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In order to supply sufficient voltage to the device, lithium-ion battery packs are usually composed of multiple batteries connected in series. However, if the capacity mismatch between the batteries occurs, it can affect the capacity of the entire battery pack. To achieve this, we need to balance the mismatched batteries. This article discusses the concept of battery balance and some precautions.

Lithium ion battery packs typically consist of one or several battery packs connected in parallel, with each pack consisting of 3 to 4 batteries connected in series. This combination method can simultaneously meet the voltage and power requirements for laptops, medical equipment, detection instruments, and industrial use. However, this commonly used configuration usually does not achieve its maximum efficiency, as if the capacity of a series connected battery does not match that of other batteries, it will reduce the capacity of the entire battery pack.

The mismatch in battery capacity includes a state of charge (SOC) mismatch and a 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 due to inadequate process control and testing methods, rather than changes in the chemical properties of lithium ions themselves. Prismatic lithium-ion batteries require stronger mechanical pressure during processing and are more prone to differences between batteries. In addition, lithium-ion polymer batteries may also experience differences between batteries due to the use of new processes.

The use of battery balancing technology can address the issue of SOC and C/E mismatch, thereby improving the performance of series lithium-ion battery packs. By balancing the battery during the initial adjustment process, the problem of battery mismatch can be corrected. Afterwards, only balancing is needed during the charging process, while C/E mismatch must be balanced during both charging and discharging processes. Although the defect rate of a battery manufacturer's product may be very low, it is still necessary for us to provide further quality assurance in order to prevent the occurrence of short battery life issues.

The meaning of battery balance

Portable devices with a working voltage of 6V or above are powered by series battery packs, in which case the total voltage of the battery pack is the sum of the voltage of each series battery. The battery pack of portable computers is usually composed of three or four batteries connected in series, with a nominal voltage of 10.8V or 14.4V. In most such applications, a single series battery pack cannot provide the energy required by the device. At present, the largest battery (such as 18650) can supply 2000mAh (milliampere hour) of energy, while computers require 50-60Whr (5000-600mAh) of energy, so it is necessary to connect three batteries in parallel for each battery in series.

Battery balancing refers to the use of differential current 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 balance. The battery balance issue is only considered when the batteries in the battery pack are connected in series and the connected batteries are equal to or greater than three levels. Battery balancing is achieved when all batteries in the battery pack meet the following two conditions:

If all batteries have the same capacity, battery balance is achieved when their relative charging state 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 weighing standard for SOC. If all batteries in an unbalanced battery pack can reach full capacity (equilibrium point) through differential charging, they can undergo normal charging and discharging without any additional adjustments, which are usually one-time. When users use a new battery, they usually require long-term charging, which actually includes a complete discharge and charging process. This process minimizes the load and maximizes the battery charging time, reducing the requirements for the battery equalization circuit.

If the capacity of the batteries is different, they are also considered 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 ensure that the SOC of batteries with different capacities is the same, differential current must be used every time a series of batteries are charged or discharged. The normal charging and discharging time is shorter than the initial charging and discharging, and requires a higher current.

When the batteries in a battery pack are unbalanced, their available capacity will decrease, and the lowest capacity battery in a series battery pack will determine the total capacity of the battery pack. In an unbalanced battery pack, one or more batteries will reach their maximum capacity while other batteries still need to be charged. During discharge, batteries that are not fully charged will be discharged before other batteries, causing the battery pack to stop supplying power earlier due to insufficient voltage.

Usually, the difference in capacity between batteries is less than 3%. If a battery in series with a lithium-ion battery pack does not meet standards or is placed too long before packaging, the voltage difference can reach 150mV after overflowing, resulting in a 13-18% decrease in the total capacity of the battery pack.

SOC balanced solution

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

The meaning of the charging state of a single battery is:

SOC=C/CTOTAL%

The capacity of a single battery means:

C=(i × t)mAh

To determine the capacity of a certain battery, we fully discharge the battery and then charge it, and measure the current at different times during the charging process until an open circuit voltage of 4.20V is reached. The optimal performance battery has a SOC of 100% and an OCV voltage of 50% in this state, commonly referred to as VMID, with a typical value of 3.67V.

In order to charge batteries with different capacities to achieve the same SOC, it is required that some batteries have a higher charge/discharge capacity than others, which requires the use of differential current. We call this process capacity/energy maximization.

Maximizing capacity/energy

Maximizing capacity/energy refers to setting all series connected batteries in a 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 capacity of batteries is different, some batteries will charge/discharge more than others. For example, suppose a battery pack has three series connected batteries, C1>C2=C3. The only way to balance this battery pack is to apply a differential charging current to the higher capacity battery (C1).

This is also necessary when the battery pack is discharged, otherwise when the battery with the smallest capacity reaches the shutdown voltage, the entire battery pack will stop discharging, while other batteries still have remaining capacity, which reduces the total capacity. Over time, the battery with the smallest capacity will experience faster performance degradation than other batteries, and after multiple charging/discharging cycles, capacity loss will be accelerated.

By matching the voltage of the series connected battery, more current will be drawn from the high capacity battery. When discharging, it is required to consume some additional voltage through equalization. When all batteries reach 0SOC, the total electrical energy obtained from the battery pack will still be increased before equalization.

Usually, the quality control of cylindrical lithium-ion batteries is good, and the difference in battery capacity does not exceed ± 3%. The input capacity is basically accurate, with a difference of no more than a few mAs (milliamperes per second). Therefore, the absolute value of battery capacity is also basically accurate, and the difference in SOC is within a few percentage points.

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