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The problem
Most small chargers sold to students are single-stage trickle chargers or raw unregulated adapters. A trickle charger left on too long overcharges the battery — gassing, water loss and warped plates. A dumb adapter undercharges it — and chronic undercharging is what sulfates plates and silently kills batteries that would otherwise last years. Proper charging needs three distinct stages: bulk to refill fast at constant current, absorption to hold ~14.4 V until the battery is genuinely full, and float at ~13.8 V to compensate self-discharge without cooking the cells. This project is a working mains-powered charger that does exactly that on an Arduino Nano controller. A 12-0-12 V centre-tapped transformer, bridge rectifier and filter bank produce the raw DC; the controller reads battery voltage and charge current, drives a pass MOSFET through the three stages, and opens a cut-off relay when absorption is complete. The design includes reverse-polarity protection and an over-temperature cut-out on the heatsink — the kind of protection thinking examiners probe in the viva. Everything is built openly: transformer, rectifier, regulator, sensing, relay logic and firmware, so the student can explain each block and its numbers.
12-0-12 V centre-tapped 3 A transformer steps mains down for the charger.
Follow the working sequence
How it works
- 230 V AC steps down through the 12-0-12 V centre-tapped transformer (3 A rated); both secondaries feed the KBPC bridge rectifier and the 2×2200 µF / 35 V filter capacitors, producing unregulated DC.
- The Arduino Nano reads battery voltage through a resistor divider and charge current through a low-side sense resistor roughly every 200 ms.
- BULK stage: while the battery is below ~14.4 V, the firmware holds the pass MOSFET on at the current-limited set-point (~1.5–2 A design target) for fast charging.
- ABSORB stage: at ~14.4 V the controller holds voltage constant; when the charge current tapers below the ~0.25 A design target, the battery is declared full and the cut-off relay opens.
- FLOAT stage: after cut-off, the controller maintains the battery at ~13.8 V through the maintenance path, compensating self-discharge without overcharging.
- The 16×2 LCD refreshes every second with voltage, current and stage; the reverse-polarity fuse and heatsink thermistor guard the power stage throughout.
Tech stack:
- Arduino Nano (ATmega328P)
- 12-0-12 V, 3 A centre-tapped transformer
- KBPC bridge rectifier · 2×2200 µF filter bank
- IRFZ44N N-MOSFET pass device on aluminium heatsink
- Resistor-divider voltage sense · low-side current sense
- 16×2 character LCD (I2C backpack)
- 12 V, 10 A cut-off relay
- Embedded C++ (Arduino framework)
| Parameter | Value |
|---|---|
| Input mains | 230 V AC, 50 Hz (design target operating range 210–250 V) |
| Transformer | 12-0-12 V, 3 A, centre-tapped (manufacturer rating) |
| Rectifier | KBPC3510, 35 A / 1000 V max (datasheet) |
| Battery type | 12 V sealed lead-acid, 4–14 Ah recommended (design target) |
| Bulk charge current | 1.5–2 A, constant current (firmware set-point, design target) |
| Absorption voltage | 14.4 V hold, cut-off on current taper below ~0.25 A (design targets) |
| Float voltage | 13.6–13.8 V (design target) |
| Cut-off relay | 12 V coil, 10 A contacts (datasheet) |
| Display | 16×2 character LCD: voltage, current, stage (component spec) |
| Over-temperature cut-out | Heatsink thermistor, ~75 °C trip (design target) |
Project features
- [Three-stage charging state machine] Bulk (constant current), absorption (constant voltage ~14.4 V design target with current-taper detection), float (~13.8 V design target) — implemented in Arduino firmware with documented transition thresholds.
- [Auto cut-off relay] A 10 A relay disconnects the battery from the charging path when the absorption criteria are met; the controller re-engages charging if the battery voltage sags below the reconnect threshold.
- [Live voltage & current display] 16×2 LCD shows measured battery voltage, charge current and the active stage (BULK / ABSORB / FLOAT), updated every second.
- [Transformer-isolated power stage] 12-0-12 V, 3 A centre-tapped transformer (manufacturer rating) feeding a KBPC-series bridge rectifier and a 2×2200 µF / 35 V filter bank.
