--- title: "Charging lead acid battery from AC source" subject: "Industrial Power Distribution and Illumination" chapter: 9 key: "semester_6/industrial_power_distribution_and_illumination:lead-acid-ac-charger" source: https://www.bctengineer.com/q/semester_6/industrial_power_distribution_and_illumination/lead-acid-ac-charger site: BCT Engineering --- # Charging lead acid battery from AC source Industrial Power Distribution and Illumination · Chapter 9: Emergency and Backup Supply Systems - Asked in: 2068 Magh (old course), 8 marks - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_6/industrial_power_distribution_and_illumination/lead-acid-ac-charger - Whole chapter: https://www.bctengineer.com/md/semester_6/industrial_power_distribution_and_illumination/chapter-9 ## Question What are the methods commonly adopted for charging a lead acid battery from an A.C. single phase source. What are the principle components of charger of the above type? ## Short answer Common methods include constant-current, constant-voltage, trickle, float, and boost charging using a step-down transformer and rectifier. Principal components are a fuse/switch, isolating transformer, diode/SCR bridge, smoothing choke/capacitor, current controller, meters, reverse-current protection, automatic cut-off, and cooling. ## Answer #### Methods of charging a lead-acid battery from a single-phase AC source Since the battery needs DC, the AC is first stepped down and rectified. The common methods are: 1. **Constant-current charging** using a transformer, rectifier and **variable series resistance** (or SCR control) to hold current at about C/10. Two-rate charging lowers the current when gassing starts. 2. **Constant-voltage charging** – rectifier output regulated at about 2.3–2.4 V/cell; current falls as the battery charges. A small series resistance limits the initial current (modified constant-voltage). 3. **Trickle charging** – small rectifier with high series resistance gives a tiny continuous current to standby batteries. 4. **Float charging** – a regulated rectifier connected in parallel with battery and load at ≈ 2.2 V/cell. 5. **Boost charging** – high-current rectifier for short periods. Rectifiers used: half-wave (small trickle chargers), **centre-tap full-wave**, or **bridge full-wave** (most common), with diodes or SCRs. ```text 230 V ─[F]─┬─────────┐ 1φ AC │ Step- │ ┌─D1─┬─D2─┐ │ down ├─┤ │ ├─[L]─[R]─(A)─┐ │ transf. │ └─D3─┴─D4─┘ +│ N ─────────┴─────────┘ │ [Battery] └───────────────−─┘ (V) across battery ``` #### Principal components of such a charger 1. **Fuse/MCB and ON-OFF switch** – protection on AC side. 2. **Step-down (isolating) transformer** – lowers 230 V to the needed level and isolates the battery from mains; tappings allow coarse voltage adjustment. 3. **Rectifier** – diode bridge (or SCR bridge for control) converting AC to DC. 4. **Filter / smoothing choke and capacitor** – reduces ripple in the charging current. 5. **Current-control device** – variable resistor, rheostat, SCR firing circuit or switch-mode regulator. 6. **Ammeter and voltmeter** – to monitor charging current and battery voltage. 7. **Reverse-current/polarity protection** – blocking diode so the battery cannot discharge back into the charger; reverse polarity alarm. 8. **Automatic cut-off or float controller** – switches to float/trickle mode at full charge to prevent overcharging. 9. **Cooling and enclosure** – heat sinks for rectifier, ventilated cabinet. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. How is constant-current charging achieved from a single-phase AC source? A. With a transformer, rectifier and variable series resistance or SCR control B. With a battery in parallel with a load C. With an autotransformer only, with no rectifier D. With a capacitor across the battery Answer: A. The AC is stepped down and rectified, and current is held at about C/10 by series resistance or SCR control. (Key point: Constant-current method) 2. Which rectifier is described as most common in these chargers? A. Half-wave rectifier B. Bridge full-wave rectifier C. Three-phase controlled rectifier D. Voltage doubler Answer: B. Half-wave is for small trickle chargers; bridge full-wave is the most common. (Key point: Rectifier type) 3. What does the step-down isolating transformer do in the charger? A. Increases the voltage to charge faster B. Smooths the ripple C. Lowers 230 V to the needed level and isolates the battery from mains D. Prevents reverse current Answer: C. It reduces the voltage and provides isolation between mains and battery. (Key point: Role of transformer) 4. What is the purpose of a blocking diode or reverse-current protection in the charger? A. To reduce ripple B. To raise the output voltage C. To measure charging current D. To prevent the battery discharging back into the charger Answer: D. If mains fails, the battery could otherwise discharge through the charger. (Key point: Blocking diode) 5. Why does a charger include an automatic cut-off or float controller? A. To switch to float/trickle mode at full charge and prevent overcharging B. To increase gassing C. To speed up the charging D. To adjust the AC supply frequency Answer: A. At full charge it reduces current to avoid overcharging and gassing. 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