The importance of lithium battery and protection circuit
Source: Sai Microelectronics Co., Ltd. - Zeng anti
At present, the development of electric tools is miniaturized, light and cordless. In the application of portable electric tools, the use of nickel cadmium batteries is becoming more and more limited. Meanwhile, as the price of non-ferrous metals such as nickel and cobalt has increased exponentially, and the application and large-scale production of new materials for power lithium ion batteries, the cost gap between lithium ion batteries and Ni Cd and Ni MH batteries has been greatly reduced. These factors strongly promote and promote the application and development of lithium ion batteries in portable electric tools.
1 Overview
Lithium ion batteries must take into account the safety of charge and discharge, and the consistency between the individual cores, in order to prevent the deterioration of the characteristics. The lithium battery with high energy density to ensure the safety of the battery, the charging excessive state, the excess energy battery temperature rise, so the electrolyte decomposition and produce gas, the internal pressure rise and produce spontaneous combustion or rupture phenomenon; on the other hand, in a state of excessive discharge, causing the battery electrolyte due to decomposition characteristics and durability deterioration reduce the number of rechargeable. The change of temperature, especially at low temperature, will lead to the two more cases. Therefore, overcharge, over discharge, overcurrent, short circuit and temperature protection are very important for lithium batteries. Therefore, protective circuits are usually designed in battery pack to protect lithium batteries.
2 Analysis
2.1, when the battery voltage is too high and the corresponding overcharge protection is normal, the lithium battery is charged to 4.2 V, and at the same time, the charging current is <0.1 C or 0.05 C, which indicates that the battery is full. The 3000 mAh battery is filled with 3 V, and the capacity curve is shown as shown in Figure 1. The battery is divided into three groups, and the effect of the charge voltage on the battery life is investigated. The charging cut-off voltage is set to 4.200 V, 4.275 V and 4.350 V, respectively, and the discharge cut-off voltage is set to 3 V uniformly. The battery charge and discharge cycle is recorded under this condition, and the change of battery capacity is recorded. As shown in Figure 2.
It is shown that the initial discharge capacity of No. 3 battery is greater than that of L and 2 due to the increase of the charging voltage. However, after about 20 cycles, the battery capacity began to decline rapidly, and the 2 battery had similar performance. But due to the slightly lower charging cut-off point, the capacity of the battery began to decay after about 130 cycles. Therefore, the higher the charging voltage of the battery, the faster the life decline (or the attenuation rate).
Reference to the requirements for the remaining 80% after the 300 cycle of the battery, No. 2 and 3 are not satisfied. No. 1 can be achieved, that is, the charge voltage of the lithium battery should not be more than 4.2 V under normal conditions. In contrast, the overcharge protection is to limit the battery voltage to this range. Because more than that, it will certainly lead to a decline in battery life. And the higher the battery voltage, the greater the risk factor, more than 4.25 V in general need special vigilance, the risk of explosion. Therefore, the discharge depth of battery capacity and number 3 battery is 100% with the charging termination voltage, that is, the battery capacity is released every time. After the 250 cycle, the capacity of the battery attenuates rapidly, that is, the greater the depth of the discharge, the faster the battery attenuates.
In general, the minimum voltage can not be lower than 2.5 V or 2.4 V. When the battery voltage is less than 2.4 V, the battery will enter deep discharge. The deeper the lithium battery is, the shorter the charging life will be, and the shorter the service life of the battery will be.
In practical applications, deep discharge is usually controlled by setting off the protection termination voltage. The setting of over discharge protection termination voltage should be referred to the discharge current of the applied equipment and the application environment, but it should be higher than 2.4 V.
2.3, the experiment of large current discharge and over current protection for lithium battery is set up: using the same lithium battery to discharge at different discharge rates, and get the discharge capacity. As shown in Figure 5. Seen from the figure, with the increase of discharge rate, discharge capacity and discharge voltage platform has the battery discharge capacity of 2C decreased, and 3C were 89.6% and 68.4% of the discharge capacity of lC, 2C, 3C 3.6 V discharge capacity above the total capacity of 64.3% and 57.2%, has a great impact on battery discharge rate of discharge the capacity of the battery.