Why can't a single-cell lithium battery protection board be used in series with multiple cells?

When using a single-cell lithium battery protection board to connect multiple cells in series, the following problems may occur.

1: Charging: Assume that a certain battery reaches the protection voltage of 4.2V first. For example, after the protection of the cout charging tube of the B protection board is activated, the internal resistance is infinite. At this time, the current is disconnected by this tube. Generally, the field effect tube on the single-cell lithium battery protection board has a very low withstand voltage, so it may be broken down (but due to the charging state, the charging voltage is subtracted from all battery voltages, and generally there will be no overvoltage phenomenon). In addition, after the protection of the B charging tube, the charging voltage will be added to the VDD end of the B protection board alone, which may cause overvoltage and damage the integrated block of the B protection board.

2: Discharge: Assuming that a battery reaches the 2.7V protection voltage first, for example, after the Dout charging tube of the A protection board is protected, the internal resistance is infinite. At this time, the current is disconnected by this tube. At this time, assuming that there are 6 batteries in the circuit, this tube will withstand a voltage of up to 25V, and the DOUT field effect tube in the green circle in the figure will be softly broken down. In this way, even if the protection is activated, there will be a few milliamperes. If it is completely damaged, a strong current will pass through and lose the protection function. In addition, after the protection of this tube, there will be a reverse voltage of up to 25V from the V- end of the A board to the VSS end of the A board, and there will be a reverse voltage of about 21V from the v- to the VDD end, which may completely cause damage to the chip.

Another point worth noting from the above figure is that the field effect tube is a bidirectional conductive device. Under normal circumstances, even without a driving voltage, the current can flow in the direction of the arrow in the icon. For example, when DOUT is at a high potential in the above figure, the current can flow in the opposite direction of the arrow. When DOUT is at a low potential, the current can only flow along the arrow, but the flow in the opposite direction is cut off. Therefore, DOUT is the discharge control terminal.

Under normal circumstances, even without high potential, the current can flow in the direction of the arrow, but there is a voltage drop of about 0.3V inside. Therefore, in order to eliminate the internal voltage drop, a high level is added, and the internal voltage drop in the direction of the arrow will be several millivolts to tens of millivolts. Therefore, the field effect tube is a bidirectional conduction control device.