where Q - the amount of electricity discharged by the battery, Ah;
I——discharge current, A; ?
T——discharge time, h.
The terminating voltage refers to the voltage at which the battery cannot operate normally when the battery is lower than the specified voltage. In other words, the battery continues to be discharged below the termination voltage, which may cause permanent damage to the battery. The rated capacity or nominal capacity of the battery is indicated by the letter C. For example, a battery with a rated capacity of 6 Ah, C = 6 Ah; a battery with a rated capacity of 24 Ah, C = 24 Ah.
The concept of capacity is essentially the representation of battery energy conversion. For example, considering the fact that the terminal voltage E=12V of the battery remains almost constant in actual use and the output energy expression W(t)=IVt=IEt, 6Ah can be understood as NP6-12 from the viewpoint of energy effect. The type of battery releases energy when the voltage at the holding terminal is constant. If discharged at a current of 6 A, it can be discharged for 1 h or discharged with a current of 1 A for 6 h.
Second, the discharge system and discharge rate
When studying batteries, a uniform discharge time is often specified, called discharge. The discharge current can be determined from the rated capacity using the given discharge system. Discharge current (A) = rated capacity of the battery (Ah) / discharge time (h), in order to compare the batteries with different capacities, the discharge current is not expressed in absolute value (amperes), but the rated capacity C and discharge time It is called a discharge rate or a discharge rate. The discharge rate of the 20h system is C/20=0.05C, and the unit is A.
Therefore, the capacity index 6Ah of the above NP6-12 type battery is measured at a discharge rate of 20 hours, that is, a discharge rate of 0.05C. For an NP6-12 battery, 0.05C is equal to a current of 0.3A.
Third, test UPS battery
The purpose of testing the UPS battery is to determine if the battery meets the UPS power requirements. This is necessary when replacing the UPS battery and determining if the original UPS battery has failed.
In the actual maintenance of the UPS, the general UPS power supply requirements for the battery: meet the original terminal voltage of the battery; the battery should have the characteristics of outputting a large current at the moment of starting the discharge; meet a certain capacity and internal resistance to ensure the inverter power supply time.
From the above requirements of the UPS power supply battery, it is impossible to determine whether the battery is good or bad by measuring the terminal voltage of the UPS battery.
1. Measure the terminal voltage of the UPS battery
(1) Offline measurement of the terminal voltage of the battery
Measuring the terminal voltage of the battery offline means that the battery is directly measured by the DC voltage file or voltmeter of the multimeter when the battery is disconnected from the original connection line. The measured battery terminal voltage is about 12V, and the minimum cannot be lower than 10.5V. A battery that is less than 10.5V is a battery that is under voltage or may have failed. If the battery is still less than 12V after charging or activating the charging back-end voltage, it is a failed battery.
(2) Online measurement of the terminal voltage of the battery
Measuring the terminal voltage of the battery online means measuring the voltage across the battery using the multimeter's DC voltage or voltmeter while the UPS is operating. The UPS of the mains supply state, because the battery is in a state of charge, the terminal voltage is greater than 12V. When the terminal voltage of the battery drops to 10.5V, the normal UPS power supply will start the battery undervoltage automatic protection circuit inside the machine, so that the UPS enters the protection state without neither the mains supply nor the inverter supply.
2. Test whether the UPS battery has the characteristics of starting a large current at the moment of starting
The switching time requirement of the backup UPS power supply from the mains supply to the inverter supply is less than 7ms, and the general design is about 4~5ms. That is to say, once the mains supply is interrupted, the UPS battery must output the current required by the load in less than 4 to 5 ms. Some failed batteries can meet the requirements of terminal voltage and capacity, but can not meet the requirements of large current in less than 4~5ms. The UPS fault caused by this battery is as follows: the UPS can successfully switch the inverter when it is no-load or light load, and the inverter fails when it is increased to the normal load.
3. Determine the internal resistance and capacity of the UPS battery
The internal resistance of a good quality UPS battery is about 20 to 30 mΩ. When the internal resistance exceeds 80 mΩ, it is necessary to perform equalization charging treatment or activation treatment on the battery. The increase in the internal resistance of the battery is inevitably accompanied by a decrease in the actual output energy, thereby manifesting a decrease in the capacity of the battery. In addition, there are other factors that cause the battery capacity to decrease, such as electrolyte loss.
Test whether the internal resistance of the battery is increased, and never use the resistance of the multimeter to measure directly. The method of indirect measurement calculation should be used. In actual maintenance, the following simple method can be used to determine whether the internal resistance of the battery is increased:
Use a good battery and a battery with an increased internal resistance to perform a series charging experiment (such as two 12V batteries in series in a 500VA UPS). During the charging process, the terminal voltages of the two batteries are simultaneously measured, and the battery with the increased internal resistance obtains a higher charging voltage than the good battery, and the difference in the charging voltage reflects the difference in internal resistance.
