The cells usually have three different shapes: cylindrical, soft bags and square hard shells. Due to the limited voltage windows and energy capacity of the battery cell (for example, 3.7-4.2V and 2000 mAh of cylindrical lithium ion batteries), they are therefore combined with the battery in series or parallel configuration to become larger into a larger one. Battery.

　　Lithium -ion batteries are usually parallel to form a "larger capacity battery", and then connect the parallel cores in series, thereby reducing the complexity of the battery pack protection. In the battery industry, first specify the number of battery connected batteries, and then the number of battery connected by parallel. For example, in the Nissan Leaf electric vehicle, two soft bag batteries are connected in series, and then connected in parallel to form a "2S2P" battery module of about 7.5V and 488Wh. The last 48 modules are combined into a 360V battery Bag.

　　Under normal operating conditions, the chemical energy in lithium -ion batteries is converted into electrical energy, and the battery has limited heat production. When the battery cell exceeds its limitation (called a safe operation area (SOA)), the conversion of electrical energy will soon exceed control and generate a lot of heat. If the heat released exceeds the heat dissipation capacity of the battery shell and cooling system, irreversible heat loss will occur, which may lead to fire and explosion. However, in order to achieve as high energy density as possible, the battery cells are tightly stacked together in geometric shapes, which exacerbates this problem.

　　Therefore, the most important task of BMS is to provide safety functions, so that batteries in battery packs do not exceed the specified limit in terms of voltage, temperature, and current.

　　Due to the differences in manufacturing and aging, the capacity or characteristics of the cells will be small. In the service life of the battery pack, these differences may cause the imbalance between the batteries, one of which is full of charging, while the other does not. When charging or discharging, the total voltage of the battery pack is usually kept within its limit. However, when the battery voltage is not monitored, some battery cells may have exceeded their limit before reaching the battery pack limit and stopping the charging. Therefore, BMS should be able to monitor the battery voltage and take appropriate security measures. Instead, when the battery is monitored, the performance of the entire battery bag is determined by the weakest battery cell. BMS can provide a battery balancing function to overcome the small differences between the cells to a certain extent. In turn, this will improve the efficiency of battery packs (such as the mileage of electric vehicles) and life. Therefore, the second task of BMS is to extend or improve the life and efficiency of the battery.

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