3.7v 2200mah 18650 lithium battery

　　Typical methods for active balancing of 3.7v 2200mah 18650 lithium battery systems

　　We often hear such remarks that Japanese batteries are good and domestic batteries are worse. The important point here is the consistency between battery cells. For vehicle endurance, capacity is the most direct and important parameter, so consistency mainly refers to capacity. Capacity is a parameter that cannot be directly measured in a short time. According to experience, people have found that the capacity of a single cell has a one-to-one correspondence with its open circuit voltage. Therefore, the focus of examining the battery consistency in the system that has been installed and operated finally falls on the cell voltage.

　　Single cell voltage is a direct measurement value and can be measured online in real time, which makes it a favorable condition for measuring the consistency level of system cells. Not only that, in common BMS management strategies, the single cell voltage value is used as a trigger condition, and there are also discharge termination conditions, charge termination conditions, etc. For a parameter in such a position, if the single cell voltage consistency difference is too large, it will directly limit the charging and discharging power of the battery pack. Based on this, people use battery balancing methods to solve the problem of excessive single cell voltage differences in battery packs that are already in operation to increase the capacity of the battery pack. Therefore, it can be inferred that the balancing method extends the cruising range and extends the battery life. A picture in the literature vividly illustrates the principle of active balancing. From this, we can see that our balancing is not very ideal, but there is no better way for the time being.

　　We usually call energy consumption balancing passive balancing, and other balancing active balancing. Although human intervention in the system is often not discussed in theory, it is indispensable in practical applications. Single cell charging balancing is a way to solve the inconsistency problem by manually charging the low-voltage battery cells separately. There are many specific implementation plans for active balancing, which can be divided into two categories from the concept: high-cutting and low-filling type and parallel balancing type. Active balancing that is usually questioned to affect battery life refers specifically to the type of active balancing that is high-cutting and low-filling. Several typical active balancing circuits are summarized below.

　　High-cutting and low-filling means transferring part of the energy of the already high-voltage battery cells to the low-voltage battery cells, thereby delaying the minimum single cell voltage from reaching the discharge. The time when the cut-off threshold and the highest single cell voltage reach the charge termination threshold is obtained to obtain the effect of the system increasing the charging and discharging of the power. However, in this process, both the high-voltage single cell and the low-voltage single cell are additionally charged and discharged. We all know that the life of a battery is called "cycle life". For this battery cell alone, it is certain that the extra charge and discharge burden will lead to the consumption of life. However, for the battery pack system, whether it generally prolongs or reduces the system life, there is no clear experimental data to prove it.

　　Balancing by cutting high and filling low includes capacitor balancing, inductor balancing, and transformer balancing. These three balancing methods include balancing during charging and balancing during static. There is also an active balancing called parallel balancing, which only works during charging. Some people also believe that balancing should be added during vehicle operation and at the end of the discharge process, but it is generally believed that the fluctuation of the system current value is relatively large. If the balancing is still based on the single cell voltage, it is likely to be misjudged and affect the balancing effect. Of course, with the development of technology, it is possible to accurately calculate the SOC directly through other means, and the balancing based on the SOC will no longer be troubled by this problem.

　　Capacitor balancing

　　Let B1 and B3 battery cells be the highest and lowest voltage cells in the group respectively. All the switches in the figure are normally open. When the equalizer issues a balancing command, the power switches S1 and Q2 are closed. At this time, the single battery B1 charges the capacitor. The duty cycle of the power switch is controlled to control the charging power and time. After the charging is completed, the switches S3 and Q4 are closed, and the capacitor charges the single battery B3. At this time, the imbalance in the battery pack is reduced and the balancing is completed.

　　Inductive balancing

　　During the charging process, the switch S is closed, and the charger charges the battery pack. At this time, all the switches on the right side of the battery pack are disconnected, and the balancing system is not turned on. Assume that the voltage of the single battery B1 begins to be significantly higher than that of other batteries and reaches the balancing threshold. At this time, the balancing system is turned on, the switches S1 and Q2 are closed, and the inductor is connected in parallel with the single battery B1, which plays a role of shunting. The inductor stores energy from the charger and the battery B1; when the switches S1 and Q2 are set to 0, and the switches Q3 and S4 are set to 1, the inductor releases a certain amount of energy to the single battery B3 in the charging process.

　　During the static process, the switch tube S is disconnected. When the voltage of the single battery B1 is higher than that of other batteries and reaches the balancing threshold, the balancing system is turned on, the switch tubes S1 and Q2 are closed, the inductor is connected in parallel with the single battery B1, and the inductor absorbs the energy of B1; when the switch tubes S1 and Q2 are disconnected and the switch tubes Q3 and S4 are closed, the inductor releases the power to the single battery B3.

　　Transformer type balancing

　　Parameter design is based on the flyback balancing transformer, that is, the transformer is used as both an energy absorption source and an energy release source, and the conversion of energy absorption and release lies in the conversion of energy between magnetic energy and electrical energy.

　　Similarly, assuming that the voltage of the single battery B1 is the highest, S1 and Q2 are set to 1, and other switch tubes are set to 0. At this time, the transformer is used as an energy absorption source, and the energy is converted from the electrical energy given by the B1 battery to magnetic energy; S1 and Q2 are set to 0, Q1 and S2 are set to 1, and the energy is transferred from the primary winding to the secondary winding, and the energy is released to the single battery B3, and the energy is converted from magnetic energy to electrical energy again.

　　Parallel balancing

　　The ideal balancing method is that all batteries have the same energy and terminal voltage, and the voltage of the single cells in the parallel battery group is always equal, because like the principle of the communicating vessel, the water columns on both sides are always horizontal, and the parallel battery also has the innate single cell voltage that spontaneously charges the battery with a low single cell voltage. However, if you want to apply this principle in a series battery group, you need to slightly change the original battery group topology.

　　As shown in the parallel topology in the figure below, each single cell has a single-pole double-throw switch relay, so n+1 relays are required in an n-series battery group.

　　The control principle is as follows: Assume that the voltage of B4 in the battery group is the highest and the voltage of B2 is the lowest, and the control relays S5, S3, Q4, and Q2 are closed. At this time, the two single cells are connected in parallel, and the two single cells are automatically balanced, and the voltage tends to be consistent. The disadvantage of this topology is that it cannot be balanced during charging, and parallel balancing can only be performed during static depolarization.

　　There are also many forms of parallel balancing. In addition to the above, we can also see the scheme shown in the figure below:

　　Parallel balancing, in general, is to shunt the charging current during the charging process, charge the low-voltage battery more, and charge the high-voltage battery less. Therefore, there is no need to "rob the rich to help the poor", avoid the extra charging and discharging burden of the highest and lowest voltage batteries, and there is no need to doubt the impact of the balancing process on the life of individual batteries and drag down the system life.

　　Balancing between modules

　　This form is rare in actual applications, but the blueprint provided by the chip supplier has already appeared in the blueprint that adjacent modules can balance each other. A schematic diagram is as follows.

　　Comparison of several balancing methods

　　Choice of active balancing

　　An industry insider has summarized a set of methods for selecting balancing methods based on his own engineering experience:

　　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 combined with the battery sampling part to do battery balancing.

　　3) For battery packs with a capacity of over 100 Ah, it may be better to use an independent charging module, because the balancing current of batteries with a capacity of over 100 Ah is around 10A. If there are more cells in series, the balancing power will be very large, and it may be safer to use external DC-DC or AC-DC balancing if the wires are connected outside the battery.

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