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In order to provide sufficient voltage to the device, lithium battery packs are usually composed of multiple batteries connected in series, but if there is a capacity mismatch between the batteries, it will affect the capacity of the entire battery pack. For this, we need to balance the mismatched batteries. This article discusses the concept of battery balancing and some precautions.

Lithium 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 of laptops, medical equipment, testing instruments, and industrial applications. However, this commonly used configuration often does not achieve its maximum effectiveness, 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 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 inadequate process control and detection methods, rather than changes in the chemical properties of lithium ions themselves. Prism shaped lithium batteries require stronger mechanical pressure during production, making it easier for differences to occur between batteries. In addition, lithium-ion polymer batteries may also exhibit differences between batteries due to the adoption of new processes.

The use of battery balancing processing technology can solve the SOC and C/E mismatch problem, thereby improving the performance of series connected lithium battery packs. By balancing the battery during the initial adjustment process, the battery mismatch problem can be corrected. Afterwards, balancing only needs to be done 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 low, it is still necessary to provide further quality assurance to avoid the problem of short battery life.

Definition of battery balance

Portable devices with a working voltage of 6V or above are powered by series connected battery packs, and in this case, the total voltage of the battery pack is the sum of the voltages of each series connected battery. 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 such applications, a single series connected battery pack cannot provide the energy required by the device. At present, the largest battery (such as 18650) can provide 2000mAh (milliampere hour) energy, while computers require 50-60Whr (5000-6000mAh) energy, so three batteries must be connected 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 connected battery pack. The current of each battery in a series connected battery pack is usually the same, so additional components and circuits must be added to the battery pack to achieve battery balancing. The battery balancing issue will only be considered when the batteries in the battery pack are connected in series and the number of connected batteries is equal to or greater than three levels. When all batteries in the battery pack meet the following two conditions, battery balancing is achieved:

If all batteries have the same capacity, then battery balancing is achieved when their relative charging states are 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. When users use new batteries, they usually need to charge the battery for a long time, which actually includes a complete discharge charge process. This process minimizes the load and maximizes battery charging time, reducing the requirements for battery balancing circuits.

If the capacities of the batteries are 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 make the SOC of batteries with different capacities the same, differential current must be used every time the series connected batteries are charged and discharged. The normal charging and discharging time is shorter than the initial charging and discharging, and requires a larger current.

When the batteries in the battery pack are unbalanced, its available capacity will decrease, and 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 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 prematurely due to insufficient voltage.

Usually, the difference in capacity between batteries is less than 3%. If one of the batteries in the series connected lithium battery pack does not meet the standards, or is left for too long before packaging, the voltage difference can reach 150mV after being fully charged, resulting in a 13-18% decrease in the total capacity of the battery pack.

SOC balancing processing

If the capacity of all batteries in the battery pack is the same, we use SOC balancing processing. When the SOC values of all batteries are the same, we consider the batteries to be balanced.

The charging status of a single battery is defined as:

SOC=C/CTOTAL%

The capacity of a single battery is defined as:

C=(i×t)mAh

To determine the capacity of a battery, we fully discharge it and then charge 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 at 50% SOC is 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 refer to this process as capacity/energy maximization.

Maximizing Capacity/Energy

Capacity/energy maximization 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 batteries connected 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 discharging the battery pack, 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. Over time, the battery with the smallest capacity will experience faster performance degradation than other batteries, accelerating capacity loss after multiple charge/discharge cycles.

By matching the voltage of the series connected batteries, more current will be drawn from high-capacity batteries. During discharge, it is required to consume some additional voltage through balancing. In the end, when all batteries reach 0SOC, the total electrical energy obtained from the battery pack will still increase relative to before balancing.

The quality control of cylindrical lithium-ion cells is usually good, and the difference in battery capacity does not exceed ± 3%. The input capacity is basically quite accurate, with a difference of no more than a few mAs (milliampere seconds). 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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