Sealed Lead Acid Battery Charger Design Example

1.Maintenance charging :

When the battery voltage is low (can be set, this circuit is preset to below 9V), the charger works in a low current maintenance charging state. The working principle is that the potential of U1C pin 9 (in-phase end) is lower than that of pin 8 (inverting end), U1C outputs a low potential, T4 is cut off. The potential of U1D pin 11 is about 0.18V. At this time, the charging current is about 250mA (the constant current circuit is composed of R14, U1D, and T1B peripheral circuits. Readers are requested to analyze the constant current principle by themselves).

2. Fast charging:

As the maintenance charging continues, the battery voltage gradually increases. When the battery voltage exceeds 9V, the charger switches to high-current fast charging mode. The potential of U1C pin 9 (in-phase end) is higher than that of pin 8 (inverting end). U1C outputs a high potential, T4 is turned on, and the potential of U1D pin 11 is about 0.48V. The charger outputs a constant current of about 1A to charge the battery.

3. Pressure-limited float charge:

When the battery is nearly fully charged, the charger automatically switches to the limited voltage floating charge state (the limited voltage floating charge voltage is set to 13.8V, if it is a 6V battery, the floating charge voltage should be set to 6.9V). At this time, the charging current will gradually decrease from the fast charging state. When the battery is fully charged, the charging current is only 10~30mA to replenish the battery power lost due to self-discharge.

4. Protection and charging indication circuit:

This circuit is equipped with a reverse polarity protection circuit, which is composed of D4, U1C, U1D, T1 and peripheral components. When the battery is reversely connected, the charger limits the output current to prevent accidents (the reader is requested to analyze the principle by himself, or email the author). The charging indicator is composed of U1A, D7 and peripheral components. When charging, D7 lights up, and when the charger enters the floating charge state, D7 goes out, indicating that charging is completed.

5. This circuit can adjust the charging current and floating charge voltage arbitrarily by slightly modifying the circuit parameters to meet the needs of batteries of different specifications.

6. The bill of materials is as follows

Note: CF = Carbon Film Resistor; MF = Metal Film Resistor; MOF = Metal Oxide Film Resistor

* indicates components that can be adjusted as needed.
In view of this, a two-stage constant current voltage-limited lead-acid battery charger was designed and manufactured.

7. The measured charging curve of the charger is as shown below.