UPS battery occupies a very important position in UPS power supply equipment. At present, the maintenance-free sealed lead-acid battery widely used in small and medium-sized UPS power supplies accounts for as much as 1/4 to 1/2 of the total cost of UPS power supplies. Not only that, the actual maintenance also shows that more than 50% of UPS power failures are related to UPS batteries. Regardless of whether it is the cause or the result of a UPS failure, the failure of the UPS battery will be directly manifested as increased internal resistance, insufficient terminal voltage, insufficient capacity, or instantaneous discharge current that does not meet the requirements for on-load start-up. Therefore, when using and maintaining UPS power supplies, it is particularly important to correctly understand UPS batteries, use UPS batteries scientifically, and master the methods of testing and selecting UPS batteries (for the convenience of explaining the problem, UPS batteries are referred to as batteries for short.)
1. Main technical indicators of UPS battery
Among the indicators for measuring UPS batteries, the rated voltage and rated capacity of the battery are the two most commonly used technical indicators.
The capacity of the battery refers to the output power of a fully charged battery when it is discharged to the cut-off voltage. In the case of constant current discharge, the capacity Q=It?
In the formula, Q——the electric quantity released by the battery, Ah?
I—discharge current, A;
t—discharging time, h.
The so-called termination voltage refers to the voltage at which the battery cannot work normally when the battery is lower than the specified voltage. In other words, if the battery continues to be discharged when the voltage is lower than the cut-off voltage, the battery may be damaged.
The rated capacity or nominal capacity of the battery is represented by the letter C. For example, for a battery with a rated capacity of 6Ah, C=6Ah; for a battery with a rated capacity of 24Ah, C=24Ah.
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 during actual use and the output energy expression W(t)=IVt=IEt, therefore, from the perspective of energy effect, it can be understood that the battery is kept at the terminal The energy is released under the condition of constant voltage, if it is discharged with a current of 6A, it can be discharged for 1h or with a current of 1A for 6h.
2. Discharge system and discharge rate
When researching batteries, a uniform discharge time is often specified, which is called a discharge system. Use the given discharge capacity to find the discharge current through the rated capacity. Discharge current (A) = rated capacity of the battery (Ah) / discharge time (h)
In order to compare batteries with different capacities, the discharge current is not expressed by the current value (ampere), but by the ratio of the rated capacity C to the discharge time, which is called the discharge rate or 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-mentioned 12V6AH battery is measured at a discharge rate of 20h, that is, a discharge rate of 0.05C. For NP6-12 battery, 0.05Cis equal to 0.3A current.
3. Test the UPS battery
The purpose of testing the UPS battery is to determine whether the battery meets the requirements of the UPS power supply. This is necessary when replacing the UPS battery and determining whether the original UPS battery has failed.
In the actual maintenance of UPS, the general UPS power supply requirements for batteries: meet the terminal voltage of the original battery; the battery should have the characteristics of outputting a large current at the moment of starting and discharging; meet a certain capacity and internal resistance to ensure inverter power supply time.
It can be seen from the above requirements of the UPS power supply for the battery that it is impossible to determine whether the battery is good or bad simply by measuring the terminal voltage of the UPS battery.
3.1 Measure the terminal voltage of the UPS battery
(1) Measure the terminal voltage of the battery offline
Off-line measurement of the terminal voltage of the battery refers to the direct measurement of the voltage at both ends of the battery using the DC voltage range of a multimeter or a voltmeter when the battery is disconnected from the original connection line. The measured battery terminal voltage is about 12V, and the minimum value cannot be lower than 10.5V. A battery with less than 10.5V is undervoltage or may have failed. If this kind of battery is charged or the terminal voltage after charging still does not reach 12V, it is a failed battery
(2) Measure the terminal voltage of the battery online
Online measurement of battery terminal voltage refers to using the DC voltage range of a multimeter or a voltmeter to measure the voltage at both ends of the battery when the UPS power supply is working. For the UPS in the state of mains power supply, because the battery is in the charging state, 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 a protection state where there is neither mains power supply nor inverter power supply.
3.2 Test whether the UPS battery has the characteristics of outputting a large current at the moment of starting.
The switching time of the backup UPS power supply from the mains power supply to the inverter power supply is required to be less than 7ms, and the general design is about 4-5ms. That is to say, once the mains power supply is interrupted, the UPS battery must output the current required by the load in less than 4-5ms. Some failed batteries can meet the requirements of
terminal voltage and capacity, but the discharge current cannot reach the requirement of high current in less than 4-5ms. The UPS failure caused by this kind of battery is manifested as: the UPS can successfully switch the inverter when it is no-load or light-loaded, and the inverter fails when it is increased to a normal load.
Identify the internal resistance and capacity of the UPS battery
The internal resistance of a good-quality UPS battery is about 20-30mΩ. When the internal resistance exceeds 80mΩ, the battery needs to be balanced or activated. The increase of the internal resistance of the battery is bound to be accompanied by a decrease in the actual outputenergy, which is manifested as a decrease in the capacity of the battery. In addition, there are other factors that cause the capacity of the battery to decrease, such as the loss of electrolyte.
