Electric vehicle battery charging protection circuit

This protection circuit can not only prevent undercharging and overcharging, but also automatically cut off the power supply if the charger does not switch to green light for ten hours to prevent serious errors. It can also automatically record the charging time with a quartz clock. In this way, you can understand the change of battery capacity and judge whether the charger does not switch to green light, so as to decide whether to send it to a repair center for processing.

The circuit principle is shown in the figure below. Press the AN button to energize the circuit. At the beginning, Q10 and Q13 of CD4060 are both 0V, and the outputs of LJ2C and LJ2D NAND gates of CD4011 are high level, so Q2 transistor 8050 is turned on, J is attracted and self-held, and the charger starts charging.

555 generates a pulse with a period of 8.789 seconds as a timing reference and adds it to CD4060 for counting. When charging, the voltage of about 1.8V taken from the red light of the charger is added to the base of transistor Q1 through the plug to turn it on. The output of U12A of CD4011 is high level and the output of U2B is low level. Therefore, if the red light is always on, even if Q10 becomes high level after counting 512 pulses, the relay J will not be disconnected. Only when Q13 becomes high level L after counting 4096 pulses (10 hours); 2C outputs 0V, Q2 is turned off, relay J is powered off and charging stops, and the circuit itself stops working. If the red light turns green after charging for a period of time, the voltage on the base of Q1 is less than the voltage required for conduction, so it is turned off, its output becomes high level, U2A output becomes low level, and U2B output becomes high level. At the same time, the rising edge of U2B is added to the RST reset terminal of CD4060 through capacitor C3, so that it starts counting from zero. When the count reaches 512 pulses (1 hour and 15 minutes), Q10 becomes high level, U2D output becomes 0V, Q2 is cut off, J is powered off, and the charging stop circuit also stops working. This plays the role of preventing under-charging and over-charging, and automatically shutting off the power if the green light does not change for more than ten hours, so it can be called a charging protector.

When LED1 is on, it means the red light voltage of the charger has been taken out and connected normally. When LED2 is on every few seconds, it means the 555 is working normally. It is used to observe the pulse cycle during debugging. When LED3 is on, it means the relay is energized and in the charging state. And the voltage of about 1.8V taken from it can be used as the power supply of the quartz clock (remove the 1.5V battery), so that it can record the time taken for charging. When debugging, first temporarily connect the 12th foot of LJ2D to the 7th foot Q4 of CD4060, without connecting the red light voltage of the charger, and see if the relay can be disconnected normally when LED2 is on for 9 times (8 cycles). If there is a "pop" sound but it is not disconnected, it is necessary to connect a capacitor of about 0.01μ from the 12th foot of CD4060 to the ground to eliminate the erroneous reset pulse generated by electromagnetic interference on the 12th foot when the relay is disconnected. The erroneous reset will cause the transistor Q2 to be turned on immediately after disconnection because Q10 and Q13 become 0V, and the relay is energized again. The capacitor should not be too large, otherwise it will affect the effect of normal reset, and it cannot be guaranteed that the counting starts from zero when the green light is on. C3 must not have leakage, and can use a 1u monolithic capacitor or a glass glaze capacitor.

Adjust RPl so that the 8 oscillation cycles of 555 are about 70 seconds. After adjusting, connect the 12th foot of CD4011 NAND gate U2D to the 15th foot Q10 of CD4060 and it will be successful.

For a charger with a constant current charge of 1.8A, 18AH can be charged in 10 hours. For batteries within 14AH, even if charging starts from the lowest allowable voltage, considering the efficiency of electrochemical conversion, 14×1_2=16.8AH needs to be charged, so the green light will be on in no more than 10 hours. It is reasonable to set 10 hours as the protection time. If the original charger is still used and replaced with a large-capacity battery such as 20AH, charging from the lowest allowable voltage and considering the conversion efficiency, the green light will be on for more than 10 hours. If 10 hours is still used as the protection time, undercharging will occur. This requires adjusting RP to lengthen the pulse period of 555 so that the protection time is greater than the time required to light the green light. For example, adjust the relay to cut off power 1 hour and 30 minutes after the green light turns on.

The protection power-off time will automatically become 8×1.5=12 hours. Using the charging protector, you can charge without any supervision, extend the battery life and save electricity.