Analysis of the working principle of lithium battery balancing circuit -Lithium - Ion Battery Equipment
The development of new energy and electric vehicles will use lithium-ion batteries with relatively high energy density. When lithium-ion batteries are used in series, in order to ensure the consistency of battery voltage, a voltage balancing circuit is inevitably used. Today I would like to share with you some battery balancing circuits that I have used in my work. I hope it will be helpful to you.
The simplest balancing circuit is load consumption balancing, that is, a resistor is connected in parallel to each battery and a switch is connected in series for control. When the voltage of a certain battery is too high, the switch is turned on and the charging current is shunted through the resistor. In this way, the charging current of the battery with high voltage is small, and the charging current of the battery with low voltage is large. In this way, the battery voltage is balanced.
However, this method can only be applied to small-capacity batteries and is not practical for large-capacity batteries.
The second equalization method I have not tried is the flying capacitor method. To put it simply, each battery is connected in parallel with a capacitor. By switching this capacitor, it can be connected in parallel to its own battery or to an adjacent battery.
When the voltage of a certain battery is too high, first connect the capacitor in parallel with the battery so that the capacitor voltage is consistent with the battery, then switch the capacitor to the adjacent battery and the capacitor discharges the battery. Achieve energy transfer.
Since capacitors do not consume energy, energy can be transferred losslessly. But this method is too cumbersome. Currently, dozens of power lithium-ion batteries are often connected in series. If this method is used, a lot of switches are needed to control it.
The first time I did balancing was to charge a power lithium-ion battery pack. Two groups of 80ah battery cells were connected in parallel, and the balancing current was required to be 10a. The balance principle I originally understood was not enough. Such a large current is equivalent to one small module. In the end, n small modules were connected in series, and each battery was connected in parallel with a small module. If the single battery voltage is low At the set value, start the corresponding parallel module to start charging the low-voltage battery, supplement energy to increase the voltage, and achieve balance.
The active equalization method can use the multi-channel output method of a transformer I mentioned earlier.
If you want to use the circuit diagram below to make a multi-output flyback power supply and use the output voltage of each module to balance the battery, I estimate that you need a lot of skills to do it, because the interleaving adjustment rate alone is It's very difficult. However, using this circuit, we can change our thinking and do not need to regulate the voltage of each output. Of course, in order to prevent open circuit damage to the output capacitor, we can make a simple primary feedback. Then, an electronic switch is connected in series between each output and the battery. Since this balance works together with the battery management system, each output only needs to be connected in series with an electronic switch and controlled by the management unit. Which voltage is the ground? This electronic switch can be turned on, and the power output will charge the battery until the voltage of all single cells reaches our expected value.(Lithium - Ion Battery Equipment)
Using this balancing method, we have done balancing of 1000AH, 7-string batteries and 300AH, 80-string batteries. After the balancing is completed, the voltage of all single cells can reach within 5mV.
Active equalization can also use energy transfer. The so-called energy transfer can either take energy from the entire set of voltages to supplement the low voltage, or it can take energy from a battery with too high voltage and feed it back to the entire set of voltages.
I have used the second method to achieve battery balancing in a communication power supply system.
What was done at that time was the balancing of 16 strings of lithium-ion batteries, divided into two groups, each group of 8 batteries connected in series. Only 6 are drawn here to describe the working principle.
If the voltage of battery b5 is too high, Q5 is controlled to work in pWM mode. When Q5 is turned on, the inductor L5 stores energy; when Q5 is turned off, the energy stored in the inductor will charge batteries b1-b4 through D5, reducing the battery voltage of b5 and raising the remaining batteries. Voltage, the same principle can be used to decompose the working process of other battery packs when the voltage is too high.
The simplest balancing circuit is load consumption balancing, that is, a resistor is connected in parallel to each battery and a switch is connected in series for control. When the voltage of a certain battery is too high, the switch is turned on and the charging current is shunted through the resistor. In this way, the charging current of the battery with high voltage is small, and the charging current of the battery with low voltage is large. In this way, the battery voltage is balanced.
However, this method can only be applied to small-capacity batteries and is not practical for large-capacity batteries.
The second equalization method I have not tried is the flying capacitor method. To put it simply, each battery is connected in parallel with a capacitor. By switching this capacitor, it can be connected in parallel to its own battery or to an adjacent battery.
When the voltage of a certain battery is too high, first connect the capacitor in parallel with the battery so that the capacitor voltage is consistent with the battery, then switch the capacitor to the adjacent battery and the capacitor discharges the battery. Achieve energy transfer.
Since capacitors do not consume energy, energy can be transferred losslessly. But this method is too cumbersome. Currently, dozens of power lithium-ion batteries are often connected in series. If this method is used, a lot of switches are needed to control it.
The first time I did balancing was to charge a power lithium-ion battery pack. Two groups of 80ah battery cells were connected in parallel, and the balancing current was required to be 10a. The balance principle I originally understood was not enough. Such a large current is equivalent to one small module. In the end, n small modules were connected in series, and each battery was connected in parallel with a small module. If the single battery voltage is low At the set value, start the corresponding parallel module to start charging the low-voltage battery, supplement energy to increase the voltage, and achieve balance.
The active equalization method can use the multi-channel output method of a transformer I mentioned earlier.
If you want to use the circuit diagram below to make a multi-output flyback power supply and use the output voltage of each module to balance the battery, I estimate that you need a lot of skills to do it, because the interleaving adjustment rate alone is It's very difficult. However, using this circuit, we can change our thinking and do not need to regulate the voltage of each output. Of course, in order to prevent open circuit damage to the output capacitor, we can make a simple primary feedback. Then, an electronic switch is connected in series between each output and the battery. Since this balance works together with the battery management system, each output only needs to be connected in series with an electronic switch and controlled by the management unit. Which voltage is the ground? This electronic switch can be turned on, and the power output will charge the battery until the voltage of all single cells reaches our expected value.(Lithium - Ion Battery Equipment)
Using this balancing method, we have done balancing of 1000AH, 7-string batteries and 300AH, 80-string batteries. After the balancing is completed, the voltage of all single cells can reach within 5mV.
Active equalization can also use energy transfer. The so-called energy transfer can either take energy from the entire set of voltages to supplement the low voltage, or it can take energy from a battery with too high voltage and feed it back to the entire set of voltages.
I have used the second method to achieve battery balancing in a communication power supply system.
What was done at that time was the balancing of 16 strings of lithium-ion batteries, divided into two groups, each group of 8 batteries connected in series. Only 6 are drawn here to describe the working principle.
If the voltage of battery b5 is too high, Q5 is controlled to work in pWM mode. When Q5 is turned on, the inductor L5 stores energy; when Q5 is turned off, the energy stored in the inductor will charge batteries b1-b4 through D5, reducing the battery voltage of b5 and raising the remaining batteries. Voltage, the same principle can be used to decompose the working process of other battery packs when the voltage is too high.
