In order to provide sufficient voltage to the device, lithium battery packs are usually made up of multiple batteries in series, but if the capacity mismatch between the batteries will affect the capacity of the entire battery pack. To this end, we need to balance the mismatched batteries. This article discusses the concept of battery balancing and some considerations.

　　In order to provide sufficient voltage to the device, lithium battery packs are usually made up of multiple batteries in series, but if the capacity mismatch between the batteries will affect the capacity of the entire battery pack. To this end, we need to balance the mismatched batteries. This article discusses the concept of battery balancing and some considerations.

　　Lithium battery packs usually consist of one or several battery packs in parallel, and each battery pack consists of 3 to 4 batteries in series. This combination can simultaneously meet the voltage and power requirements of notebook computers, medical equipment, test instruments, and industrial applications. However, this commonly used configuration usually does not exert its maximum effect, because if the capacity of a series battery does not match other batteries, the capacity of the entire battery pack will be reduced.

　　"The mismatch of battery capacity includes state of charge (SOC) mismatch and capacity/energy (C/E) mismatch. In both cases, the total capacity of the battery pack can only reach the capacity of the weakest battery. In most cases, the cause of battery mismatch is imperfect process control and detection methods, rather than changes in the chemical properties of lithium ion itself. Prismatic lithium batteries (LiIonprismaticcell) require stronger mechanical pressure during production, and differences are more likely to occur between batteries. In addition, lithium-ion polymer batteries will also have differences between batteries due to the use of new processes.

　　Using battery equalization processing technology can solve the problem of SOC and C/E mismatch, thereby improving the performance of series-connected lithium battery packs. The battery mismatch problem can be rectified by balancing the battery during the initial adjustment process. After that, it only needs to be balanced during the charging process, while the C/E mismatch must be balanced during the charging and discharging processes. Although the defect rate of a battery manufacturer may be very low, in order to avoid the problem of too short battery life, we still need to provide further quality assurance.

　　The definition of battery balance

　　Portable devices with a working voltage of 6V or above are powered by a series battery pack. In this case, the total voltage of the battery pack is the sum of the voltages of the batteries in series. The battery pack of a portable computer is usually composed of three or four batteries connected in series, with a nominal voltage of 10.8V or 14.4V. In most of these applications, a single battery pack in series cannot provide the energy required by the device. At present, the largest battery (such as 18650) can provide 2,000mAh (milliampere hour) of energy, and the computer needs 50-60Whr (5,000-6,000mAh) of energy, so it is necessary to connect three batteries in parallel for each battery connected in series.

　　Battery balancing refers to the use of differential currents for different batteries (or battery packs) in a series battery pack. The current of each battery in a series battery pack is usually the same, so additional components and circuits must be added to the battery pack to achieve battery balancing. Only when the batteries in the battery pack are connected in series and the batteries in series are equal to or greater than three levels, the battery balancing problem will be considered. When all the batteries in the battery pack meet the following two conditions, battery balancing is achieved:

　　1. If all batteries have the same capacity, then the battery balance is achieved when their relative state of charge is the same. SOC is usually expressed as a percentage of current capacity to rated capacity. Therefore, open circuit voltage (OCV) can be used as a measure of SOC. If all the batteries in an unbalanced battery pack can reach full capacity (equilibrium point) through differential charging, they can be charged and discharged normally without any additional adjustments, which are usually one-time adjustments. When a user uses a new battery, he usually needs to charge the battery for a long time. This process actually includes a complete discharge-charge. This process minimizes the load and maximizes the battery charging time, reducing the requirements for the battery balancing circuit.

　　2. If the battery capacity is different, they are considered to be balanced when the SOC is the same. But SOC is only a relative value, and the absolute value of each battery capacity is different. In order to make the SOC of batteries with different capacities the same, a differential current must be used every time the batteries in series are charged and discharged. Normal charging and discharging time is shorter than the initial charging and discharging, and requires more current.

　　When the batteries in the battery pack are unbalanced, its usable capacity will decrease. The battery with the lowest capacity in the series battery pack will determine the total capacity of the battery pack. In an unbalanced battery pack, one or several batteries will reach their maximum capacity while the other batteries still need to be charged. When discharging, the battery that is not fully charged will be discharged before other batteries, so that the battery pack will stop power supply early due to insufficient voltage.

　　Generally, the difference in capacity between batteries is less than 3%. If a battery in the series lithium battery pack is not up to standard, or placed too long before packaging, the voltage difference can reach 150mV after being fully charged, which will reduce the total capacity of the battery pack by 13-18%.

　　SOC equalization processing

　　If all the batteries in the battery pack have the same capacity, we will use the SOC equalization process. When the SOC values of all batteries are the same, we consider the batteries to be balanced.

　　The state of charge of a single battery is defined as:

　　SOC=C/CTOTAL%

　　The capacity of a single battery is defined as:

　　C=(i×t)mAh

　　In order to determine the capacity of a certain battery, we fully discharge the battery and then recharge it, and measure the current at different times during the charging process until it reaches an open circuit voltage of 4.20V. The SOC of the best performance battery in this state is 100%, and the OCV voltage with a SOC of 50% is usually called VMID, and its typical value is 3.67V.

　　In order to charge batteries with different capacities to achieve the same SOC, some batteries must be charged/discharged more than others, which must use differential current. We call this process capacity/energy maximization.

　　Maximize capacity/energy

　　Maximizing capacity/energy refers to setting all batteries in series in the battery pack to the same SOC, even if their capacities are different. Manage SOC at all times to maximize the output energy of the battery pack. In order to maximize the output energy, all batteries must be fully charged. That is, the SOC of all batteries must be 100%. If the battery capacity is different, some batteries will charge/discharge more than others. For example, suppose a battery pack has three batteries in series, C1>C2=C3. The only way to balance this battery pack is to apply a differential charging current to the higher capacity battery (C1).

　　The same must be done when the battery pack is discharged. Otherwise, when the battery with the smallest capacity reaches the cut-off voltage, the entire battery pack will stop discharging, while other batteries still have remaining capacity, which reduces the total capacity. In the long run, the battery with the smallest capacity will degrade faster than other batteries, and will accelerate capacity loss after multiple charge/discharge cycles.

　　By matching the voltage of the battery in series, more current will be drawn from the high-capacity battery. When discharging, it is required to consume some extra voltage through equalization. In the end, when all batteries reach 0SOC, the total electric energy obtained from the battery pack will still increase compared to before equalization.

　　Generally, the quality control of cylindrical lithium-ion batteries (cylindricalcell) is usually better, and the difference in battery capacity does not exceed ±3%. The input capacity is basically relatively accurate, and the difference does not exceed a few mAs (milliamp·s). Therefore, the absolute value of the battery capacity is basically accurate, and the difference in SOC is within a few percentage points.