Lead-acid battery overcharge and over-discharge protection circuit

Commercially available emergency lights all have overcharge protection, but often do not have discharge protection. If a lead-acid battery is over-discharged, the lead sulfate crystals will form larger bodies, which not only increases the resistance of the plates, but also makes it difficult to restore them during charging, directly affecting the capacity and life of the battery. The circuit provided here can protect the battery from overcharge and over-discharge. When the AC power supply is valid, the load is powered by the regulator; if the AC power supply fails, the load automatically switches to the 6V battery. When the AC power supply is restored, the load is powered by the regulator again, and the battery begins to charge.

The entire circuit can be divided into four parts: power supply, switching circuit, over-discharge protection circuit and over-charge protection circuit.

The AC power supplied by the power supply is stepped down by transformer X1, rectified by bridge rectifier BR1, filtered by capacitor C1, and then stabilized into 6V power supply by chip 7806 (IC1).

The switching circuit is composed of transistor T1 and relay RL1. When the AC power supply is valid, T1 is turned on, RL1 (6V, 100Ω) is pulled in, and the green LED1 lights up, indicating that the AC power supply is valid. At the same time, the output of the regulator is connected to the load through the normally open contact N/O of RL1 and the normally closed contact N/C of RL3, and the 6V battery starts to charge through the normally closed contact N/C of RL2.

When the AC power fails, T1 is turned off and RL1 is released. As a result, the load is powered by the normally closed contact N/C of RL1 from the 6V battery. At this time, LED1 turns off, indicating that the AC power no longer exists.

The over-discharge protection circuit of the battery is composed of components such as IC3, T3, and RL3. When the battery is over-discharged (lower than 5.5v), the voltage on the inverting input terminal (2) of IC3 is higher than the voltage on its non-inverting input terminal (3). At this time, the output of IC3 is low, T3 is turned on, RL3 (5V, 100Ω is pulled in, and the load is disconnected from the 6V battery due to the separation of the contacts N/C, thus avoiding over-discharge. At the same time, LED3 lights up, indicating that the battery is in an over-discharge state. When the AC power is restored, the battery starts to charge through the contacts N/C of RJ2. When the battery voltage reaches 5.5v, the output of IC3 returns to a high potential, T3 is turned off, RL3 is released, and the load is connected to the output of the regulator again.

The battery overcharge protection circuit is composed of IC2, T2, RL2 and other components. When the AC power supply is effective and the battery voltage is lower than 6.6V, because the voltage on the inverting input terminal ② of IC2 is higher than the voltage on the non-inverting input terminal ③, the output of IC2 is low, T2 is cut off, and RL2 (6V. 100Ω) remains in the released state. At this time, the battery continues to charge through the N/C contact of RL2. Once the battery voltage reaches 6.6V, the output of IC2 becomes high, T2 is turned on, RJ2 is closed, and charging stops. At this time, LED2 lights up, indicating that the battery is in an overcharged state. In the figure, D2 and D3 protect RL2 and RL3 respectively from the impact of back electromotive force.