A: structure and lithium battery voltage, capacity of the composition of the way
Lithium battery can be divided into a cell and module and battery pack.
Batteries are usually there are three kinds of different shape: cylindrical, soft bag and square scale. Due to the limited available voltage window and energy capacity of batteries (for example, a cylindrical lithium-ion batteries of 3.7-4.2 V and 2000 ma), so they are by put batteries in series or parallel configurations to combine into a bigger battery.
Lithium ion battery is usually first in parallel, thus forming a "larger capacity battery, then put the batteries in series, in parallel to reduce the complexity of the battery pack to protect. In the battery industry, first specify the serial number of batteries, and then the number of batteries in parallel. In the nissan Leaf electric cars, for example, the two soft package of batteries together, and then parallel, form a 7.5 V and 488 wh "2 s2p" battery module, the last 48 such modules are combined into a 360 V battery pack.
2, BMS needs
Under normal operating conditions, chemical energy into electrical energy in lithium ion batteries, battery heat production co., LTD. When batteries beyond its limit (referred to as the safe operating area (SOA), electric energy conversion control and generate a large amount of heat is soon. If the heat released more than battery shell and the cooling system cooling capacity, occurs irreversible thermal runaway, could lead to fire and explosion. As much as possible in order to achieve high energy density, however, batteries with geometrical shape tightly stacked together, exacerbated this problem.
BMS, therefore, the most important task is to provide a safe function, make the battery in the battery pack in terms of voltage, temperature and current shall not exceed the specified limit.
Because of the difference of manufacturing and aging, the capacity of the batteries or features of the minute differences. In the service life of the battery pack, these differences may lead to imbalance between batteries, one of the batteries fully charged, and the other batteries is not. In charging or discharging, battery pack total pressure is usually stay within their limits, when batteries voltage has not been monitoring, however, before the battery pack limit and stop charging and discharging some batteries may be beyond their limits. So BMS should be able to monitor the batteries voltage and take appropriate safety measures. On the contrary, when the batteries are monitoring, the whole performance of the battery pack is determined by the weakest batteries. BMS can provide batteries balance function, to a certain extent, overcome the small differences between the batteries. This, in turn, will improve the efficiency of the use of the battery pack (such as electric car mileage), and longevity. So BMS the second task is to extend or improve the battery life and efficiency.
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