- [Current-limited bulk output] Charging current is limited by the current-sense resistor and firmware set-point, design target ~1.5–2 A, suitable for 4–14 Ah sealed lead-acid batteries.
- [Reverse-polarity protection] Series fuse plus polarity-guard relay wiring so a reversed battery cannot drive current back into the controller or the power stage.
- [Over-temperature cut-out] Thermistor on the pass-transistor heatsink pauses charging above the design limit (~75 °C design target) until the heatsink cools.
- [Documented calibration procedure] Step-by-step charge-cycle procedure using a multimeter and a discharged battery, run during your calibration to trim the firmware set-points.
What is included
- Working charger prototype (transformer, controller board, cut-off relay, LCD, battery leads)
- Arduino firmware source with the documented three-stage state machine and trimmable set-points
- Circuit schematic and wiring diagram (PDF)
- Charge-cycle calibration procedure (multimeter + discharged battery steps)
- Project report PDF (background, rectifier/regulator theory, methodology, design targets)
- PPT presentation for final review
- Viva Q&A preparation document (rectification, three-stage charging, sensing, relay logic, protection)
Limitations & prerequisites
- 12 V lead-acid only (flooded, AGM or gel) — NOT for Li-ion, LiFePO4 or 24 V systems; those need different voltage profiles and chemistries.
- All stage voltages and the 1.5–2 A current limit are design targets set during the build; no charge curve was measured on this exact unit before delivery — the buyer runs the documented calibration procedure with their own battery.
- Efficiency is a design estimate (~75–80% for the transformer stage), not a measured claim.
- Large automotive batteries (>40 Ah) would charge extremely slowly; this design targets 4–14 Ah packs — a bigger transformer is a different design.
- The firmware does not attempt recovery of batteries below ~9 V terminal voltage; deeply sulfated batteries need bench recovery first.
- The 230 V mains side must be used with the isolating enclosure and fuse as documented; it is not touch-safe when open.
Frequently Asked Questions
Why three stages instead of a fixed charging voltage?
Because a battery is not a resistor. Bulk at constant current refills fast; absorption holds ~14.4 V until the current tapers so the battery is genuinely full; float at ~13.8 V only compensates self-discharge. A single fixed voltage either undercharges (sulfation) or overcharges (gassing, water loss).
How does the auto cut-off actually work?
During absorption the controller holds ~14.4 V and watches the charge current. When it tapers below the ~0.25 A design threshold, the firmware declares the battery full and opens the cut-off relay. If the battery later sags below the reconnect threshold, charging restarts.
Can it charge my car battery?
Physically it connects and charges, but a car battery is 35–45 Ah and this design targets 4–14 Ah — it would take very long. Bigger batteries need a bigger transformer and a revised current set-point.
What happens if I connect the battery backwards?
The series fuse and polarity-guard relay wiring prevent any reverse current from reaching the controller or the power stage — the fault shows up as zero current on the LCD.
Can I trim the voltage set-points?
Yes — every threshold (absorption voltage, cut-off current, float voltage, reconnect point) is a named constant at the top of the firmware, and the delivery includes a calibration procedure using a multimeter and a discharged battery.
Is this project suitable for a final-year project?
Yes — for Electrical and Electronics programs. It covers mains power supplies, rectification and filtering, voltage regulation, current sensing, embedded state-machine design and protection engineering, all in one demonstrable build. Suitable for B.E./B.Tech final-year projects in Electrical, Electronics and IoT.
Components & software requirements
- Arduino Nano (ATmega328P)
- 12-0-12 V, 3 A centre-tapped transformer
- KBPC bridge rectifier · 2×2200 µF filter bank
- IRFZ44N N-MOSFET pass device on aluminium heatsink
- Resistor-divider voltage sense · low-side current sense
- 16×2 character LCD (I2C backpack)
- 12 V, 10 A cut-off relay
- Embedded C++ (Arduino framework)
Delivery information
Built-to-order project. Delivery timeline is shared after order confirmation based on current queue.
Support terms
Complete documentation, setup guide, and viva preparation included. Support for setup and explanation provided.
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