If the battery is only insufficient in capacity, the main performance is that the time for the UPS reversible power supply is shortened, and the load capacity of the UPS, the switching between the mains supply and the inverter power supply are not affected.
Fourth, the scientific use of UPS batteries
Scientific use of UPS batteries is to clarify the correct use of the battery, extend the life of the battery, and make the most of its role.
1. Control the charging voltage to prevent overvoltage charging
For a battery with a terminal voltage of 12V, the normal float voltage is between 13.5 and 13.8V. The float voltage is too low, the battery charge is not full, and the float voltage is too high, which will cause overvoltage charging. When the float voltage exceeds 14V, it is considered to be overvoltage charging. Overpressure charging causes water in the electrolyte to be separated into hydrogen and oxygen to overflow, shortening the life of the battery.
2. Control the charging current to prevent overcurrent charging
The ideal charging current should adopt the method of staged constant current charging, that is, use a large current in the initial stage of charging. After charging for a certain period of time, change to a smaller current, and switch to a smaller current at the end of charging. The design of the charging current is generally 0.1C, which can be considered as overcurrent charging when the charging current exceeds 0.3C. Over-current charging can cause the battery plates to bend and the active material to fall off, causing damage to the battery. ?
3. Prevent UPS battery over-current discharge
The actual discharge capacity of the battery is related to the discharge current. The larger the discharge current, the lower the efficiency of the battery. For example, when the discharge current is 0.4C, the discharge to the termination voltage is 1 hour and 50 minutes, the actual output capacity is 17.6 Ah, and the efficiency is 73.3%. When the discharge current is 7C, the time to discharge to the termination voltage is only 20s, the actual output capacity is 0.93Ah, and the efficiency is 3.9%. Therefore, large current discharge should be avoided to improve the efficiency of the battery. General circuit design and user selection of load, to protect the UPS battery inverter discharge current does not exceed 2C.
4. Prevent UPS battery deep discharge Although the small current discharge can improve the efficiency of the battery, when it is discharged for a long time with a very small current (less than 0.05C), the actual discharge capacity of the battery will exceed its rated capacity, resulting in serious battery. Deep discharge. According to the manufacturer's data, when the battery discharge depth is 100%, the actual battery life is about 200 to 250 charge and discharge cycles; when the discharge depth is 50%, it is about 500 to 600 charge and discharge cycles. Therefore, when using the UPS, it is necessary to avoid heavy-duty over-current discharge, and to avoid deep discharge of the battery caused by long-time light-load inverter.
5. Regular operation of UPS
In areas where the mains supply is not stopped for a long time, the user must turn off the UPS AC input at regular intervals, for example, 3 months, and use the UPS battery inverter to supply power. This regular experimental operation helps to extend battery life. A normally used UPS has a battery life of no more than 5 years.
Fifth, the skills and methods of maintaining UPS batteries
UPS batteries are generally maintenance-free batteries, but in some cases it is necessary to maintain the UPS battery, and it has practical significance.
1. Under-voltage battery charging tips
Some UPS battery undervoltage is caused by battery discharge due to damage to the final drive circuit of the UPS inverter. If the battery is connected to the original circuit in time after repairing the circuit failure, the battery will still be restored. The problem is that an undervoltage battery cannot make the UPS start up successfully, that is, switch to the mains (charge) state. At this point, the following solutions can be used:
(1) After starting the UPS to the mains state with a good battery, remove the battery and replace it with the under-voltage battery to be charged. Note: When replacing the battery, the UPS is required to run at no load. After the UPS enters the mains state, as long as the input mains supply is normal, removing the battery will not affect the mains supply status.
(2) Charge the under-voltage battery to above 10.5V and then connect to the original UPS circuit to start the UPS successfully. To charge the under-voltage battery, you can use the +12V power supply in the microcomputer power supply to directly charge the battery. Pay attention to the charging current during charging and determine whether to add a current limiting resistor based on the measured actual charging current.
2. Battery activation treatment
Activation treatment refers to equalization of the battery. The following conditions can cause the internal resistance of the battery to increase, the terminal voltage to be too low, or the capacity to decrease. These batteries need to be recharged to restore their original performance specifications.
(1) A battery that has been left unused for a long time and exceeds the static storage time. At normal temperature, the static storage time of a typical UPS battery is 9 months. When the temperature is 31 to 40 ° C, the static storage time is 5 months.
(2) A battery that failed to be charged in time after discharge.
(3) Long-term work in the floating state (that is, the UPS is working in the mains state for a long time) and exceeds the static storage time.
(4) Inadvertently discharge, put the battery terminal voltage below the termination voltage. The equalization charging current is generally selected to be 0.3 C or slightly less than 0.3 C. For batteries with a rated voltage of 12V, the equalization charging voltage is generally selected to be 14.5V. For users who do not have a dedicated charger, refer to the above data to lap up the circuit required to resurrect the old battery.
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