Arduino Battery Checker (Arduino UNO + 16×2 LCD)

A tiny battery checker that measures open-circuit voltage with an Arduino UNO, shows the result on a 16×2 LCD and over Serial, and (optionally) lights a 3‑LED “fuel gauge.”

• Uses the UNO’s INTERNAL ~1.1 V ADC reference for high resolution.
• Displays mV for very weak cells and DEAD / LOW / GOOD / NEW status.
• Includes presence hysteresis so the status doesn’t flicker when you touch the cell to the clips.

Hardware

From the Arduino Starter Kit:

• Arduino UNO
• 16×2 LCD (parallel)
• Breadboard + jumpers
• Resistors: 10 kΩ ×2 (divider), 220 Ω ×3 (LEDs)
• 10 kΩ potentiometer (LCD contrast) – or tie VO to GND for max contrast
• LEDs: red, yellow, green (optional)

Wiring

Battery sense (2:1 divider):

Battery + ─ 10k ──●── 10k ── GND
                  │
                  └── A0 (Arduino)
Battery − ─────────────────── GND

Images:

LCD (16‑pin parallel, 4‑bit mode):

• 1 VSS → GND
• 2 VDD → 5 V
• 3 VO → middle of 10 k pot; pot ends → 5 V & GND (or VO→GND for max contrast)
• 4 RS → D12
• 5 RW → GND
• 6 E → D11
• 11 D4 → D5
• 12 D5 → D4
• 13 D6 → D3
• 14 D7 → D2
• 15 A (LED+) → 5 V (add 220 Ω if too bright)
• 16 K (LED−) → GND

LED bar (optional):

• D9 → 220 Ω → RED → GND
• D8 → 220 Ω → YEL → GND
• D7 → 220 Ω → GRN → GND

⚠️ Never connect the battery to the Arduino 5 V pin.

Build & Upload

1. Open arduino-battery-checker/arduino-battery-checker.ino in the Arduino IDE.
2. Select Board: Arduino/Genuino UNO and the correct Port.
3. Upload. Open Serial Monitor @ 115200 baud to see logs.

Calibrate (optional but recommended)

The UNO’s internal reference varies by board. To tighten absolute accuracy:

1. Measure a test cell with a multimeter → V_true.
2. Note the sketch’s reading → V_disp.
3. Compute: NEW_VREF = OLD_VREF * (V_true / V_disp)
4. Update VREF_CAL in code and re-upload.

Resolution is ~2.1 mV at the battery per ADC count (with the 2:1 divider).

Status thresholds

• NEW ≥ 1.50 V
• GOOD ≥ 1.30 V
• LOW ≥ 1.10 V
• DEAD < 1.10 V (LCD also shows mV for values < 1.000 V)

These are heuristic for alkaline. NiMH behaves differently; open‑circuit voltage alone is a rough guide.

Next steps

• Add a 10 Ω / 2 W load resistor + MOSFET to test under load for a far better state‑of‑charge estimate.
• Add a buzzer for DEAD.
• Add a button to switch ALK/NiMH modes and adjust thresholds.

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License

MIT © 2025 Brian Dooley