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The UCC3957 lithium-ion battery charging and discharging control chip can provide protection against overcharging, discharging, and overcurrent for 3 or 4 lithium-ion battery packs. Specifically, the chip samples the voltage of each battery in the battery pack and compares it with the internal precision reference voltage. When any battery cell is in an overvoltage or undervoltage state, the chip will perform corresponding control to prevent further charging or discharging. Its typical application circuit is shown in Figure 8. In the figure, Q1 and Q2 are P-channel MOSFET tubes, which respectively control the charging and discharging currents.

Principle diagram of 3.7V lithium battery protection board

(1) Connection of battery pack

Pay attention to the sequence when connecting the battery pack to the IC. The bottom end of the battery pack is connected to the AN4 end of UCC3957 (U1), and the top end is connected to the VDD end. The connection points of every two batteries are connected to the AN1 to AN3 ends in corresponding order.

When the battery pack consists of 3 batteries, pin ② (CLCNT end) of U1 is connected to pin 16 (DVDD end), and pin ⑥ (AN3 end) is connected to pin ⑦ (AN4 end); When the battery pack consists of 4 batteries, pin ② is grounded (i.e. connected to the AN4 terminal).

(2) Discharge

U1 has intelligent discharge function. When discharging, pin 13 of U1 outputs a low level, and the discharge switch Q2 conducts. The lithium battery pack supplies power to the load through the diodes in Q2 and Q1. When the required current for the load is high, the voltage drop at both ends of the current detection resistor RS is also high. When it exceeds 15mV (corresponding to a discharge current of 0.6A), pin ③ of U1 outputs a low level, and the charging switch Q1 conducts, thereby improving the discharge capacity of the battery pack.

(3) Undervoltage protection

When any battery is detected to be over discharged (below the undervoltage threshold), pins ③ and 13 of U1 output a high level, while turning off Q1, Q2 and U1 enter sleep mode. At this time, the working current of the chip is only 3.5 μ A. Only when the voltage of pin ③ rises to VDD, the chip will exit sleep mode after detecting it.

(4) Charging

When the charger is connected, switch S1 is closed, pin 9 (CHGEN end) of U1 is connected to pin 16 (DVDD), pin 3 of U1 outputs a low level, charging switch Q1 is connected, and the battery pack is charged.

During charging, if U1 is in a sleep state, the discharge switch Q2 is still turned off, and the charging current flows through the diode inside Q2 to charge the battery pack. When the voltage of each battery is higher than the undervoltage ON value, Q2 conducts.

(5) Overcurrent protection

In order to adapt to large capacitive loads, UCC3957 has two overcurrent threshold voltages, and each threshold voltage can be set with different delay times, that is, using a two-stage overcurrent protection mode. This type of secondary overcurrent protection can provide fast response to short circuits and also withstand certain surge currents in the battery pack to prevent unnecessary overcurrent protection actions caused by large filtering capacitor capacity.

The current detection resistor RS is connected between pin ⑦ (AN4) and pin ⑧ (BATLO) of U1. When the pressure drop at both ends of RS exceeds a certain threshold, overcurrent protection enters intermittent mode. In this mode, the discharge switch tube Q2 turns off and on periodically until the fault is eliminated. Once the fault is resolved, the chip automatically returns to its normal working state.

The first level overcurrent protection threshold is 0.15V (corresponding to an output current of 6A), and the duration exceeds the time set by U1 (set by the capacitor C4 between U1's pin 10 (CDLY1) and ground). If U1 enters intermittent operation mode, the duty cycle of its output pulse is about 6%, that is, the turn off time of the switch is about 16 times the conduction time.

The second level overcurrent threshold is 0.375V (corresponding to an output current of 15A), and the duration exceeds the time set by U1 (set by the capacitor C3 between U1's 14 pins (CDLY2) and ground). If U1 enters intermittent operation mode, the duty cycle of its output pulse is less than 1%, that is, the turn off time of the switch is about 100 times the conduction time.

(6) Overvoltage protection

If the charging voltage of a certain battery exceeds the charging threshold, pin ③ of U1 outputs a high level, and the charging switch Q1 is turned off, entering an overvoltage protection state.

In addition, if the connection between the battery pack and pins ④ to ⑥ (AN1-AN3) of U1 is broken, U1 will also enter overvoltage protection state.

Circuit diagram of lithium battery 3.7V protection board modification

Currently, domestic lithium batteries have varying levels of quality at 3.7V, with a discharge voltage of around 2.8V being the limit. If it reaches 2.5V, a good battery can still be charged several times. Ordinary batteries are basically scrapped. The protective board I bought has two types of chips (DW01 and 8205A): DW01 sampling chip and 8205A power driver chip. DW01 sampling: Overdischarge voltage is between 2.35v~2.5v, and overshoot is between 4.0v~4.19v. If you buy the protective board for these two chips, domestic lithium batteries with a voltage of 3.7V will be basically scrapped, and even those without scrapping cannot be charged a few times.

Solution:

① A 1N5822 (Schottky diode) is connected in parallel at the head and tail, with a forward voltage of 0.52v. Adding 2.35v equals 2.87v.

② 1N5822 is connected in series to B+or B-pole.

③ Then connect the 3.7V battery in series to B or+B -, and even though the protection board is 2.35V, the actual battery voltage is 2.87V. Effectively protect over discharge.

Explanation: Batteries cannot be soldered onto protective plates. If soldered, there will be a 0.52V loss during charging. The best method is to not solder and use a standard 3.7V charger for charging, which can fully charge the battery without over discharging during use.

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