Multi-string 18650 lithium rechargeable battery charging solution

　　With the advantages of high energy density, small size and light weight, lithium-ion batteries have completely replaced other batteries in the mobile phone and notebook computer markets, with a share of almost 100%. Currently, lithium-ion batteries are rapidly extending into power tools and other applications, and their broad market prospects are increasingly recognized by the industry. However, compared with nickel metal hydride, nickel cadmium, 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. This article will discuss a new charging solution from the perspective of the charger to improve the safety of lithium-ion batteries, extend battery life, and reduce the cost of the charger.

　　In the process of using batteries, we often hear a saying from the battery industry: "Battery damage is rare during use, and more is damaged by charging." We can understand this sentence as incorrect charging conditions or methods will more easily damage the battery and reduce the battery life. Taking the 18650 cobalt oxide 18650 lithium rechargeable battery as an example, when charging is overheated, around 70°C: the electrolyte interface (SEI) module begins to decompose and generates heat; around 120°C: the electrolyte and positive electrode begin to thermally decompose, causing gas evolution and a rapid rise in temperature. ; At about 260℃: the battery explodes. Or charging overvoltage, with an overvoltage of 5.5V, it is easy for lithium metal to precipitate, the solvent to be oxidized, the temperature to rise, creating a vicious cycle, and even the battery catches fire or explodes. Therefore, let’s discuss the following important issues regarding how to charge.

　　Why do you need the precharge function?

　　The battery operating voltage ranges from 2.5V (carbon negative battery: 3V, 0% charge) to 4.2V (100% charge). When the voltage is less than 2.5V, battery discharge is terminated. At the same time, because the discharge circuit is closed, the current loss of the internal protection circuit is also reduced to a minimum. Of course, due to different internal materials in actual applications, 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 battery safety; when the operating voltage of the single battery drops below 3.0V, we can consider it to be in an over-discharge state, and the discharge circuit is terminated to protect the battery safety. Therefore, when the battery is not in use, it should be charged to 20% capacity and then stored in a moisture-proof manner.

　　Since lithium-ion batteries have a high energy ratio, overcharging and over-discharging must be strictly avoided during battery use. Over-discharge will make it difficult to recover the active material. At this time, if you directly enter the fast charging mode (large current), it will cause damage to the battery, affect its service life, and may cause safety hazards. It is necessary to first charge to 2.5V to 3.0V with a small current (C/10), and then convert to fast charging.

　　Although lithium-ion batteries are currently equipped with protective plates in applications, under normal circumstances, the chance of over-discharge is very small, but without the precharge function, over-discharge may still occur in these two situations. hidden dangers. One is the failure of the protective board, and the other is the self-discharge rate after being left for a long time (5%-10%/month). Therefore, small current precharging can effectively solve the charging problem of over-discharged batteries.

　　However, charging current is not always better. Taking a single 18650 lithium rechargeable battery as an example, its charging methods include constant current and constant voltage charging processes. The constant voltage is usually 4.2V (taking LiCoO2 batteries 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 choose an appropriate constant current value for charging.

　　Voltage accuracy requirements during constant voltage charging

　　As a high-energy-density battery, overcharging will cause great harm to lithium-ion batteries, which may cause expansion, leakage or even explosion. Moreover, overcharging can easily cause the electrolyte in the battery to react faster and shorten the service life of the battery. Therefore, accurate constant voltage charging is of great significance to the service life of lithium-ion batteries.

　　In order to fully charge the battery, it is necessary to ensure that the accuracy of the constant voltage value and the end voltage value is within 1%. Taking lithium cobalt oxide batteries as an example, it is best to be as close to 4.2V as possible but not exceeding 4.2V. This high-precision voltage charging method can reduce the dissolution of cobalt and stabilize the layered structure of LiCoO2, making it The coating does not undergo phase change, improves cycle performance, and maintains high capacity. In addition, even a slight overvoltage will cause changes in two phenomena, a reduction in the initial capacity of the battery and a reduction in the cycle life of the battery.

　　When multiple lithium-ion batteries are connected in series, in order to ensure maximum battery capacity and life, sometimes the accuracy is even required to be within 0.5%. Therefore, precise control of charging voltage is a key technology for 18650 lithium rechargeable battery chargers.

　　At present, people have such a misunderstanding about the charging voltage of lithium batteries. They think that with the battery protection board, there is no need to care about the voltage accuracy. This is not advisable. Because the purpose of the battery protection board is to provide timely protection against possible accidents, it considers more safety factors than performance factors. For example, taking a 4.2V battery cell as an example, the overvoltage protection parameter of the protection board is 4.30V (some may require 4.4V). If it is overcharged every time and 4.30V is used as the charging cut-off point, the battery capacity will be very high. Fast decaying.

　　Why you need a charging timer

　　A charger manufacturer once said that they often encountered charger users who came to return their products, saying that the charger was broken. The reason was that the battery was not fully charged after charging for a day, and the charger did not turn on the light and kept the red light. But when the manufacturer actually measured the charger, it was found to be normal and in line with factory requirements. What's the problem? This is mainly because the charger does not take into account the self-discharge of the battery as it ages. If the charge cut-off current setting is too small, the aging battery will never reach the charging completion set point, causing the user to misjudge that the charger is broken.

　　The function of the charging timer is to prevent damaged or excessively cycled lithium batteries. During the cut-off period of charging, due to excessive self-discharge, it is difficult for the battery to enter the EOC state (higher than the judged current). On the one hand, it brings problems to the user. On the other hand, misjudgment that the battery is not fully charged may also cause the battery to overheat, expand, and even be dangerous due to charging for too long.

　　Considering these factors, the new multi-string 18650 lithium rechargeable battery charging chip OZ8981 launched by O2Micro is a complete solution. OZ8981 is a dedicated charging management integrated chip with precise voltage, current output and multiple protections, and provides six-stage charging control mode, making system design easy and low-cost. It is mainly targeted at multi-cell 18650 lithium rechargeable battery packs used in light electric vehicles, electric bicycles and power tools.

　　The cost-effective and reliable OZ8981 contains a single-chip integrated charge controller that enables efficient error amplifier output. It supports six stages of intelligent charging control: 0V pulse charging, pre-charging, constant current charging, constant voltage charging, cut-off charging, and automatic recharging. Supports flexible setting of pre-charge starting voltage, constant current charging value, constant voltage charging value and cut-off charging current value.

　　In addition, OZ8981 has high-precision charging voltage (<1%) and current (<5%) output; through external resistor adjustment, the voltage output accuracy can be <0.5%. Supports dual charging timer protection: pre-charging timing, constant voltage charging timing (maximum 5 hours, or not in use). Supports dual temperature protection: chip internal temperature protection (115℃), external over-temperature protection (default: 44℃) and low-temperature protection (default: 2℃). The external temperature protection point can be flexibly set externally. Supports charging overvoltage protection, overcurrent protection, and short circuit protection. Supports automatic battery access detection and direct LED display of charging status. The device comes in a universal package SOp16.

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