Analysis of the working principle of lithium battery equalization circuit

The simplest balance circuit is load consumption balance, 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 high-voltage battery is small, and the charging current of the low-voltage battery is large. In this way, the battery voltage is balanced. But this method can only be applied to small-capacity batteries, which is impractical for large-capacity batteries. The second balance method I haven't tried is the flying capacitor method. To put it simply, each battery has a capacitor in parallel. This capacitor can be connected in parallel to its own battery or adjacent batteries by switching. When the voltage of a certain battery is too high, first connect the capacitor in parallel with the battery, the capacitor voltage is consistent with the battery, and then the capacitor is switched to the adjacent battery, and the capacitor discharges the battery. Realize the transfer of energy. Since capacitors do not consume energy, lossless energy transfer can be achieved. But this method is too cumbersome. At present, there are dozens of power lithium-ion batteries connected in series. If this method is adopted, many switches are required to control. The first time to balance is to charge a power lithium-ion battery pack. Two sets of batteries with a battery capacity of 80ah are connected in parallel, and the balance current is required to be 10a. It turns out that the principle of a little equalization is not enough. Such a large current is equivalent to a small module. In the end, n small modules are used in series, and each battery is connected in parallel. If the voltage of the single battery is low At the set value, start the corresponding parallel module, start charging the low-voltage battery, supplement the energy to increase the voltage, and achieve equilibrium. The active equalization method can use the multiple output method of a transformer that I mentioned earlier. If you want to use the following circuit diagram to make a multi-output flyback power supply, and use the output voltage of each module to balance the battery, I guess you need a lot of skill, because you only need to interweave the adjustment rate. It's hard. However, using this circuit, we can change our mindset, and each output should not be regulated. Of course, in order to guard against open circuit damage to the output capacitor, we can do a simple primary-side feedback. Then connect an electronic switch in series between each output and the battery. Since this equalization works with the battery management system, each output only needs to be connected in series with an electronic switch, which can be controlled by the management unit. This electronic switch can be turned on, and there is a power output to charge the battery until the voltage of all single cells reaches our expected value. Using this equalization method, the balance of 1000AH, 7 series batteries and 300AH, 80 series batteries has been done. After the equalization is completed, the voltage of all single cells can reach within 5mV. Active balancing 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 take energy from the over-voltage battery to feed back the entire set of voltages. I have used the second method to achieve battery balance in a communication power supply system. The schematic diagram of the circuit is as follows: What was done at the time was the equalization of 16 strings of lithium batteries, which were divided into two groups with 8 batteries in series in each group. Only 6 of them are drawn here to describe the working principle. If the voltage of battery B5 is too high, control Q5 to work in PWM mode. When Q5 is turned on, the inductor L5 stores energy; when Q5 is closed, 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 the remaining battery packs when the voltage is too high. During the experiment, the two groups were balanced in this way. When there is a deviation between the two groups, two-way DC-DC can be used for energy conversion, so that the number of modules used is small and the design is more convenient. I did not use two-way DC-DC, but simply used energy consumption to balance the batteries between the two groups. Judging from the final experimental results, the battery balance is quite good. In the equalization process, if supplying one charging module for each battery feels like a killer, the energy consumption type does not meet the technical requirements, that is, active equalization, then the above-mentioned method of transformer with multiple outputs, Maybe it's more suitable for your needs, just use a suitable transformer and do a multi-output flyback power supply with primary side feedback and current limiting. In fact, with the development of the use of power lithium-ion batteries, not only balance, but also the protection of battery overcharge and overdischarge, that is, the use of the protection board we often say will become more and more extensive. We know that the original 18650 batteries, more than a dozen series of protection boards with IC is very common, to achieve short circuit, overcharge protection, over discharge protection. But if it's dozens of batteries, I don't know if any netizens who have been in contact with this information can share it together. This is the four battery balancing methods that I have experimented so far. The balanced battery ranges from 2AH to 1000AH, and the number of cells in series ranges from 7 strings to 120 strings. Disclaimer: Some pictures and content of articles published on this site are from the Internet. If there is any infringement, please contact to delete. Previous article: Modeling decomposition of lithium battery injection and infiltration process