Battery Selector

Battery Selector

Guides battery chemistry and charging circuit selection for embedded projects.

Resources

This skill includes bundled tools and references:

• scripts/compare_batteries.py - Battery comparison calculator with 15+ battery types
• references/safety-guidelines.md - Comprehensive safety guide for all chemistries

Quick Start

Interactive selection:

uv run --no-project scripts/compare_batteries.py --interactive

Command line:

# Find battery for 50mA project, 24h runtime
uv run --no-project scripts/compare_batteries.py --current 50 --hours 24

# Require rechargeable
uv run --no-project scripts/compare_batteries.py --current 100 --hours 12 --rechargeable

# List all batteries in database
uv run --no-project scripts/compare_batteries.py --list

When to Use

• "What battery should I use?"
• "How do I charge this project?"
• "Lithium vs alkaline?"
• "Is this battery safe?"
• Planning portable/battery-powered projects

Decision Flowchart

START
  │
  ▼
Is project rechargeable? ──No──► Alkaline/Lithium Primary
  │                              (Disposable batteries)
  Yes
  │
  ▼
What voltage does MCU need?
  │
  ├── 5V ──► LiPo + Boost converter
  │          OR 3x/4x NiMH
  │
  ├── 3.3V ──► Single LiPo (3.0-4.2V)
  │            Directly compatible!
  │
  └── 12V+ ──► Multi-cell LiPo pack
               OR Lead-acid
  │
  ▼
How much current?
  │
  ├── <50mA ──► Small LiPo (500-1000mAh)
  │             OR Coin cell (CR2032)
  │
  ├── 50-500mA ──► Standard LiPo (1000-3000mAh)
  │                OR 18650 cells
  │
  └── >500mA ──► Large LiPo (3000mAh+)
               OR Multiple 18650s
               External power recommended

Battery Chemistry Comparison

Quick Reference

Alkaline (AA/AAA/9V)

Pros:

• Cheap, available everywhere
• No charging circuit needed
• Very safe
• Long shelf life (5-10 years)

Cons:

• Not rechargeable (e-waste!)
• Voltage drops as discharged
• Poor at high current
• Heavy for capacity

Best For:

• Low-power projects (<20mA average)
• Beginner projects
• Remote/deployment where charging impractical
• Backup power

Voltage Configurations:

Cells   Voltage    Use With
────────────────────────────
2x AA   3.0V       3.3V MCUs (with LDO)
3x AA   4.5V       5V MCUs (direct or LDO)
4x AA   6.0V       5V MCUs (with regulator)
9V      9.0V       With 5V/3.3V regulator

NiMH (AA/AAA Rechargeable)

Pros:

• Rechargeable (500-1000 cycles)
• Same size as alkaline
• Safer than lithium
• No memory effect

Cons:

• Lower voltage (1.2V vs 1.5V)
• Self-discharge (~20%/month)
• Need proper charger
• Heavier than LiPo

Best For:

• Projects replacing disposable batteries
• Educational settings
• Where LiPo is too risky
• Budget rechargeable solution

Charging:

• Use dedicated NiMH charger
• Don't mix brands/capacities
• Eneloop/Eneloop Pro recommended

LiPo / Li-ion (3.7V)

Pros:

• High energy density (light + powerful)
• Rechargeable (300-500 cycles)
• Flat discharge curve
• Many form factors

Cons:

• ⚠️ Fire risk if abused
• Needs protection circuit
• Temperature sensitive
• Ages even unused

Best For:

• Most portable projects
• Weight-sensitive applications
• When you need runtime
• Professional builds

Critical Safety Rules:

✅ DO:
- Use protected cells with BMS
- Store at 40-60% charge
- Use proper TP4056/similar charger
- Monitor temperature during charge
- Use battery with JST-PH connector (prevents polarity swap)

❌ DON'T:
- Puncture, crush, or bend
- Charge below 0°C
- Discharge below 3.0V
- Leave charging unattended (first few times)
- Use damaged/puffy batteries

LiFePO4 (3.2V)

Pros:

• Much safer than LiPo (no thermal runaway)
• Longer cycle life (2000+ cycles)
• Flat discharge curve
• Tolerates abuse better

Cons:

• Lower energy density
• Lower voltage (may need boost)
• More expensive
• Less common in small sizes

Best For:

• Safety-critical applications
• Outdoor/rugged deployments
• Long-term installations
• When LiPo risk unacceptable

CR2032 / Coin Cells

Pros:

• Tiny and light
• Long shelf life
• 3V output (direct to 3.3V MCU)

Cons:

• Very low capacity (220mAh)
• Poor high-current performance
• Not rechargeable
• ⚠️ Danger if swallowed

Best For:

• Ultra-low power only (<10µA average)
• RTC backup
• Tiny sensors
• Keyfobs, beacons

Current Limits:

Continuous: <2mA
Pulse: <15mA (brief)

DON'T use for: WiFi, Bluetooth, motors, LEDs

Voltage Regulation

3.3V Systems (ESP32, RP2040)

Single LiPo → 3.3V:

LiPo outputs 3.0-4.2V
Most 3.3V MCUs tolerate this range directly!

