Battery charger circuit and charging methods

Battery charger

A battery charger converts AC mains into controlled DC and drives current into a secondary battery in the direction opposite to discharge, restoring its chemical energy. A typical single-phase charger has:

1. Step-down transformer – reduces 230 V to a suitable level and isolates the battery from mains.
2. Rectifier – bridge of diodes or thyristors (SCRs) to convert AC to DC.
3. Filter/choke – smooths the DC current.
4. Regulator/control – series resistance, SCR phase control or switch-mode regulator to set constant current or constant voltage.
5. Protection and metering – fuse/MCB, reverse-polarity diode, ammeter, voltmeter, overcharge cut-off.

230 V ─►[Fuse]─►[Step-down ]─►[Bridge   ]─►[Choke/ ]─┐
  AC            [transformer]  [rectifier]  [regul. ] │
                                                  (A) │
                                    ┌──(V)──┐         │
                                    └─+Batt−┘◄────────┘

The SCR-controlled version measures battery voltage and current and adjusts the firing angle to give the required charging profile.

Method 1: Constant-current charging

• The charging current is held constant (about 1/10 of the Ah rating, e.g. 10 A for 100 Ah) by adjusting a series resistance or the rectifier output while battery voltage rises from about 2.0 to 2.6 V/cell.
• Charging time ≈ Ah to be restored / current, plus about 10–20% for losses.
• Two-rate variant: high current until gassing begins (~2.4 V/cell), then a lower finishing rate.
• Advantages: simple, accurate control of charge given. Disadvantage: slow; overcharge and gassing if not stopped.

Method 2: Constant-voltage charging

• A constant voltage of about 2.3–2.4 V per cell is applied. At first the current is high because the battery EMF is low; it falls automatically as the EMF rises, approaching a small value at full charge.
• A current-limit resistor or electronic limit protects against the initial surge.
• Advantages: faster charging (about 80% in a few hours), less risk of overcharging, no supervision.
• Used in automobiles and modern CC–CV chargers.

Const. current      Const. voltage
 I │──────┐          I │\
   │      │            │ \__
   │      └─ t         │    ‾‾───── t