Battery Technology

Common Battery Chemistries

Comparison Overview

Quick Selection Guide

Smartphones/Laptops/Wearables → Li-Po (thin, custom shapes, lightweight)
Electric Vehicles → LFP (cheap, safe) or NMC (long range)
Energy Storage/Solar → LFP (long life, safe)
RC Models → Li-Po (high discharge rate)
UPS/Car Starting → Lead-Acid (cheap, durable)
Remote Controls/Toys → NiMH or Alkaline

Li-Ion / Li-Po Detailed

Voltage vs. State of Charge

Typical Li-Ion (NMC/LCO):

4.2V = 100%
4.0V ≈ 80%
3.8V ≈ 50%
3.7V = Nominal Voltage (~37%)
3.6V ≈ 20%
3.5V ≈ 10%
3.0V = 0% (Discharge cutoff, going lower causes damage)

Non-linear! Voltage drops quickly from 4.2→3.8V, then stays flat from 3.8→3.5V.
Do not estimate capacity linearly using 3.5~4.2V.

Factors Affecting Lifespan

Cycle Life Killers (in order of severity):

1. High Temperature (>40°C) — The biggest killer
   - Storing at full charge at high temps: loses 35% capacity in one year
   - Storing at full charge at room temp: loses 20% capacity in one year
   - Storing at 40% charge at room temp: loses only 4% capacity in one year

2. Storing at Full or Empty Charge
   - For long-term storage: charge to 40~60%, store in a cool place
   - Avoid storing at 100% or <10% for long periods

3. Deep Discharge
   - Cycling 100%→0%→100% every time: ~300 cycles
   - Cycling 80%→30%→80% every time: possibly 1000+ cycles

4. High Current
   - 1C charging: Standard
   - 2C charging: Accelerates aging
   - Fast charging damages batteries, but convenience > battery life for most people

Extending Lifespan:
  Stop charging at 80% (some phones/laptops have this feature)
  Don't drain until automatic shutdown
  Avoid playing heavy games while charging (heat + electrical stress)

Protection Board (PCM/BMS)

Single-cell protection board (present in every Li-Ion battery pack):
  Overcharge protection: >4.25V ± 0.05V → cuts off charging
  Over-discharge protection: <2.5~3.0V → cuts off discharging
  Overcurrent/Short circuit: Depends on design
  Some include temperature protection (NTC)

Warning: Never bypass the protection board to charge directly!
     Unprotected lithium batteries = potential fire bombs

LiFePO₄ (LFP)

Why LFP is Exploding in Popularity

Advantages:
  ✓ Extremely safe (rarely catches fire) → Top choice for energy storage/home use
  ✓ Ultra-long cycle life (2000~5000 cycles vs. 300~500 for Li-Ion)
  ✓ Low cost (cobalt-free; cobalt is expensive)
  ✓ Cobalt-free = more ethical (cobalt mining has severe human rights issues)

Disadvantages:
  ✗ Lower energy density (20~30% lower than NMC)
  ✗ Lower voltage (3.2V, requires more cells in series)
  ✗ Poor low-temperature performance (capacity drops sharply below 0°C)
  ✗ Flat voltage curve (difficult to estimate SOC via voltage)

Applications: BYD Blade Battery, Tesla Standard Range, Energy Storage Stations, RV Batteries

Charging Characteristics

Full Charge: 3.65V (not 4.2V!)
Nominal: 3.2V
Discharge Cutoff: 2.5V

CC/CV charging, CV stage voltage = 3.60~3.65V
After full charge, voltage rests back to ~3.35V

Using a standard Li-Ion charger for LFP = Overcharge = Dangerous!
(LFP requires a dedicated charger or an adjustable charger)

Battery Series and Parallel Connections

Series (Increase Voltage)

Two 18650 cells in series:
  [3.7V 3000mAh] →+ [3.7V 3000mAh] →+  = 7.4V 3000mAh

Capacity remains the same, voltages add up.
Requires balanced charging (BMS).
Cells must:
  - Be the same model and batch (capacity matched)
  - Have the same voltage (charged to the same voltage before assembly)
  - Have the same internal resistance (same aging level)

Parallel (Increase Capacity)

Two 18650 cells in parallel:
  [3.7V 3000mAh] ─┬─ = 3.7V 6000mAh
  [3.7V 3000mAh] ─┘

Voltage remains the same, capacities add up.
No balancing needed (self-balancing).
However, voltages must be identical before connecting! (Voltage difference >0.1V → high current)

Safety Warnings

❌ Mixing batteries of different brands, ages, or models
   (Can lead to overcharge/over-discharge, worst case: fire)

❌ Welding standard 18650 cells directly (High heat damages cells)
   ✅ Use a spot welder with nickel strips

❌ Running without a protection board
   ✅ Every series string must have a BMS

❌ Mechanical crushing/piercing
   ✅ Secure with hard casing or brackets

Fire Response:
  Lithium battery fire → Use Class D extinguisher or large amounts of water to cool
  (Batteries provide their own oxidizer, so CO2 extinguishers are ineffective!)

Common Battery Specifications

Cylindrical Cells

14500: 14mm × 50mm  (AA size, Li-Ion 3.7V)
18650: 18mm × 65mm  (Most common, laptops/power banks/Tesla)
21700: 21mm × 70mm  (Higher capacity, Tesla's new standard)
26650: 26mm × 65mm  (Higher current)

18650 Capacity: Typical 2000~3500mAh
  With protection board vs. unprotected (flat top)
  Power type (high discharge) vs. Capacity type

Pouch Cells

Li-Po Common Form: Square pouch
  Thin (1~10mm), any shape
  Highest energy density
  Swelling = Time to replace (gas generation, dangerous)

Common Capacity Markings: mAh (phones) or Wh (laptops)
  Wh = V × Ah
  Example: 60Wh laptop battery ≈ 60/11.1 = 5400mAh (3S)

Model Meanings

18650-30Q: Samsung 3000mAh 15A
18650-VTC6: Sony/Murata 3000mAh 30A (High discharge)
18650-GA: Sanyo/Panasonic 3500mAh 10A (Capacity type)
21700-50E: Samsung 5000mAh 10A

Brand Recommendations: Samsung, Sony/Murata, Panasonic/Sanyo, LG
        Domestic (China): EVE, BAK, Lishen (Quality improving)
        Absolutely Avoid: Ultrafire and various "fire" knockoff brands (capacity inflated 10x+)

Keywords: Li-Ion, Li-Po, LiFePO4, LFP, CC/CV, BMS, 18650, 21700, Balancing, Spot Welding