Option 1: Direct connection (if MCU allows)
   LiPo(+) → 3.3V/VIN pin
   
Option 2: LDO for clean 3.3V
   LiPo(+) → [AMS1117-3.3] → 3.3V pin
   (Need 4V min input for AMS1117)

Better: Use HT7333 LDO (low dropout, low quiescent)
   Works from 3.3V input!

5V Systems (Arduino UNO/Nano)

LiPo → 5V:

Option 1: Boost converter
   LiPo(+) → [MT3608] → 5V → VIN pin
   
Option 2: PowerBoost module (Adafruit)
   Includes charging + boost + protection
   
Option 3: USB power bank
   Already regulated 5V + charging built-in

Charging Solutions

TP4056 Module (Most Popular)

┌─────────────────────────────┐
│  TP4056 with Protection     │
│                             │
│  [USB-C] ─► [TP4056] ─► [DW01+FS8205] ─► [B+/B-]
│   IN         Charger     Protection      To Battery
│                             │
│  Features:                  │
│  - 1A max charge current    │
│  - Overcharge protection    │
│  - Overdischarge protect    │
│  - Short circuit protect    │
│  - LED charge indicator     │
└─────────────────────────────┘

Wiring:
  B+ → LiPo positive
  B- → LiPo negative
  OUT+ → Load/MCU positive
  OUT- → Load/MCU negative

⚠️ Get module WITH protection (6 pins, not 4 pins)

Adafruit PowerBoost 500C/1000C

Premium solution with:

• LiPo charging via USB
• 5V boost output (500mA or 1A)
• Low battery indicator
• Load sharing (charge while running)

DIY Charging Don'ts

❌ Never charge LiPo with a constant voltage supply
❌ Never charge LiPo with a phone charger directly
❌ Never charge at >1C rate (e.g., 1000mAh → max 1A)
❌ Never charge frozen batteries

Battery Sizing Calculator

Step 1: Determine average current (from power-budget-calculator)
        I_avg = _____ mA

Step 2: Determine required runtime
        T_required = _____ hours

Step 3: Calculate minimum capacity
        C_min = I_avg × T_required × 1.25 (safety factor)
        C_min = _____ × _____ × 1.25
        C_min = _____ mAh

Step 4: Select battery
        Choose capacity ≥ C_min
        Consider: size, weight, form factor

Example:

Project: Weather station
I_avg: 15mA
T_required: 48 hours (2 days between charges)

C_min = 15 × 48 × 1.25 = 900mAh

Selection: 1000mAh LiPo (gives ~67 hours actual)

Common Mistakes

1. Using Wrong Charger

❌ "My 9V adapter should work"
   LiPo needs CC-CV charging at 4.2V max!
   
✅ Use TP4056 or dedicated LiPo charger

2. No Low-Voltage Cutoff

❌ Draining LiPo below 3.0V
   Permanently damages the cell!
   
✅ Use protection module OR monitor in code:
   if (batteryVoltage < 3.2) {
       enterDeepSleep();  // Protect battery
   }

3. Ignoring Inrush Current

❌ Battery can't handle WiFi TX spike (500mA)
   Causes brownout/reset
   
✅ Add 100-470µF capacitor near MCU
✅ Size battery for peak current, not just average

4. No Reverse Polarity Protection

❌ Swapping battery wires = magic smoke

✅ Use JST-PH connectors (keyed)
✅ Add protection diode or P-FET

Recommended Setups by Project Type

Low-Power Sensor Node

Battery: 18650 (3000mAh) or LiPo 2000mAh
MCU: ESP32 with deep sleep
Charger: TP4056 with protection
Runtime: Weeks to months

Handheld Device

Battery: LiPo 1000-2000mAh flat pack
MCU: Any
Charger: PowerBoost or TP4056 + boost
Runtime: Hours to days

Robot/High Current

Battery: 2S or 3S LiPo pack (7.4V or 11.1V)
Regulator: Buck converter to 5V
Charger: Balance charger (external)
Runtime: Minutes to hours

Ultra-Low Power Beacon

Battery: CR2032 or 2x AA
MCU: ESP32-C3 or ATtiny with deep sleep
No charger needed
Runtime: Months to years

Quick Selection Table

Shared Output Contract

Use the shared Arduino skill contract: state assumptions, required tools and versions, implementation steps, tests/evidence by proof stage, known limitations, and recovery/security notes.