Practical case: Read battery with ADC and resistive divider

Materials

• 1 × Microcontroller board with ADC input (e.g., 3.3 V ref)
• 1 × Battery (e.g., 9 V) and holder
• 1 × R1 = 220 kΩ (top of divider)
• 1 × R2 = 120 kΩ (bottom of divider)
• 1 × C1 = 100 nF ceramic (optional RC filter to ground)
• 1 × Breadboard
• 6 × Jumper wires
• 1 × Multimeter

Wiring guide

• Share ground: connect the battery negative to the microcontroller GND.
• Build the divider:
• Connect R1 from +VBAT (battery +) to the ADC tap node.
• Connect R2 from the ADC tap node to GND.
• Optional filter:
• Connect C1 from the ADC tap node to GND (in parallel with R2).
• Connect the ADC:
• Wire the ADC tap node to an ADC input pin (e.g., A0).
• Abbreviations used:
• VBAT: battery positive voltage (measure at the dot labeled VBAT).
• VADC: voltage at the ADC tap node (measure at the dot labeled VADC).
• Double-check polarity before powering:
• Battery + goes only to the top of R1; battery − to GND.
• Ensure the ADC reference (Vref) is known (e.g., 3.3 V or 5 V).
• Divider math (for later verification):
• VADC = VBAT × R2 / (R1 + R2)
• With R1 = 220 kΩ and R2 = 120 kΩ: VADC ≈ VBAT × 0.3529
• Recover battery voltage: VBAT ≈ VADC × 2.8333

Schematic

              +VBAT  Batería Li‑ion 1S (3.0–4.2 V)
                │
                ● VBAT
                │
               ┌┴┐
               │ │        R1 = 27 kΩ (alto del divisor)
               │ │
               └┬┘
                │───────────────► A0 (ADC) del microcontrolador
                ● VADC
                │
                ├─────────┬───────── (nodo del divisor y filtro)
                │         │
               ┌┴┐       ┌┴┐
               │ │       │ │
               │ │       │ │
               └┬┘       └┬┘
        R2 = 100 kΩ       C1 = 100 nF
     (bajo del divisor)   (filtro a GND)
                │         │
                └─────────┴─────────
                          │
                          ● GND
                          │
                         GND

Measurements and tests

• Pre-checks:

  • Verify with the multimeter that VBAT is within expected range (e.g., ~9 V on a fresh 9 V battery).
  • Confirm microcontroller GND is continuous to battery negative.

• Divider verification (multimeter):

  • Measure VBAT: red probe at ● VBAT, black probe at ● GND.
  • Measure VADC: red probe at ● VADC, black probe at ● GND.
  • Check ratio: VADC / VBAT should be close to 0.3529. If not, recheck wiring and resistor values.

• ADC read sanity check:

  • Know your ADC: VADC = ADC_count × Vref / (2^N − 1), where N is ADC resolution.
  • Compute VBAT_est = VADC × (R1 + R2) / R2 ≈ VADC × 2.8333.
  • Compare VBAT_est (from ADC) to VBAT (from multimeter). Expect small differences due to tolerance and ADC error.

• Stability test (with and without C1):

  • If readings bounce, fit C1 and see if VADC becomes steadier.
  • Ensure ADC sampling time is adequate for the divider’s source impedance (R1 || R2 ≈ 78.3 kΩ).

Common mistakes

• Using too small resistor values, causing unnecessary battery drain.
• Forgetting common ground between battery and microcontroller.
• Exceeding ADC maximum input voltage (if the ratio is wrong).
• Placing C1 in series instead of in parallel from VADC to GND.

Safety notes

• Never connect the battery directly to the ADC pin.
• Double-check polarity before powering.
• If measuring higher-voltage packs (e.g., >12 V), recalculate resistor values to keep VADC ≤ Vref.

Improvements

• Add a series 1 kΩ resistor from the VADC node to the ADC pin for ESD/overcurrent protection.
• Add a 3.6 V TVS diode or clamp to protect the ADC from transients.
• Calibrate in firmware using measured R1 and R2 values and ADC reference for higher accuracy.
• Use lower-tolerance resistors (e.g., 1%) to reduce scaling error.

More Practical Cases on Prometeo.blog

Carlos Núñez Zorrilla

Electronics & Computer Engineer

Telecommunications Electronics Engineer and Computer Engineer (official degrees in Spain).

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