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 scripts/compare_batteries.py --interactive
Command line:
# Find battery for 50mA project, 24h runtime
uv run scripts/compare_batteries.py --current 50 --hours 24
# Require rechargeable
uv run scripts/compare_batteries.py --current 100 --hours 12 --rechargeable
# List all batteries in database
uv run 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 |