Multi String Lithium Ion Battery Charging Solution

With the advantages of high energy density, small volume and light weight, lithium-ion batteries have completely replaced other batteries in the mobile phone and notebook computer market, accounting for almost 100%. At present, lithium-ion battery is rapidly extending to power tools and other applications, and its broad market prospect has been recognized by the industry. However, compared with Ni MH, Ni Cd and lead-acid batteries, in order to promote the application and development of lithium-ion batteries faster, its safety and service life must be continuously improved. From the perspective of charger, this paper will discuss a new charging solution to improve the safety of lithium-ion battery, prolong the service life of battery and reduce the cost of charger.

In the process of using the battery, we often hear the saying of the battery industry: "the battery is less broken in use, and more are charged". This sentence can be understood as that incorrect charging conditions or methods will more easily damage the battery and reduce the service life of the battery. Taking 18650 lithium cobalt oxide battery as an example, when the charging temperature is too high, at about 70 ℃, the electrolyte interface (SEI) mode begins to decompose and heat; About 120 ℃: electrolyte and positive electrode begin thermal decomposition, resulting in gas evolution and rapid temperature rise; At about 260 ℃: battery explosion. Or charging overvoltage. In terms of overvoltage of 5.5V, it is easy to precipitate lithium metal, oxidize the solvent, increase the temperature, produce a vicious cycle, and even the battery catches fire and explodes. Therefore, for how to charge, let's discuss the following important issues.

Why is precharge required?

The working voltage of the battery ranges from 2.5V (carbon negative battery: 3V, charge 0%) to 4.2V (charge 100%). When the voltage is less than 2.5V, the battery discharge is terminated. At the same time, because the discharge circuit is closed, the current loss of the internal protection circuit is also minimized. Of course, in practical application, due to different internal materials, the discharge termination voltage can be in the range of 2.5v-3.0v. When the voltage exceeds 4.2V, the charging circuit is terminated to protect the safety of the battery; When the working voltage of the single battery drops below 3.0V, we can consider the over discharge state and the discharge circuit is terminated to protect the safety of the battery. Therefore, when the battery is not in use, the battery should be charged with 20% of the power, and then stored against moisture.

Due to the high energy ratio of lithium-ion battery, overcharge and over discharge should be strictly avoided in the use of battery. Over discharge will make it difficult to recover active substances. At this time, if you directly enter the fast charging mode (high current), it will damage the battery, affect the service life and may bring potential safety hazards. It is necessary to charge 2.5V to 3.0V with small current (C / 10) and then convert to fast charge.

At present, lithium-ion batteries are equipped with protective plates in applications, and the probability of over discharge is very small under normal circumstances, but without precharge function, it may still bring hidden dangers of over discharge in these two cases. The first is the failure of the protection plate, and the second is the self discharge rate after being placed for a long time (5% - 10% / month). Therefore, low current precharge can effectively solve the charging problem of over discharge battery.

However, the greater the charging current, the better. Taking the single lithium ion battery as an example, its charging methods include constant current and constant voltage charging processes. The constant voltage is usually 4.2V (taking LiCoO2 battery as an example), and the constant current setting value is 0.1C 1C. Although high current charging will shorten the charging time, it will also shorten the battery life cycle and reduce the capacity. Therefore, we need to select the appropriate constant current value for charging.

The following is the relationship curve between different current charging and battery capacity of a 4.2v/900mah LiCoO2 cell (Fig. 1). We can see that after about 500 charging and discharging cycles, the battery capacity of low current charging is significantly greater than that of high current charging.

Voltage accuracy requirements during constant voltage charging

As a high energy density battery, overcharge will cause great harm to lithium-ion battery, which may expand, leak and even explode. Moreover, overcharge is easy to accelerate the reaction of electrolytic substances in the battery and shorten the service life of the battery. Therefore, accurate constant voltage charging is of great significance for the service life of lithium-ion battery.

In order to fully charge, the accuracy of constant voltage value and termination voltage value shall be within 1%. Taking lithium cobalt oxide battery as an example, it is best to approach 4.2V as much as possible, but not more than 4.2V. This high-precision voltage charging method can reduce the dissolution of cobalt, stabilize the layered structure of LiCoO2, prevent its coating from phase change, improve the cycle performance and maintain high capacity. In addition, even a slight overvoltage will bring about changes in two phenomena, the reduction of the initial capacity of the battery and the reduction of the cycle life of the battery.

When multiple lithium-ion batteries are connected in series, in order to ensure the maximum battery capacity and service life, sometimes the accuracy is even required to be less than 0.5%. Therefore, the precision control of charging voltage is a key technology of lithium-ion battery charger.

At present, people have such a misunderstanding about the charging voltage of lithium battery that with the battery protection board, there is no need to care about the voltage accuracy, which is not desirable. Because the purpose of battery protection board is to protect possible accidents in time, it considers more safety factors than performance factors. For example, taking a 4.2V cell as an example, the overvoltage protection parameter of the protection board is 4.30v (some may be 4.4V). If it is overcharged every time and takes 4.30v as the charging cut-off point, the battery capacity will decay quickly.

Why do I need a charging timer

A charger manufacturer once said that they used to encounter charger users to return goods, saying that the charger was broken because the battery was not full after charging for a day, and the charger did not turn the light, which was always a red light. But when the manufacturer actually measured the charger, it was found that it was normal and met the factory requirements. What's the problem? This is mainly because the charger does not consider that the self discharge becomes larger after the aging of the battery. If the current setting of cut-off charging is too small, the aging battery will not reach the set point of charging completion, so that the user will misjudge that the charger is broken.

The function of the charging timer is to prevent the damaged or over cycled lithium battery from entering the EOC state (higher than the judgment current) due to excessive self discharge during the charging cut-off period. On the one hand, it will lead to the misjudgment that the battery is not full. On the other hand, it may also lead to overheating, expansion and even danger due to charging for a long time.

Considering these factors, the new multi string lithium-ion battery charging chip oz8981 launched by O2micro is a perfect solution. Oz8981 is a special integrated chip for charging management. It has accurate voltage, current output and multiple protection, and provides six-stage charging control mode. The system design is convenient and the cost is low. It is mainly used for multiple lithium-ion battery packs on light electric vehicles, electric bicycles and electric tools.

The cost-effective and reliable oz8981 includes a single-chip integrated charging controller, which can realize efficient error amplifier output. It supports six stages of intelligent charging control: 0V pulse charging, precharge, constant current charging, constant voltage charging, cut-off charging and automatic recharging. It supports flexible setting of precharge starting voltage, constant current charging value, constant voltage charging value and cut-off charging current value.

In addition, oz8981 has high accurate charging voltage (Figure 4 shows the charging curve of oz8981 lithium ion battery. Combined with the front-end PWM chip, oz8981 will help users quickly realize the design of safe, efficient and low-cost lithium-ion battery charger.

Multi String Lithium Ion Battery Charging Solution 1

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