Backup power automatic charger circuit

Backup power supply is a temporary power supply equipment prepared for power outage or other reasons. Common backup power sources include small generators and various batteries. Among them, the economical and durable lead-acid battery is the first choice for backup power supply. After comprehensive consideration, the author designed an automatic charger for the lead-acid battery backup power supply . The circuit is as shown in the attached figure (the mains voltage reduction and rectification part is omitted). 
 

The core part of the circuit is the "hysteresis comparator" composed of NE555. R8, R9, RP1 and RP2 constitute the sampling circuit. LED 1-LED3 are charging status indicators. The charging of the battery is connected with a relay, making the on and off more reliable. S1 and S2 are touch switches that can be used to manually control the charging process, making the circuit more flexible and convenient.
The following focuses on the working principle, debugging method and installation process of the circuit.
1. Working principle
The core of this circuit is the NE555 time base circuit. When the battery is in an undervoltage state (such as 1OV), the voltage output by the sampling circuit is lower than the lower limit of the "hysteresis comparator" formed by the NE555. At this time, the ③ pin of NE555 outputs high level, VT1 is turned on, the relay is closed (connected to J-2), and the power supply charges the battery through R6; after a certain period of time, the battery voltage gradually increases with the charging process. When the high At the preset voltage value (such as 13.7V), the voltage output by the sampling circuit is higher than the upper limit of the "hysteresis comparator" formed by NE555. At this time, the ③ pin of NE555 outputs low level, VT1 is cut off, the relay is released (connected to J-1), and the power supply provides weak supplementary current to the battery through R7 and LED3. If the battery is discharged, when the battery voltage is lower than the set lower limit again, the circuit will switch to the charging state again. Therefore, there is a hysteresis in this process, which is what "hysteresis" means. When the battery voltage is between the upper and lower limits of hysteresis after sampling, no matter what state the circuit is in, pressing S1 will force the circuit to enter the charging state; pressing S2 will force the circuit to exit charging. This function is very meaningful for emergency replenishment and energy saving during idle time. According to the working principle, when LED2 lights up, it means that it is charging; when LED2 goes out and LED1 and LED3 light up, it means that the battery is in a normal state of charge. The brightness of LED3 can also be used to determine how much the battery is charged.
2. Debugging method
First, disconnect the circuit at the "X" mark, connect the power supply, and use a digital meter to measure the voltage at pin 5 of NE555 which should be around 8V. At this time, replace the battery with a standard regulated power supply and connect it to terminals a and b. Note that the voltage polarity is: a positive b negative. Adjust the standard regulated power supply voltage to the predetermined maximum value of the corresponding battery voltage (such as 13.7V), and then adjust RP2 so that the voltage output of the sliding terminal of RP2 is equal to the measured voltage value of pin ⑤ (such as 8V). Then reduce the standard regulated power supply voltage to the predetermined minimum value of the corresponding battery voltage (such as 10.5V), and adjust RP1 so that the output of the sliding terminal of RP1 is half of the voltage value of pin ⑤ (such as 4V). After the voltage is basically adjusted, slowly adjust the voltage of the standard regulated power supply from about 8V to 15V, and then slowly reduce it to 8V. At the same time, use a digital meter to track the voltages across terminals a and b. You will find that LED2 lights up first. When the voltage rises to about 13.7V, LED2 goes out and LED 1 lights up. When it drops to about 10.5V, the circuit flips back to the original state. When the voltage is between 10.5V and 13.7V, you can try pressing S1 or S2, and you will naturally understand its function, and finally connect the disconnected point.
3. Installation process
In order to ensure accurate and safe circuit operation, attention should be paid to the separate wiring of the charging circuit and the control circuit. The charging circuit has a large current, so short and thick wires should be used for connection; the control circuit should try not to "overlap" with the charging circuit. When making a printed circuit board, try to keep the ⑤ and ① pins of the NE555 close to the "ground" of the sampling circuit, and the ground wire should be wide and thick. Generally, as long as they are connected correctly, they can work normally. In the circuit, R6 is the current limiting resistor, which determines the size of the charging current. For a 12Ah battery, it is appropriate to adjust the charging current to 0.7A. RP1 and RP2 should use precision multi-turn potentiometers as much as possible.