How to connect the lithium battery of the protection board in series?

by:CTECHi     2021-08-10

The lithium battery protection board implements overcharge protection and recovery protection, overdischarge protection and overdischarge protection recovery control, charge and discharge overcurrent protection, short circuit protection, protection delay setting, temperature protection and other functions for the battery. The Chinese Chuangfa Warmly reminds everyone that a single-cell lithium battery protection board cannot be used in series with multiple cells. When using a single-cell lithium battery protection board for multiple cells in series, the following problems will occur:   1: Charging: Assuming that a certain battery reaches first 4.2V protection voltage. For example, after the protection of the cout charging tube of the B protection board, the internal resistance is infinite. At this time, the current is disconnected by this tube. Generally, the field effect tube on the protection board of a single-cell lithium battery has a very low withstand voltage, so It may be broken down (but due to the state of charge, the charging voltage is subtracted from all battery voltages, and there is generally no overvoltage phenomenon). In addition, after the B charging tube is protected, the charging voltage will be added to the VDD terminal of the B protection board. Overpressure may occur, causing damage to the integrated block of the B protection board. 2: Discharge: Assume that a certain battery reaches the 2.7V protection voltage first. For example, after the protection of the Dout charging tube on the A protection board, the internal resistance is infinite. At this time, the current is disconnected by this tube. At this time, it is assumed that there are 6 sections in the circuit. Battery, then this tube will withstand a voltage of up to 25V, the DOUT field effect tube in the green circle in the figure will be softly broken down, so even if the protection is activated, there will be a few milliamperes. If it is completely damaged, it will be very The strong current passes through, and the protection is lost. In addition, after the protection of this tube, a back pressure of up to 25V will appear from the V- terminal of the A board to the VSS terminal of the A board, and a back pressure of about 21V will also appear from the v- to the VDD terminal, which may completely cause the chip to be damaged. In addition, it is worth noting that the above figure is analyzed. The FET is a bidirectional device. Under normal circumstances, even if the driving voltage is not applied, the current can flow in the direction of the arrow in the icon. For example, when dout is high in the figure above, the current can be reversed. The arrow flows in the reverse direction. When DOUT is low, the current can only flow along the arrow, but the flow in the reverse arrow direction is blocked. Therefore DOUT is the discharge control terminal. Under normal circumstances, even if the high potential is not applied, it can flow in the direction of the arrow, but there is a pressure drop of about 0.3V. Therefore, in order to eliminate the internal pressure drop, add a high level, then the internal pressure drop along the arrow direction will almost be A few millivolts-tens of millivolts. So the FET is a bidirectional control device.

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