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

Chemistry	Voltage	Rechargeable	Energy Density	Cost	Safety
Alkaline	1.5V/cell	No	Medium	Low	Very safe
Lithium Primary	3V	No	High	Medium	Safe
NiMH	1.2V/cell	Yes	Medium	Medium	Safe
LiPo/Li-ion	3.7V	Yes	Very High	Medium	⚠️ Needs care
LiFePO4	3.2V	Yes	High	High	Safer than LiPo
Lead-acid	2V/cell	Yes	Low	Low	⚠️ Acid hazard

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

Project Type	Best Battery	Capacity	Charger
Simple Arduino	4x AA	2500mAh	None
ESP32 portable	18650	2600mAh	TP4056
Wearable	Small LiPo	500mAh	TP4056
Robot	2S LiPo	2200mAh	Balance
Ultra-low power	CR2032	220mAh	None
Solar project	LiFePO4	3200mAh	MPPT

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.