To test whether the internal resistance of the battery has increased, it is never possible to use the resistance gear of the multimeter to directly measure it. The method of indirect measurement and calculation should be used. During actual maintenance, the following simple methods can be used to determine whether the internal resistance of the battery has increased:
Use a good battery and a battery with suspected increased internal resistance to do a series charging experiment (for example, two 12V batteries are used in series in a 500VA UPS). During the charging process, the terminal voltage of the two batteries is measured and compared at the
same time. The charging voltage obtained by the battery with increased internal resistance is
higher than that of the good battery. The difference in charging voltage reflects the degree of
difference in internal resistance. the
If the capacity of the battery is only insufficient, the main performance is that the time for UPS
inverter power supply is shortened, while the load capacity of UPS, the switching between mains
power supply and inverter power supply, etc. will not be affected.
4. Scientific use of UPS batteries
The scientific use of UPS batteries is to clarify the correct use of the battery, prolong the life of the battery, and make it play its biggest role.
4.1 Control the charging voltage to prevent overvoltage charging
For a battery with a terminal voltage of 12V, the normal floating charge voltage is between 13.5 and 13.8V. If the floating charging voltage is too low, the battery will not be fully charged, and if the floating charging voltage is too high, it will cause overvoltage charging. When the floating charging voltage exceeds 14V, it is considered as overvoltage charging. Overvoltage charging will cause the water in the electrolyte to be separated into hydrogen and oxygen and overflow, shortening the life of the battery.
4.2 Control the charging current to prevent over-current charging
The ideal charging current should adopt a staged constant current charging method, that is, use a larger current at the beginning of charging, change to a smaller current after charging for a certain period of time, and use a smaller current at the end of charging. The charging current is generally designed to be 0.1C. When the charging current exceeds 0.3C, it can be considered as over-current charging. Over-current charging will cause the battery plate to bend, the active material will fall off, and the battery will be damaged.
4.3 Prevent UPS battery from over-current discharge
The actual released capacity of the battery is related to the discharge current. The higher the discharge current, the lower the efficiency of the battery. For example, when the discharge current of a 12V/24Ah battery is 0.4C, the discharge time to the cut-off voltage is 1 hour and 50 minutes, the actual output capacity is 17.6Ah, and the efficiency is 73.3%. When the discharge current is 7C, the time to discharge to the cut-off 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 loads must protect the UPS battery inverter discharge current from exceeding 2C.
4.4 Prevent deep discharge of UPS battery
Although discharging with a small current can improve the efficiency of the battery, when it is discharged with a small current (less than 0.05C) for a long time, the actual discharge capacity of the battery will exceed its rated capacity, resulting in serious deep discharge of the battery. According to the manufacturer's data, when the battery discharge depth is 100%, the actual
service life of the battery is about 200-250 charge-discharge cycles; when the discharge depth is 50%, it is about 500-600 charge-discharge cycles. Therefore, when using UPS, it is necessary to avoid heavy-load over-current discharge, and to avoid deep discharge of the battery caused by long-term light-load inverter.
4.5 Operate the UPS regularly
In areas where the mains power does not stop for a long time, the user must manually shut down the UPS AC input at regular intervals, such as 3 months, and use the UPS battery to invert the power supply. This regular experimental operation helps to extend battery life. Generally, the battery life of UPS in normal use does not exceed 5 years.
5. Skills and methods of maintaining UPS batteries
UPS batteries are generally maintenance-free batteries, but in some cases it is necessary to maintain UPS batteries and has practical significance.
5.1 Charging skills for undervoltage batteries
Some UPS battery undervoltage is caused by battery discharge due to damage to the drive circuit at the end of the UPS inverter. If the battery is connected to the original circuit for charging in time after the circuit failure is repaired, the battery will still be restored to its original state. The problem is that an undervoltage battery cannot make the UPS start successfully, that is, switch to the mains (charging) state. At this point, the following solutions can be used:
(1) Use a good battery to start the UPS to the mains state, then remove the good battery and replace it with an undervoltage battery to be charged. Note: When replacing the battery, the UPS is required to run without load. Generally, after the UPS enters the state of mains power, as long as the mains input remains normal, removing the battery will not affect the state of the mains power supply.
(2) Charge the undervoltage battery to above 10.5V first, and then connect it to the original UPS circuit, so that the UPS can be started successfully. To charge the undervoltage battery, the +12V power supply in the microcomputer power supply can be used to charge the battery directly. Pay attention to observe the charging current during charging, and determine whether to add a current-limiting resistor according to the actual measured charging current.
5.2 Activation treatment of battery
Activation treatment refers to the equalization charging of the battery. The following situations will lead to increased internal resistance of the battery, too low terminal voltage or reduced capacity. These batteries need to be charged to restore their original performance indicators.
(1) Batteries that have been left unused for a long time and have exceeded the static storage time.
In normal temperature environment, the static storage time of general UPS battery is 9 months. When the temperature is 31-40°C, the static storage time is 5 months.
(2) Batteries that cannot be charged in time after discharge.
(3) Long-term work in the floating charge state (that is, the UPS works in the mains state for a long time) and exceeds the static storage time.
(4) Accidentally discharge, put the battery terminal voltage below the cut-off voltage. Theequalizing charging current is generally chosen to be 0.3C or slightly less than 0.3C. For a battery with a rated voltage of 12V, the balanced charging voltage is generally 14.5V. Users who do not have a dedicated charger can refer to the above data to build the circuit needed to revive the old battery.
