Do you understand the characteristics of parallel circuits?
November 21, 2019
For a long time, both domestic and foreign, whether it is a communication system or a UPS system, people are accustomed to use two sets of batteries in parallel to use with a UPS or a communication device. I don't know if it is because of the habitual force or for other reasons. This parallel use has become a principle that designers and users must follow, but the author believes that it is not necessary, as long as the user can follow the battery. The manufacturer's instruction manual is good for battery maintenance. It is enough to use only one set of batteries, not only enough, but also the effect of this group of batteries (such as battery stability, reliability, balance, especially battery The service life, etc.) is much better than when two sets of batteries are used in parallel. This is especially true for valve-regulated sealed lead-acid batteries. So, why is the author's positive proposition (or even disapproval) not to use the battery packs in parallel, and what are the pros and cons of using them in parallel?
First let's review the characteristics of the parallel circuit. In a parallel circuit, the total voltage is equal to the shunt voltage. That is to say, the charging voltage applied to each of the two groups of batteries connected in parallel is equal to the total charging voltage, that is, U total = U1 = U2. According to the formula of I=U/R, it can be known through calculation that I1≠I2 (because the internal resistance of the two sets of batteries is definitely not the same, that is, R1≠R2, in the case of U1=U2, I1 is definitely obtained. ≠I2 results). That is to say, in the case of the same size of the charging voltage, the battery packs used in parallel between the two groups have different charging currents for each group, and the charging current is small, the internal resistance is small, and the internal resistance is small. The current is large. In this way, it is possible that the battery packs with a small charging current are often in a state of insufficient charging. Over time, the battery may be more sulfated due to long-term loss of power, and the internal resistance is increased, and the charging current is further increased. Small, due to such a vicious circle, the life of this battery is greatly shortened. This is not the case with just a set of batteries. This point is enough to show that the battery pack single-use use is far better than parallel use. Therefore, the author suggests that users should not use two sets of batteries in parallel when they can meet the needs of the equipment with a set of batteries. Otherwise, the battery life will be shortened, the cost of use will be increased, and the overall performance of the battery will be reduced. This kind of labor and money should not be done. If the power of the device is large, if the two groups of batteries are still not connected in parallel to meet the power requirements of the device, and more than two groups, such as three groups, four groups, or even more groups of batteries are used in parallel, it is even more unnecessary. The parallel use of two sets of batteries has brought many disadvantages. The parallel use of more battery packs is more complicated and more disadvantageous. In this case, it is necessary to use a large-capacity battery that can meet the power requirements of the equipment. If there is no large-capacity specification in the 12V series battery, you can use 2V series batteries, 2V series batteries, various large capacity. There are, you can say how big you can make it. As far as I know, the current 2V series batteries in China can reach 6000Ah.
Of course, it is understandable that the designer and the user can improve the reliability of the standby power supply. In the event of an AC power failure, when one of the two sets of batteries cannot be powered, another battery can be secured. Even dry? ? ? It is also worthwhile to pay for the work of the people. If we consider the parallel use of the battery pack from this point of view, the author only agrees to use up to two sets of batteries in parallel. If more than two groups are connected in parallel, it is absolutely harmful. If you are not using two sets of batteries in parallel, please also follow the following principles: First, the batteries used in parallel must be produced by the same manufacturer, and the same type, the same size of the battery; the second is used in parallel The battery must be in the same state of the old and new; the third is that the same batch number is shipped at the same time; the fourth is that it is installed at the same time.
The lead-acid battery is a positive electrode, liquid mass transfer limited water electrochemical system. This system will generate gas (hydrogen evolution, oxygen evolution) during operation, resulting in water loss. Therefore, maintenance of the water replenishment is required.
Maintenance-free (meaning no need to add water and rehydration) is the most simple instinct requirement of people. In the process of achieving maintenance-free lead-acid battery, it has gone through a long and tortuous road, including the use of catalytic hydrogen elimination and auxiliary electrodes. .
4, use the power supply peak charging
For users who have long-term UPS power supply with low-voltage power supply or frequent power failure, in order to prevent premature damage of the battery due to insufficient long-term charging, the battery should be fully charged (such as late-night time) to charge the battery to ensure that the battery is discharged every time. There is enough charging time after that. After the battery is deeply discharged, it takes at least 10 to 12 hours to recharge to 90% of the rated capacity.
