Battery Selection

Learning Goals 5 min

L03-46 introduced power budgeting. Now you pick batteries for the specific demands of a low-power IoT node. By the end of this lesson you will:

1. Compare battery chemistries by energy density, peak current, shelf life, rechargeability, cost.
2. Pick the right cell for: a 2-year sensor node, a 1-day wearable, a 5-second alarm beacon.
3. Add a voltage divider on A0 to monitor battery state-of-charge from the sketch.

Warm-Up 10 min

From L03-46, you know to compute runtime = capacity / average current. Today we compare options.

The five chemistries

For each: think about energy density (mAh per gram), peak current (peaks vs cruise), recharge cycles, leak rate.

New Concept · Pick batteries by use case 25 min

Use case 1 — 2-year sensor node (low duty cycle)

Sensor wakes every 15 minutes, samples, transmits, sleeps. Average current with proper sleep: ~30 µA.

Math: 30 µA × 8760 h × 2 years = 530 mAh consumed.

Battery options:

• AA alkaline (2500 mAh): 2 × AA in series = 3 V, but a single AA is 1.5 V — too low for a 3.3 V chip without a boost converter. Best: 2 × AA in series, runtime ~3 years.
• CR123A primary (1500 mAh, 3 V): right voltage, 5+ years runtime. Used in commercial IoT.
• LiPo + tiny solar (L04-40): infinite.

Use case 2 — 1-day wearable (high duty cycle)

Wearable with BLE + IMU + screen + button. Average current: ~30 mA.

Battery needs: 30 mA × 16 h (day of use) = 480 mAh.

Best: small LiPo (500–1000 mAh), 3.7 V — same voltage range as a phone, easy to charge via USB + TP4056 charger module.

Use case 3 — Beacon (rare, short transmits)

Sends a BLE advertising packet once every 30 s for 5 ms. Average current: ~50 µA average. Battery: CR2032 (225 mAh) → 4500 hours = 6 months. Many Bluetooth iBeacons run on CR2032.

Charging

Rechargeable cells need a smart charger. For LiPo + Li-ion, the standard is the TP4056 module — a £1 USB-C charger board that handles constant-current then constant-voltage charging safely. Don't skip; a dumb charge will set the cell on fire.

Battery monitor (voltage divider)

// Divider: battery+ -> 10k -> A0 -> 10k -> GND
// A0 sees half the battery voltage.
float readBatteryVoltage() {
  int raw = analogRead(A0);
  float vAtPin = raw * (3.3 / 1023.0);   // assume 3.3 V VREF
  return vAtPin * 2;                      // undo the 1:1 divider
}

void loop() {
  float v = readBatteryVoltage();
  Serial.print("battery "); Serial.print(v); Serial.println(" V");

  if (v < 3.4) {                          // LiPo near-dead
    digitalWrite(LOW_BATT_LED, HIGH);
  }
  delay(5000);
}

Bigger resistors (100k + 100k) mean less current draw (33 µA at 3.3 V), but slower response. For battery monitoring, slow is fine.

Boost / buck regulators

Sometimes the battery voltage doesn't match what you need:

• 1 × AA (1.5 V) → 3.3 V: needs a boost converter.
• 2 × Li-ion (7.4 V) → 5 V: needs a buck.
• LiPo (3.7–4.2 V) → 3.3 V: tiny LDO or buck.

The cheapest hobby boost/buck modules (£1–3) are 80–90% efficient. Higher-quality TI / ADI modules reach 95%.

Worked Example · Spec a battery for the magic wand 20 min

Recall the wand from L04-36: Nano 33 BLE Sense + occasional BLE bursts + IMU sampling.

Estimate average current

• Idle (sleep): 1 mA average if you implement proper sleep.
• Inference + transmit: 30 mA for ~150 ms per gesture.
• ~6 gestures per minute typical use.
• Average: 1 mA + (30 mA × 0.15 s × 6 / 60 s) ≈ 1.45 mA.

Pick battery

Want: small, > 6 hours runtime, rechargeable, 3.7 V (matches Nano 33 BLE Sense's VBAT).

Options:

• 500 mAh LiPo (matchbox size): 500 / 1.45 ≈ 345 hours = 14 days. Way overspec.
• 200 mAh LiPo (lipstick-sized): 200 / 1.45 ≈ 138 hours = 5 days. Comfortable.
• CR2032: 225 mAh × 3 V ≈ similar energy to 200 mAh LiPo at 3.7 V. But peak current at 5 mA limits it. With a 100 µF bulk cap to handle BLE bursts, viable for ~5 days.

Pick the 200 mAh LiPo + TP4056 charger. Charge weekly via USB.

Add battery monitor

Voltage divider + A0 + low-battery LED on the wand tip. When V drops below 3.5 V, LED flashes red on every gesture.

Try It Yourself 15 min

Mini-Challenge · Match battery to project 10 min

For each project, pick the battery + justify:

1. L03-23 BT Car (50 mA cruise, 500 mA peak).
2. L03-38 Doorbell (5 mA active, 5 µA sleep, ~10 presses/day).
3. L04-12 IoT Room Monitor (continuous WiFi, ~80 mA).

Recap 5 min

Battery choice = chemistry × capacity × voltage × peak current × rechargeable × cost. CR2032 for tiny low-rate beacons. AA / NiMH for years of low-rate sensing. LiPo for wearables. Always include charge management + over-discharge protection. Tomorrow we wire all of this into a complete low-power sensor node.

Energy density

Capacity per mass (Wh/kg). LiPo > NiMH > alkaline > coin cell.

Self-discharge

Capacity lost over time even with no load. Alkalines ~3%/year; NiMH ~30%/year (LSD types: 10%); LiPo ~3–5%/year.

C-rate

The discharge rate expressed as a multiple of capacity. 1C = full capacity in 1 hour. 10C = 1/10 of an hour.

Peak current

The max current a battery can supply briefly. Coin cells ~5 mA; LiPo 5–30 A.

TP4056

The classic cheap USB-C LiPo charger module. £1; safe for ~500 mA charge current.

Boost / buck regulator

DC-DC converter that steps voltage up (boost) or down (buck). Use to bridge battery voltage to chip needs.

Battery monitor

Voltage divider + ADC + threshold. Reports V_BAT for user display or auto-shutdown.

Over-discharge protection

Software cutoff that stops drawing when V drops below the cell's safe limit. Required for LiPo.

Homework 5 min

1. Pick a battery for one of your projects. Write the spec rationale in your notebook.
2. Add a battery monitor to that project. Plot V_BAT over a few hours.
3. Read ahead to ARD-L04-39 (Low-Power Sensor Node).