Simple battery charging protection circuit

Here is a protection circuit for overvoltage when charging a +12V battery, as shown in the attached figure. The external charger circuit (not shown here) at the A and B ends in the figure charges the +12V battery. Q1, D2, and D1 in the figure form a protection circuit, where D1 (IN4743) is a voltage regulator (+13.5V), and D1 and D2 together form the bias circuit of the transistor Q1. R1 is the current limiting resistor of the Q1 tube.

The A and B ends of the circuit in the figure are the working load (LOAD) of the battery. The load can be disconnected when charging, and can be connected again after the charging is completed so that the power supply can work normally.

When the circuit is charged with an external charger, if the maximum output voltage of the charger is ≤14.2-14.4V, the +12V battery will slowly rise from the normal undervoltage (10.5V) state at the beginning of charging. When the battery voltage reaches the breakdown voltage of the voltage regulator tube D1 (+13.5V), the D1 tube begins to conduct, and the Q1 tube is also turned on, causing the voltage at the A and B terminals to drop. According to the circuit parameters in the figure, the maximum value of the voltage at the A and B terminals will not be greater than 14.4V. When the battery is fully charged, the protection circuit will put the battery in the eddy current charging state, which enables the battery to obtain the charging protection function.