5, pay attention to the choice of charger
Maintenance-free sealed batteries for UPS power supplies cannot be charged with a thyristor-type quick charger. This is because such a charger can cause the battery to be in a poor charging state with both instantaneous overcurrent charging and instantaneous overvoltage charging. This state will greatly reduce the usable capacity of the battery, and in the case of serious, the battery will be scrapped. When using the UPS power supply of the constant voltage cut-off charging circuit, be careful not to set the battery voltage too low to protect the operating point too low. Otherwise, it will easily generate overcurrent charging at the beginning of charging. Of course, it is best to charge the charger with both constant current and constant voltage.
6, to ensure the power supply ambient temperature
The capacity available for the battery is closely related to the ambient temperature. Under normal circumstances, the performance parameters of the battery are calibrated at room temperature of 20 ° C. When the temperature is lower than 20 ° C, the available capacity of the storage will be reduced, and when the temperature is higher than 20 ° C, it is available. The capacity used will increase slightly. Different types of batteries from different manufacturers are affected by temperature. According to statistics, at -20 °C, the available capacity of the battery can only reach about 60% of the nominal capacity. It can be seen that the influence of temperature cannot be ignored.
Of course, to extend the life of the battery pack should not only pay attention to maintenance and use, but also should consider the load characteristics (resistance, inductance, capacitance) and size when selecting. Do not leave the battery in an excessively light load for a long time, so as to prevent the battery from being discharged due to the battery discharge current being too small.
There are usually two ways.
The first method is to estimate the internal resistance of the battery by measuring the instantaneous short-circuit current of the battery to determine whether the battery is sufficient. The second method is to use a current meter in series with a resistor with a suitable resistance to calculate the battery by measuring the discharge current of the battery. Internal resistance to determine whether the battery is fully charged.
The biggest advantage of the first method is that it is simple. The large current file of the multimeter can directly determine the power of the dry battery. The disadvantage is that the test current is very large, far exceeding the limit value of the allowable discharge current of the dry battery, which affects the use of the dry battery to a certain extent. life. The advantage of the second method is that the test current is small, the safety is good, and generally does not adversely affect the service life of the dry battery, and the disadvantage is that it is troublesome.
The author used the MF47 multimeter to test and compare a new No. 2 dry battery and an old No. 2 dry battery by the above two methods. Suppose ro is the internal resistance of the dry battery, RO is the internal resistance of the ammeter. When using the second test method, RF is an additional series resistance with a resistance of 3 ohms and a power of 2W.
The measured results are as follows. The new No. 2 battery E=1.58V (measured with 2.5V DC voltage), the internal resistance of the voltmeter is 50k ohm, which is much larger than ro, so it can be approximated that 1.58V is the electromotive force of the battery, or open circuit voltage. When using the first method, the multimeter is set to 5A DC current, the internal resistance of the meter is RO=0.06 ohms, and the measured current is 3.3A. So ro+RO=1.58V÷3.3A≈0.48 ohms, ro=0.48-0.06=0.42 ohms. With the second method, the measured current is 0.395A, RF+ro+RO=1.58V÷0.395A=4 ohms, and the current 500mA internal resistance is 0.6 ohms, so ro=4-3-0.6=0.4 ohms.
When the old No. 2 battery was measured by the first method, the open circuit voltage E=1.2V was measured first, the internal resistance of the meter was RO=6 ohm, the reading was 6.5 mA, and the multimeter was set to 50 mA DC current file, ro+RO=1.2V÷0.0065 A ≈ 184.6 ohms, ro = 184.6-6 = 178.6 ohms. Using the second method, the measured current was 6.3 mA, ro + RO + RF = 1.2 V ÷ 0.0063 A = 190.5 ohms, and ro = 190.5-6-3 = 181.5 ohms.
Obviously the results of the two test methods are basically the same. The slight difference in the final calculation results is caused by many factors such as reading error, resistance RF error and contact resistance. This small error does not affect the judgment of battery power. If the capacity of the battery under test is small and the voltage is high, the resistance of the RF should be adjusted to increase.