A lithium battery pack is usually connected in parallel by one or several battery packs, each battery pack consisting of 3 to 4 batteries connected in series. This combination can meet the voltage and power requirements of notebooks, medical devices, test instruments, and industrial applications. However, the general configuration of such applications usually does not perform its best, because if the capacity of one of the series batteries does not match the other batteries, the capacity of the entire battery pack will be reduced.
Battery capacity mismatches include 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 imperfections in process control and detection methods, rather than changes in the chemical properties of lithium ions themselves. A prismatic lithium battery (LiIon prismaTIc cell) requires more mechanical pressure during production, and the difference between the batteries is more likely to occur. In addition, lithium-ion polymer batteries also exhibit differences between batteries due to the adoption of new processes.
The battery equalization processing technology can solve the SOC and C/E mismatch problems, thereby improving the performance of the series lithium battery pack. The battery mismatch problem can be corrected by equalizing the battery during the initial adjustment process. After that, it is only necessary to balance during the charging process, and the C/E mismatch must be equalized during the charging and discharging processes. Although the defect rate of a battery manufacturer may be low, in order to avoid the problem of short battery life, we still need to provide further quality assurance.
Battery balance definition
Portable devices operating at 6V or higher are powered by a series battery pack, in which case the total voltage of the battery pack is the sum of the voltages of the series connected cells. The battery pack of a portable computer is usually made up 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 does not provide the energy required by the device. At present, the largest battery (such as 18650) can provide 2,000mAh (milliampere-hour) energy, while the computer needs 50-60Whr (5,000-6,000mAh) energy, so three batteries in series must be connected in parallel. battery.
Battery equalization refers to the use of differential currents for different batteries (or battery packs) in a series battery pack. The current in each cell in the series battery pack is usually the same, so additional components and circuitry must be added to the battery pack to achieve battery equalization. The battery equalization problem is only considered when the batteries in the battery pack are connected in series and the series battery is equal to or greater than three. Battery equalization is achieved when all of the batteries in the battery pack meet the following two conditions:
1. If all batteries have the same capacity, then battery equalization is achieved when they are in the same state of charge. The SOC is usually expressed as a percentage of current capacity to rated capacity, so open circuit voltage (OCV) can be used as a measure of SOC. If all of the batteries in an unbalanced battery pack can reach full capacity (equalization point) by differential charging, they can perform normal charging and discharging without any additional adjustments. Usually this adjustment is one-off. When a user uses a new battery, it is usually required to charge the battery for a long time. This process actually includes a complete discharge-charge. This process minimizes the load and maximizes battery charging time, reducing the need for battery equalization circuitry.
2. If the capacities of the batteries are different, they are considered to be balanced when the SOCs are the same. But the SOC is only a relative value, and the absolute value of each battery capacity is different. In order to make the SOCs of batteries having different capacities the same, a differential current must be used each time the series battery is charged and discharged. The normal charge and discharge time is shorter than the initial charge and discharge, and requires more current.
When the battery in the battery pack is unbalanced, its available capacity will decrease, and the lowest capacity battery in the 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 when other batteries still need to be recharged. When discharging, the battery that is not fully charged will be discharged before the other batteries, so that the battery pack stops supplying power in advance due to insufficient voltage.
Typically, the difference in capacity between batteries is less than 3%. If a battery in a series lithium battery pack is out of standard or placed for too long before packaging, the voltage difference can reach 150mV after being fully charged, thus reducing the total capacity of the battery pack by 13-18%.
SOC equalization processing
If the capacity of all the batteries in the battery pack is the same, we use SOC equalization. When all the SOC values ​​of the batteries are the same, we think that the batteries are 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&TImes;t)mAh
To determine the capacity of a battery, we fully discharge the battery and then recharge it, and measure the current at different times during the charging process until an open circuit voltage of 4.20V is reached. The best performance battery has a SOC of 100% in this state and an OCV voltage of 50% SOC is commonly referred to as VMID, which is typically 3.67V.
In order to charge batteries of different capacities to achieve the same SOC, it is required that some batteries must have more charge/discharge than other batteries, which must use differential current. We call this process capacity/energy maximization.
Capacity/energy maximization
Capacity/energy maximization means setting all series cells in the battery pack to the same SOC, even if they have different capacities. The SOC is managed at all times to maximize the output energy of the battery pack. In order to maximize 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 equalize this battery pack is to apply a differential charging current to the higher capacity battery (C1).
This must also be done 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, and at this time, the remaining capacity of the other battery will be reduced, thus reducing the total capacity. Over time, the smallest battery will degrade faster than other batteries, and will accelerate capacity loss after multiple charge/discharge cycles.
By matching the voltage of the series battery, more current will be drawn from the high capacity battery. When discharging, it is required to consume some extra voltage by equalization. At the end, when all the batteries reach 0SOC, the total electric energy obtained from the battery pack will still increase before the equalization.
Generally, the quality control of a cylindrical lithium ion battery is generally good, and the difference in battery capacity does not exceed ± 3%. The input capacity is basically accurate, with no more than a few mAs (mA·s). Therefore, the absolute value of the battery capacity is also basically accurate, and the difference in SOC is within a few percentage points.
Chandelier Floor Lamp,Crystal Chandelier,Luxury Chandelier,European Classic Chandelier
GUANGDONG LAVIUS LIGHTING CO., LTD. , https://www.laviuslighting.com