How to ensure overcurrent protection for lithium battery

Lithium battery charge and discharge control chip UCC3957 can provide protection on overcharge, over discharge and over current for 3 or 4 lithium battery packs. Specifically, chip samples each battery voltage in battery pack and internally precision reference voltage compared. When any battery is under overvoltage or under voltage, the chip will controlled accordingly to prevent further charging or discharging. The typical application circuit shown in Figure 8. In figure, Q1 and Q2 are P-channel MOSFETs that control the charging and discharging currents, respectively.

3.7v lithium battery protection on board schematic

(1) Battery pack connection

Pay attention to the order of the battery pack and IC connection. The bottom of the battery pack connected to the AN4 end of the UCC3957 (U1), and the top end connected to the VDD terminal. The connection point of each two batteries connected to the AN1 to AN3 terminals in the corresponding order.

When the battery pack is 3 batteries, the 2 pin (CLCNT end) of U1 is connected to 16 pin (DVDD end), and 6 pin (AN3 end) is connected with 7 pin (AN4 end); when the battery pack is 4 batteries When the 2 feet are grounded (ie connected to the AN4 end).

(2) Discharge

U1 has a smart discharge function. When discharging, the 13 pin of U1 outputs a low level, the discharge switch Q2 turned on, and the lithium battery pack supplies power to the load via diodes in Q2 and Q1. When the current required by the load is large, the voltage drop across the current detecting resistor RS is also large. When it exceeds 15mV (corresponding to the discharge current of 0.6A), the 3 pin of U1 outputs a low level, and the charging switch Q1 leads. Pass, thereby improving the discharge capacity of the battery pack.

(3) Under voltage protection

When it is detected that any battery is over-discharged (below the under-voltage threshold), the 3 pin and pin 13 of U1 output a high level, and at the same time, Q1 is turned off, Q2 and U1 enter a sleep state, and the operating current of the chip is only It is 3.5 μA. Only when the voltage of pin 3 rises to VDD, the chip will not exit the sleep state after it detected.

(4) Charging

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

During charging, if U1 is in a sleep state, the discharge switch Q2 still turned off, and the charging current charges the battery pack through the diode in Q2. When the voltage of each battery is higher than the under voltage ON value, Q2 is turned on.

(5) Overcurrent protection

To accommodate large capacitive loads, the UCC3957 features two overcurrent threshold voltages, each of which can be set to a different delay time, using a two-stage overcurrent protection mode. This two-stage overcurrent protection not only provides a fast response to the short circuit, but also allows the battery pack to withstand a certain inrush current to prevent unnecessary overcurrent protection due to the large capacity of the filter capacitor.

The current sense resistor RS is connected between pin 7 (AN4) and pin 8 (BATLO) of U1. When the voltage drop across the RS exceeds a certain threshold, the overcurrent protection enters the intermittent mode. In this mode, the discharge switch Q2 periodically turned off and on until the fault removed. Once the fault removed, the chip automatically returns to normal operation.

First level of overcurrent protection threshold is 0.15V, (corresponding output current is 6A). Duration exceeds time set by U1 (set by U1's 10 pin (CDLY1) and ground capacitance C4), then U1 Entering intermittent mode, duty cycle of output pulse is about 6%, that is, the off time of the switch is about 16 times of the on time.

The second-stage overcurrent threshold is 0.375V, (corresponding output current is 15A). the duration exceeds the time set by U1 (set by the 14-pin (CDLY2) of U1 and the capacitance C3 between ground), then U1 enters In the intermittent mode of operation, the duty cycle of the output pulse is less than 1%, that is, the turn-off time of the switch is approximately 100 times the on-time.

(6) Overvoltage protection

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

In addition, if the battery pack disconnected from the 4 to 6 pins (AN1-AN3) of U1, U1 will also enter the overvoltage protection state.

Lithium battery 3.7v protection on board modification circuit diagram

Now domestic lithium battery, 3.7v is not good, the discharge voltage is about 2.8v the limit, if it is 2.5v, good battery can charge several times. The general battery scrapped. I bought the protection board; there are two kinds of chips (DW01, 8205A), DW01 sampling chip, and 8205A power driver chip. DW01 sampling: over-discharge voltage is 2.35v~2.5v, overshooting 4.0v~4.19v. If you buy the protection board of these two chips, the domestic lithium battery 3.7v scrapped, and it can't be charged several times without scrapping.

Solution:

1 head and tail parallel 1N5822 (Scotty diode), 1N5822 forward voltage 0.52v, plus 2.35v equals 2.87v.

21N5822 connected in series to the B+ or B-pole.

3 Then connect the 3.7v battery in series on B or +B-, then the protection board is at 2.35v but the actual battery voltage is 2.87v. Effective protection against over-discharge.

Noted: battery cannot soldered on the protection board. If it soldered, the battery will lose 0.52v when charging. The best way is to not solder, use the standard 3.7V charger to charge, so that the battery can fully charge, and it will not over-discharge when used

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