3.7V Lithium Polymer Battery

　　3.7V Lithium Polymer Battery balancing processing technology solves the problem of SOC and C/E mismatch

　　In order to provide sufficient voltage for devices, 3.7V Lithium Polymer Battery packs are usually composed of multiple batteries connected in series. However, if the capacity mismatch between batteries will affect the capacity of the entire battery pack. To do this, we need to equalize the mismatched cells. This article discusses the concept of cell balancing and some considerations.

　　3.7V Lithium Polymer Battery packs usually consist of one or several battery packs connected in parallel, with each battery pack consisting of 3 to 4 cells connected in series. This combination can simultaneously meet the voltage and power requirements required by notebook computers, medical equipment, test instruments and industrial applications. However, this commonly used configuration often does not work to its full potential because if the capacity of one series cell does not match the capacity of the other cells, it will reduce the capacity of the entire battery pack.

　　Battery capacity mismatch includes state of charge (SOC) mismatch and capacity/energy (C/E) mismatch. In both cases, the total capacity of the battery pack is only up to the capacity of the weakest cell. In most cases, the cause of battery mismatch is imperfect process control and detection methods, rather than changes in the chemical properties of lithium ions themselves. Prismatic lithium batteries (LiIon prismatic cells) 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.

　　The use of battery balancing technology can solve the SOC and C/E mismatch problem, thereby improving the performance of series 3.7V Lithium Polymer Battery packs. The battery mismatch problem can be corrected by equalizing the battery during the initial adjustment process. After that, it only needs to be equalized during the charging process, while the C/E mismatch must be equalized during both the charging and discharging processes. Although the product defect rate may be very low for a certain battery manufacturer, in order to avoid the problem of too short battery life, it is still necessary for us to provide further quality assurance.

　　Definition of cell balancing

　　Portable devices with an operating voltage of 6V or above are powered by series-connected battery packs. In this case, the total voltage of the battery pack is the sum of the voltages of the series-connected batteries. 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 connected in series cannot provide the energy required by the device. The current largest batteries (such as 18650) can provide 2,000mAh (milliamp hours) of energy, and computers require 50-60Whr (5,000-6,000mAh) of energy, so each battery in series must be connected in parallel with three batteries.

　　Cell balancing refers to the use of differential currents for different cells (or battery packs) in a series battery pack. The current drawn by each cell in a series battery pack is usually the same, so additional components and circuitry must be added to the battery pack to achieve cell balancing. The battery balancing issue will only be considered when the batteries in the battery pack are connected in series and the series connected batteries are equal to or greater than level three. When all batteries in the battery pack meet the following two conditions, battery balancing is achieved:

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

　　2. If the batteries have different capacities, they are 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 for batteries with different capacities to have the same SOC, a differential current must be used every time a series battery is charged or discharged. The normal charging and discharging time is shorter than the initial charging and discharging, and requires a larger current.

　　When the cells in a battery pack are unbalanced, its available capacity will be reduced, 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 cells will reach maximum capacity while the other cells still need to be charged. When discharging, the battery that is not fully charged will be discharged before other batteries, causing the battery pack to stop supplying power early due to insufficient voltage.

　　Typically, the difference in capacity between batteries is less than 3%. If a cell in a series-connected 3.7V Lithium Polymer Battery pack is substandard or left for too long before packaging, the voltage difference can reach 150mV after full charging, thus reducing the total capacity of the battery pack by 13-18%.

　　SOC equalization processing

　　If all batteries in the battery pack have the same capacity, we use SOC balancing. 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=(it)mAh

　　To determine the capacity of a battery, we fully discharge and then recharge the battery, taking current measurements at various times during the charge process until an open circuit voltage of 4.20V is reached. The best performance battery has an SOC of 100% in this state. The OCV voltage at which the SOC is 50% is usually called VMID, and its typical value is 3.67V.

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

　　Capacity/Energy Maximization

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

　　This must also be done when the battery pack is discharging, 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, thus reducing the total capacity. Over time, the battery with the smallest capacity will degrade faster than other batteries, resulting in accelerated capacity loss over multiple charge/discharge cycles.

　　By matching the voltage of the cells in series, more current will be drawn from the high capacity battery. During discharging, some extra voltage is required to be consumed through balancing. In the end, when all batteries reach 0SOC, the total power obtained from the battery pack will still increase compared to before balancing.

　　Generally, the quality control of cylindrical lithium-ion batteries (cylindricalcell) is usually better, and the battery capacity difference does not exceed ±3%. The input capacity is basically relatively accurate, and the difference is no more than a few mAs (milliamp seconds). Therefore, the absolute value of battery capacity is basically accurate, and the difference in SOC is within a few percentage points.

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