Analysis of the working principle of 102450 battery balancing circuit

The development of new energy and electric vehicles will use lithium-ion batteries with relatively high energy density. In the process of using lithium-ion batteries in series, in order to ensure the consistency of battery voltage, a voltage balancing circuit will inevitably be used. Today, I will share with you that I have used several battery balancing circuits in my work, and I hope it will be helpful to everyone.

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 battery is too high, turn on the switch, 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 it is not practical for large-capacity batteries.

Schematic diagram of load consumption balancing

The second balancing method I have not tried is the flying capacitor method. Simply put, each battery is connected in parallel with a capacitor, and this capacitor can be connected in parallel to the battery itself or to the adjacent battery through the switch.

When the voltage of a battery is too high, first connect the capacitor in parallel with the battery, and the capacitor voltage is consistent with the battery, then switch the capacitor to the adjacent battery, and the capacitor discharges the battery. Energy transfer is achieved.

Since the capacitor does not consume energy, lossless energy transfer can be achieved. But this method is too cumbersome. Today's power lithium-ion batteries are often connected in series with dozens of batteries. If this method is used, many switches are required to control it.

The working principle diagram of the flying capacitor method only draws the balancing principle diagram of two adjacent batteries

The first time I did balancing, I did a charging of a power lithium-ion battery pack. Two groups of 80ah battery capacity were connected in parallel, and the balancing current was required to be 10a. The principle of balancing that I knew a little about was not enough. Such a large current is equivalent to a small module one by one. In the end, n small modules were really used in series, and each battery was connected in parallel with a small module. If the voltage of the single battery is lower than the set value, the corresponding parallel module is started to start charging the low-voltage battery, replenish energy and increase the voltage to achieve balancing.

The following figure is a schematic diagram of the balancing circuit used at that time. The DC-DC input bus can be either the battery voltage or the DC input supplied by other modules, and it can be flexibly configured according to the requirements.

The active balancing method can adopt the method of transformer multi-channel output mentioned earlier.

If you want to use the circuit diagram below to make a multi-channel output flyback power supply and use the output voltage of each module to balance the battery, I guess you need a deep skill to do it, because the interleaving adjustment rate alone is difficult. However, using this circuit, we can change our thinking. Each output does not need voltage regulation. Of course, in order to prevent open circuit damage to the output capacitor, we can do a simple primary feedback. Then connect an electronic switch in series between each output and the battery. Since this balancing works with the battery management system, each output only needs to be connected in series with an electronic switch, which is controlled by the management unit. We can turn on this electronic switch at any voltage, and there is power output to charge the battery until the voltage of all single cells reaches our expected value.

Using this balancing method, I have done balancing of 1000AH, 7 strings of batteries and 300AH, 80 strings of batteries. After balancing, the voltage of all single cells can reach less than 5mV.

Structure diagram of multi-winding transformer method

Active balancing can also use the method of energy transfer. The so-called energy transfer can be either to take energy from the entire group voltage to supplement the low voltage, or to take energy from the battery with too high voltage to feed back to the entire group voltage.

I have used the second method to achieve battery balancing in a communication power supply system. The circuit schematic is as follows:

At that time, I did the balancing of 16 strings of lithium-ion batteries, which were divided into two groups, each group of 8 batteries in series, and only 6 were 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, inductor L5 stores energy; when Q5 is closed, the energy stored in the inductor will charge batteries b1-b4 through D5, reduce the voltage of battery b5 and raise the voltage of other batteries. The same principle can be used to decompose the working process when the voltage of other battery packs is too high.

During the experiment, the two groups were balanced in this way. When there is a deviation between the two groups, bidirectional DC-DC can be used for energy conversion, which uses fewer modules and is more convenient to design.

I did not use bidirectional DC-DC at the time, but simply used energy consumption to balance the batteries between the two groups. From the final experimental results, the battery balance is quite good.

In the balancing process, if it feels like a sledgehammer to supply a charging module to each battery, and the energy consumption type does not meet the technical requirements, that is, active balancing is required, then the transformer one-to-multiple output method mentioned above may be more suitable for you. Use a suitable transformer to make a multi-output flyback power supply with primary feedback current limiting.

In fact, with the development of the use of power lithium-ion batteries, not only balancing, but also the protection of battery overcharge and over-discharge, that is, the use of protection boards we often say will become more and more extensive. We know that the original 18650 battery cells, more than a dozen strings of protection boards are very common with ICs to achieve short circuit, overcharge protection, and over-discharge protection. But what if there are dozens of battery cells in series? I don't know if there are any netizens who have come into contact with this information. You can communicate with me.

These are the four battery balancing methods I have tried so far. The balanced batteries range from 2AH to 1000AH, and the number of series cells ranges from 7 to 120.

My personal feelings are as follows:

1. For battery packs within 10AH, the energy consumption type may be a better choice, and the control is simple.

2. For battery packs of dozens of AH, it should be feasible to use a one-to-many flyback transformer and combine it with the battery sampling part to do battery balancing.

3. For battery packs of hundreds of AH, it may be better to use an independent charging module, because the balancing current of batteries of hundreds of AH is about 10A. If the number of series cells is more, the balancing power is very large, and the wires are connected to the outside of the battery. It may be safer to use external DC-DC or AC-DC balancing.

At present, balancing takes the consistency of battery voltage as the end condition of balancing, but as the SOC calculation becomes more and more accurate, balancing with consistent capacity should be the trend of future development.

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