The following problems occur when using a single-cell lithium battery protection board for multi-section series use.

1. Charging: Assume that a certain battery first reaches 4.2V protection voltage. For example, after the B protection board cout charging tube protection action, the internal resistance is infinite. At this time, the current is disconnected by this tube. Generally, the single-cell lithium battery protection board The FET voltage is very low, so it may be broken down (but due to the state of charge, the charging voltage is reduced by all the battery voltage, generally no overpressure phenomenon), and after the B charging tube is protected, the charging voltage will be single. Adding to the VDD terminal of the B protection board may cause overvoltage, which may cause damage to the B protection board integrated block.

2. Discharge: Assume that a certain battery first reaches the protection voltage of 2.7V. For example, after the protective circuit of the Dout charging tube of 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 the tube will withstand up to 25V, the DOUT FET in the green circle will be soft-punched, so even if the protection action, there will be a few milliamps, if it is completely damaged, there will be very A strong current passes through and loses protection. In addition, after the protection of this tube, the V-terminus of the A board has a back pressure of up to 25V on the VSS end of the A board, and the back pressure of the V-to VDD end also appears around 21V, which may completely cause the chip to be damaged.

In addition, it is worth noting that the FET is a double-conducting device. Under normal conditions, even if no driving voltage is applied, the current can flow in the direction of the arrow inside the icon. For example, when dout is high in the above figure, the current can be reversed. The arrow flows in the opposite direction. When DOUT is low, the current can only flow along the arrow, but the flow in the direction of the reverse arrow is cut off. Therefore DOUT is the discharge control terminal.

Under normal circumstances, even if the potential is not increased, it 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 and add a high level, the internal pressure drop along the direction of the arrow will almost A few millivolts - tens of millivolts. So the FET is a two-way control device.