Battery charging and protection circuit

     1. Li+ battery protection circuit

    Although lithium-ion (Li+) batteries have many advantages such as high energy density, long service life, no memory effect, low self-discharge, and high single-cell battery voltage, strict attention must be paid to overvoltage protection, overdischarge protection, and overvoltage protection when using them. Current protection, and the accuracy requirements for the protection circuit are also high. The circuit shown in Figure 1 is a complete Li+ battery protector composed of MAX1666. MAX1666S/V/X can provide protection for 2-cell/3-cell/4-cell Li+ battery packs respectively, including: overcharge protection, over-discharge protection, battery mismatch protection and over-current protection. The overvoltage detection function can also effectively prevent overcharging of any battery in the battery pack. When the battery voltage exceeds the set threshold, a high level is output, and the field effect transistors Q2 and Q3 are disconnected to terminate the charging process.Output low level to send an alarm signal to the battery pack controller. Battery maximum voltage thresholdDetermined by external resistors R1 and R2, its settable range is 4V~4.4V.

   The undervoltage detection circuit can be used to prevent the battery from over-discharging. When the battery voltage is detected to be lower than the set thresholdWhen, UVO, , outputs high level,Maintain low level, switch tubes Q3 and Q1 are open circuit, Q2 is turned on, and the battery is in trickle charging state, allowing the battery voltage to be restored. Battery low voltage detection thresholdIt can be set through R3 and R4, and the setting range is 2V~3V.

    When any two cells in the battery pack are mismatched,, , U-VO and are all high level, and the corresponding switch tubes Q1~Q3 are all disconnected.is low to send an interrupt signal to the controller. Adaptation voltage detection thresholdIt can be set by R5 and R6.

    2. Compact Li+ battery charger

   Lithium-ion batteries (Li+) are widely used in mobile phones, PDAs, notebook computers and other products due to their high energy density and high performance. The circuit shown in Figure 2 is a compact single-cell Li+ battery charging circuit. The wall adapter in the picture is a 6VDC/800mA current-limiting voltage source. The MAX1679 has a built-in charge termination detection circuit and charging process controller. Insert the battery or supply Whenever the charger is powered on, it will start a charging process. A complete charging process includes: ① Initial charging process; during this process, the system charges the battery with a small charging current, so that the battery voltage is greater than 2.5V at the usage rate. If the temperature range exceeds 2.5℃ to 47.5℃, the charger is in waiting state. ② Fast charging process; when fast charging starts, MAX1679 turns on the external P-channel field effect transistor, and the fast charging current is determined by the external current-limiting charging power supply. Once the system detects that the battery voltage reaches the Li+ battery charge termination threshold voltage, fast charging ends. The charge termination threshold voltage is determined by the resistorSure. ③Pulse charging process; after the fast charging process, the system enters the pulse charging process. MAX1679 detects the battery voltage every 2ms. When the battery voltage is less than the termination threshold, the external P-channel field effect transistor is turned on. When the battery voltage is greater than the termination threshold, P The communication channel field effect transistor (P1) is disconnected. When the pulse charging process is near the end, the disconnection time of the P channel field effect transistor is much longer than the on time.

    In addition to battery voltage detection, the MAX1679 also has timeout detection and temperature detection functions, which can provide secondary protection for the battery. When it is necessary to detect the temperature, a thermistor should be connected between the THERM pin and the BATT- pin, and the thermistor should be installed close to the battery. When the TSEL pin is connected to BATT+, ADJ or GND, the corresponding charging limit times are 2.8 hours, 3.75 hours and 6.25 hours. When selecting P1, parameters such as drain-source breakdown voltage, minimum conduction threshold voltage, rated current, and power loss should be considered. The drain-source breakdown voltage must be at least 25% higher than the wall adapter voltage in the open circuit state.

    The LED in Figure 2 is used to indicate the charging status. The corresponding relationship between the LED flashing frequency and the charging status is listed in Table 1.

     3. Nicd battery charger

    Nickel-cadmium (Nicd) batteries are the earliest used rechargeable batteries. Their energy density and weight density are relatively low. However, due to their low cost, they are still used in many products, such as cordless phones, portable meters, etc. Nicd battery charge termination detection methods generally use -△V detection, timeout detection, battery temperature detection and battery temperature rise rate detection. In the fast charging mode, -△V detection is usually used in conjunction with timeout and temperature detection.

    Figure 3 shows a Nicd battery stand-alone charger using MAX713, which can charge 1 to 16 batteries. In the figure, MAX713 has a built-in charge termination detection algorithm (-△V detection, timeout detection, battery temperature detection). After fast charging, it can automatically switch to trickle charging to supplement the self-discharge of the Nicd battery. DCIN is connected to the output of the wall adapter. The minimum output voltage of the wall adapter should be higher than: 2V+ (1.9V×number of battery cells); the maximum voltage depends on the breakdown voltage of the P-channel MOSFET and the withstand voltage value of the input bypass capacitor. . When DCIN is lower than 15V, the DRV pin of MAX713 is directly connected to Q1 and Q2; when DCIN is higher than 15V, it is connected to Q3; R4 is used to provide appropriate voltage swing for Q1 and Q2. Current-limiting diode D4 extends the input voltage range and provides a fixed 8mA current to the MAX713 internal shunt regulator. If the wall adapter provides a narrower output voltage range, the resistor can be used in place of D4. The required charging current can be calculated based on the required charging rate and battery capacity., by the formulaDeterminable current limiting resistorthe size of. Changing the connection method of PGM0~PGM4 can set the number of battery cells and charging limit time (refer to Table 2 and Table 3 ). In Figure 3, the inductance of L1 is 220μH, and the saturation current is 1.5A. This circuit does not have strict requirements on the inductance value. The larger the inductance value, the smaller the current ripple.

    When charging a Nicd battery, incomplete discharge will cause the cadmium anode of the battery to turn into cadmium oxide, causing the battery terminal voltage to drop. In order to eliminate the memory effect of the battery, the preprocessing circuit in the dotted box in Figure 3 can make the battery Completely discharged. If MAX712 is used instead of MAX713 and the circuit in the dotted box is removed, Figure 3 can also be used for charging NiMH batteries.