Simple and easy to make 12V battery automatic charging circuit

The 12V battery automatic charging circuit will automatically monitor the battery voltage. When the battery voltage is lower than 11V, the circuit will automatically charge the battery until the battery is fully charged (about 14.4V to 14.7V).

At the same time, the charging circuit is automatically turned off after full charge, and the battery is charged again when the battery voltage is lower than 11V again.

Circuit principle description: See the circuit diagram. After the battery is connected to the circuit, because the base of Q1 is connected to capacitor Cl and the battery voltage cannot reach 14.7V, D1 and Q1 are cut off, R3 provides base current for Q2, Q2 is saturated and turned on, relay J1 is energized, and the city power is stepped down by transformer T and rectified by full bridge to charge the battery. When the battery voltage is fully charged, its voltage will reach about 14.7V, then D1 and Q1 are turned on, Q2 is cut off, and the contact of relay J1 is automatically disconnected, and charging is completed.

After charging, the battery voltage will gradually decrease over time after being discharged by the load. By choosing the resistance value of VR1 reasonably, even when the battery voltage drops to 12V, D1 is cut off, but due to the existence of R2, Q1 continues to maintain a saturated conduction state until the battery voltage continues to drop to the point where the current flowing to Ql through R2 is insufficient to support Ql conduction. Ql will be cut off immediately. At this time, Q2 is saturated and turned on, relay Jl is energized, and the AC power is stepped down by transformer T and rectified by the full bridge to charge the battery again.

The voltage regulator D1 is selected to be around 13.5V~14V, plus the voltage across R1 and the BE junction voltage of 01, it is just around 14.4V~14.7V, which is used as the shutdown voltage of charging monitoring. If there is no suitable voltage regulator, two voltage regulators can be used in series instead. VR1 is the charging start voltage adjustment potentiometer, and the charging start voltage can be adjusted to around 11V by adjusting VR1.

Component selection: Ql is C1815, β≥100Q2 is s8050, B≥100.