u003cpu003eAbstract: Conduct experiments on ordinary rechargeable lithium batteries to study the impact of charging, discharging and temperature conditions on the life of lithium batteries. Through the summary of experimental data, the necessity of lithium battery protection for batteries and reasonable parameter ranges are discussed. u003c/pu003eu003cpu003eKeywords: lithium-ion batteries, power tools, lithium battery protection overcharge and overdischargeu003c/pu003eu003cpu003eu003c/pu003eu003cpu003e0 Introductionu003c/pu003eu003cpu003eCurrently, power tools The development of the company presents a trend of miniaturization, lightness, and cordlessness. In the application of portable power tools, the use of nickel-cadmium batteries is being increasingly restricted. At the same time, due to the doubling of the price of non-ferrous metals such as nickel and cobalt, and the application of new materials and large-scale production of power-type lithium-ion batteries, the cost gap between lithium-ion batteries and nickel-cadmium and nickel-hydrogen batteries has been greatly reduced. These factors have strongly promoted the application and development of lithium-ion batteries in portable power tools. u003c/pu003eu003cpu003eu003c/pu003eu003cpu003e1 Overviewu003c/pu003eu003cpu003eLithium-ion batteries must consider the safety during charging and discharging and the consistency between individual cells to prevent deterioration of characteristics. The high power density of lithium is difficult to ensure the safety of the battery. In the overcharged state, the battery will have excess energy after the temperature rises, so the electrolyte will decompose and produce gas, which will easily increase the internal pressure and cause spontaneous combustion or rupture; on the contrary, in over-discharge In the state, the electrolyte is decomposed to deteriorate battery characteristics and durability, reducing the number of recharges. However, temperature changes, especially at low temperatures, will make the above two situations more likely to occur. Therefore, the overcharge, overdischarge, overcurrent, short circuit and temperature protection of lithium batteries are very important, so protection circuits are usually designed in the battery pack
to protect the lithium batteries. u003c/pu003eu003cpu003eu003c/pu003eu003cpu003e2Analysisu003c/pu003eu003cpu003e2.1 The battery voltage is too high and the corresponding overcharge protection Under normal circumstances, the lithium battery is charged to 4.2 V, and at the same time, the charging currentu003c At 0.1 C or 0.05 C, the battery is fully charged. Fully charge a 3000 mAh battery from 3.0 V to obtain a capacity curve as shown in Figure 1: Divide the batteries into three groups and examine the impact of charging voltage on battery life. The charge cut-off voltage was set to 4.200 V, 4.275 V, and 4.350 V, respectively, and the discharge cut-off voltage was set to 3.0 V. Under this condition, the battery charge-discharge cycle was performed and the change of battery capacity was recorded. as shown in picture 2. u003c/pu003eu003cpu003eu003c/pu003eu003cpu003eAs shown in the figure, due to the increase in charging voltage, the initial discharge capacity of No. 3 battery is greater than that of No. 1 and No. 2. However, after about 20 cycles, the battery capacity began to drop sharply; the No. 2 battery had a similar performance, but because the charge cut-off point was slightly lower, the battery capacity began to decay after about 130 cycles. Therefore, the higher the charging voltage of the battery, the faster the lifetime decline (or decay rate). u003c/pu003eu003cpu003eu003c/pu003eu003cpu003eRefer to the requirement for the remaining 80% power of the battery after 300 cycles, both No. 2 and No. 3 cannot be met. No. 1 can be reached, that is, the charging voltage of the lithium battery should not be greater than 4.2 V under normal conditions. Correspondingly, the overcharge protection limits the battery voltage beyond this range. Because exceeding this range, it will definitely lead to a decrease in battery life. And the higher the battery voltage, the greater the risk factor. Generally, if it exceeds 4.25 V, special caution is required, and there may be a danger of explosion. Therefore, combined with the end-of-charge voltage, the battery capacity and the depth of discharge of the No. 3 battery are 100%, that is, the battery capacity is discharged every time. After 250 cycles, the capacity of the battery decays extremely fast, that is, the greater the depth of discharge, the faster the battery decays. u003c/pu003eu003cpu003eu003c/pu003eu003cpu003eGenerally, the termination voltage cannot be lower than 2.5 V or 2.4 V. When the battery voltage is lower than 2.4 V, the battery will enter a deep discharge. The deeper the discharge depth of the lithium battery, the shorter its charging life will be, which will result in a shorter battery life. u003c/pu003eu003cpu003eIn practical applications, the over-discharge protection termination voltage is usually set to control deep discharge. The setting of the over-discharge protection termination voltage needs to refer to the discharge current of the application equipment and the application environment, but it should be higher than 2.4 V. u003c/pu003eu003cpu003eu003c/pu003eu003cpu003e2.3 Experimental settings for high-current discharge and over-current protection of lithium batteries: Use the same lithium battery pack
to discharge at different discharge rates to obtain the dischargeable capacity. As shown in Figure 5. It can be seen from the figure that as the discharge rate increases, the discharge capacity and discharge voltage plateau of the battery decrease. The discharge capacity of 2C and 3C are respectively 89.6% and 68.4% of the discharge capacity of 1C, and 3.6 V for 2C and 3C discharge. The above capacity accounts for 64.3% and 57.2% of the total capacity, indicating that the battery discharge rate has a great influence on the battery discharge capacity. ..... For more content, please check the source document: Download information [File size: 1.97 MB Download times: Times] Click to download the file: File.pdfu003cpu003eu003c/pu003e 4 Conclusion As a battery for electric tools, lithium battery The most feared in use is overcharge and overdischarge. Once overcharged or overdischarged, it will cause the battery capacity to decrease, life span, and then to damage. Generally, the common deflagration phenomenon in lithium batteries is mostly caused by overcharging and overdischarging the battery. Therefore, in the use of lithium batteries, even if it is only one battery, a corresponding protection circuit should be provided. This is an indispensable measure to ensure the safety, performance and service life of the lithium battery. This article is for communication only, please do not use it for commercial purposesu003c/pu003eu003c/pu003e
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