18650 lithium battery 3.7 v

　　Charge and discharge protection solution for multi-cell 18650 lithium battery 3.7 v

　　Lithium-ion battery is a widely used rechargeable battery. It has high operating voltage, small size, light weight, high energy density, long cycle life, can be fully charged in a short period of time, and has an allowable discharge temperature range. wide advantages. In addition, 18650 lithium battery 3.7 v have the advantages of small self-discharge current, no memory effect, and no environmental pollution. Its global supply continues to increase. According to a report by a market research company, the global supply of lithium-ion rechargeable batteries in 2007 increased by 17% compared with the previous year. As the use of 18650 lithium battery 3.7 v expands, the charge and discharge protection of 18650 lithium battery 3.7 v becomes increasingly important.

　　Lithium-ion battery protection

　　The safety of lithium-ion battery-powered equipment is currently the most concerning issue, so its protection is very important. The protection of 18650 lithium battery 3.7 v mainly includes overcharge protection, over-discharge protection, over-current and short-circuit protection, etc.

　　1 Overcharge protection

　　When the charger overcharges the lithium-ion battery, the charging state needs to be terminated to prevent the internal pressure from rising due to temperature rise. To this end, the protection device needs to monitor the battery voltage. When it reaches the battery overcharge voltage, the overcharge protection function is activated and charging is stopped.

　　2 Over-discharge protection

　　In order to prevent the over-discharge state of the lithium-ion battery, when the lithium-ion battery voltage is lower than its over-discharge voltage detection point, the over-discharge protection is activated, the discharge is terminated, and the battery is kept in a low quiescent current standby mode.

　　3 Overcurrent and short circuit protection

　　When the discharge current of the lithium-ion battery is too large or a short circuit occurs, the protection device will activate the overcurrent protection function.

　　Protection of multi-section lithium-ion circuits

　　The rated voltage of a single lithium-ion battery is 3.6V, which cannot meet the needs of high-voltage power supply situations. Therefore, multiple 18650 lithium battery 3.7 v need to be used in series. To this end, various relevant power management control integrated circuit manufacturers have launched their own multi-cell lithium-ion battery (battery pack) protection integrated circuit chips, such as Seiko Technology Co., Ltd. (SII)'s S-8204B (S-8204B belongs to S -8204 series, another product of this series is S-8204A. The difference between the two is that S-8204A works with P-channel MOSFET, and S-8204B works with N-channel MOSFET). The characteristic of this type of product is to monitor the charge and discharge status of 3 or 4-cell 18650 lithium battery 3.7 v, and can achieve overcharge, over-discharge and over-current protection.

　　Taking S-8204B as an example, it can perform high-precision detection of the voltage of each lithium-ion battery and has a 3-stage overcurrent detection function. The overcharge detection delay time, overdischarge detection delay time, and discharge overcurrent can be set through an external capacitor. Detection delay time 1 and discharge overcurrent detection delay time 2 can also be used to switch 3/4-cell 18650 lithium battery 3.7 v in series through the SEL terminal. However, its biggest feature is that it can be used in cascade. This function of S-8204B will be explained in detail in the next section.

　　Protection chip cascade

　　The battery protection chip mentioned above can protect up to 4 18650 lithium battery 3.7 v. However, many applications require 5 to 12 18650 lithium battery 3.7 v to work in series, such as power tools, electric bicycles and UPS. How to solve this problem? The answer is simple, just use multiple lithium battery protection chips at the same time. As shown in Figure 1, two protection chips are connected in series, and two N-channel MOSFETs are used as control switches to protect 8-cell 18650 lithium battery 3.7 v. Three protection chips are connected in series to protect 12-cell 18650 lithium battery 3.7 v. This series connection of multiple protection chips is a cascade of protection chips. Taking S-8204B as an example, two S-8204Bs are used together and 2 N-channel MOSFETs are used to control the low-voltage side. In this way, 6 to 8-cell control can be realized by selecting 3-cell and 4-cell functions with a single IC. Protection of series connected 18650 lithium battery 3.7 v. If five 18650 lithium battery 3.7 v are connected in series, an S-8204B can be used in series with other lithium-ion battery protection chips to achieve the protection function. This flexible combination of multiple protection chips can protect any number of 18650 lithium battery 3.7 v.

　　Figure 1 Cascade of multi-cell 18650 lithium battery 3.7 v

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