Charging Protocols and Fast Charging

Charging Fundamentals

The Essence of a Charger

Charger (Adapter) = AC-DC Power Supply + Protocol Chip

Legacy Chargers: Fixed 5V output
Fast Chargers: Dynamically adjust voltage/current based on device request

Nominal Power ≠ Constant Output Power
  65W Charger = "Capable of outputting up to 65W", actual power is determined by the device

USB Power Delivery (PD)

PD Versions

PD Voltage Levels

SPR (Standard Power Range, ≤100W):
  5V/3A  = 15W
  9V/3A  = 27W
  15V/3A = 45W
  20V/5A = 100W

EPR (Extended Power Range, >100W, PD 3.1):
  28V/5A = 140W
  36V/5A = 180W
  48V/5A = 240W

Note: Cables must also support the corresponding power!
      Standard Type-C cable = 3A (60W @20V)
      EPR cable = 5A (100W+), must have an E-Marker chip

PPS (Programmable Power Supply) — PD 3.0

Traditional PD: Can only select fixed voltage steps (5V → 9V → 15V → 20V)
       Device steps down voltage internally → generates heat

PPS: Device controls charger output in real-time
  Voltage: 3.3V~21V, 20mV steps
  Current: 0~5A, 50mA steps
  → Battery connected directly to charger output!
  → No internal step-down needed → No heat → Faster charging

Galaxy S20+ Fast Charging = PPS
Pixel Fast Charging = PPS
Most Android Fast Charging = PPS

Proprietary Fast Charging Protocols

Qualcomm Quick Charge (QC)

QC 2.0: 5/9/12/20V (negotiated via D+/D-)
QC 3.0: 3.6~20V, 200mV steps (INOV)
QC 4.0/4+: Compatible with USB PD, adds QC features on top of PD
QC 5.0: 100W+, dual-cell series charging

After QC 4.0+, it is essentially PD + QC enhancements
Phones are also shifting towards PD/PPS; QC's influence is declining

Domestic Proprietary Protocols

OPPO/VOOC/SuperVOOC: Low voltage, high current (5V/6A~10V/6.5A)
  → Charger controls battery directly, phone stays cool
  → Requires dedicated cable and charger

Huawei SuperCharge: Hybrid of VOOC + PD
  Early versions: 5V/4.5A or 4.5V/5A
  Later shifted towards PD/PPS

Xiaomi Turbo Charge: PD/PPS + proprietary enhancements

Current Status: Everyone is converging towards PD + PPS
                 However, proprietary "ultra-fast charging" (>100W) remains non-standard

Proprietary Protocols vs. PD

Charging Speed (Actual):
  Proprietary 100W+ > PD 65W > PD 18W > 5V/2A

Interoperability:
  PD chargers can charge most devices (15~100W)
  Proprietary charger + other brand phone = Slow charging (may be only 5V/2A)

Recommendation:
  Buying a charger: Prioritize PD + PPS (better compatibility)
  Multiple devices: GaN multi-port PD charger (one charger for multiple uses)

Li-Ion Charging Curve

CC/CV Charging Method (Foundation for all Li-ion batteries)

Stage 1: Trickle Charging
  When battery voltage < 3.0V, pre-charge with small current (~0.1C)

Stage 2: Constant Current (CC)
  Charge at a set current (typically 0.5C~1C)
  Voltage gradually rises

Stage 3: Constant Voltage (CV)
  Voltage reaches 4.2V (or 4.35V for high-voltage batteries)
  Current gradually decreases
  Current < 0.05C → Charging complete

  Voltage
4.2V ┤     ┌──────────
     │    ╱
     │   ╱
     │  ╱
3.0V ┤─╱
     └─────────────→ Time
     Trickle  CC    CV

C-rate: 1C = Current equal to the battery's capacity value
  Example: 3000mAh battery, 1C = 3A charging current
      0.5C = 1.5A

The Essence of Fast Charging

Traditional: Slow CC+CV throughout → Safe but slow
Fast Charging: Large current CC at low battery levels → Gradually slows down after ~50%

"0-50% in 15 minutes" is possible
"0-100% in 15 minutes" is impossible (CV stage is unavoidable)

Fast charging accelerates the CC stage
CV stage cannot be accelerated (due to battery safety requirements)

BMS (Battery Management System)

Basic Functions

Protection:
  - Overcharge protection (each cell > 4.25V → disconnect)
  - Over-discharge protection (each cell < 2.7~3.0V → disconnect)
  - Overcurrent/short-circuit protection
  - Temperature protection (0~45°C charging, -20~60°C discharging)

Balancing (Multi-cell):
  - Passive balancing: Discharge high-voltage cells via resistors
  - Active balancing: Transfer energy from high-voltage cells to low-voltage cells

Metering (Gauge):
  - Coulomb Counter: Integrate current → Accurate SOC
  - Voltage lookup table: Rough, not accurate enough

Multi-cell Batteries

Single-cell Li-Ion: 3.7V nominal, 4.2V full
  1S = 3.7V, suitable for low-power devices

Two-cell series (2S): 7.4V nominal, 8.4V full
  Common in: Walkie-talkies, RC models, handheld devices

Three-cell series (3S): 11.1V nominal
Four-cell series (4S): 14.8V nominal
  Common in: Laptop batteries, power tools

Multi-cell batteries must have balancing charging!
Otherwise, one cell may overcharge → catch fire/explosion

Wireless Charging

Qi (Inductive)

Based on electromagnetic induction (two coils)
Frequency: 110~205 kHz
Power:
  BPP (Base): 5W
  EPP (Extended): 15W
  MPP (Qi2/Magnetic): 15W (Apple MagSafe protocol)
  Qi2: Open standard based on Apple MagSafe

Efficiency: Typically 70~80%
Alignment: Coils must be aligned → Qi2 adds magnets (MagSafe mode)

Other Wireless Charging

Apple MagSafe: 15W (Qi2 compatible), magnetic alignment
Samsung Fast Wireless: 15W (proprietary extension)
Xiaomi/OPPO Proprietary: Up to 50~100W (but generates significant heat)

AirFuel (Resonant): Longer distance, simultaneous charging for multiple devices, but not widely adopted

Fast Charging Power Selection Reference

Device              Typical Fast Charging Power
──────────────────────────────────────
TWS Earbud Case     5W (No fast charging needed)
Smartwatch          5~10W
Smartphone          18~65W (Mainly PD/PPS)
Tablet              20~45W
Ultrabook           45~65W PD
All-around Laptop   65~100W PD
Gaming Laptop       100~240W PD 3.1 or proprietary charger
Small IoT           5W (Micro USB or Type-C 5V)

Keywords: USB PD, PPS, QC, CC/CV, BMS, C-rate, Balancing Charging, Qi, Wireless Charging