The CC/CV Charging Protocol Explained
Think of filling a balloon: at first, you pump air fast (constant current — CC phase). As the balloon approaches full, you reduce airflow to prevent bursting (constant voltage — CV phase). A LiPo cell behaves similarly — its chemical kinetics limit how fast it can absorb charge as it approaches full capacity.
| Phase | Charger Action | Cell Voltage | Duration |
|---|---|---|---|
| CC (Constant Current) | Fixed 1C current output | 3.0V → 4.2V rising | ~70–80% of charge time |
| CV (Constant Voltage) | Fixed 4.2V output, current tapers | 4.2V constant | ~20–30% of charge time |
| Termination | Charger cuts off | 4.2V (100% SoC) | Current < 5% of 1C |
C-Rating: How Much Current Can Your LiPo Deliver?
The C-rating is a measure of charge/discharge rate relative to pack capacity:
Max discharge current = C-rating × Capacity (Ah) Example: 2200mAh 25C pack → 25 × 2.2A = 55A max continuous
For charging: a 1C charge rate on a 3000mAh pack = 3.0A charging current, achieving full charge in approximately 1 hour. Higher charge rates (2C, 3C) are possible with appropriate chargers but generate more heat and degrade cell chemistry faster — stick to 1C for maximum longevity.
Battery Management System (BMS) Functions
Overcharge Protection
Disconnects charge path when any cell exceeds 4.25V, preventing electrolyte decomposition and thermal runaway.
Over-discharge Protection
Disconnects load when any cell drops below 2.5–3.0V, preventing irreversible capacity loss and copper plating.
Overcurrent / Short Circuit
Disconnects within microseconds when discharge current exceeds rated limit, protecting against hard shorts.
Cell Balancing
During charging, bleeds charge from higher-voltage cells to equalize all cells at 4.20V ±10mV.
Temperature Monitoring
NTC thermistor monitors cell temperature; disconnects at >60°C during charge or >80°C during discharge.
State of Charge (SoC)
Coulomb counting tracks mAh in/out to estimate remaining charge percentage displayed on fuel gauges.
LiPo Safety Rules: The Non-Negotiables
- Never charge unattended — stay nearby and watch for heat or swelling
- Never charge a puffed (swollen) battery — safely discharge and dispose
- Never discharge below 3.0V per cell (set low-voltage cutoff on ESC/BMS)
- Store at 3.7–3.85V per cell in a fireproof LiPo safe bag
- Never charge below 0°C — lithium plating causes permanent damage
- Never puncture, crush, submerge in water, or expose to flames
Frequently Asked Questions
How does CC/CV LiPo charging work?
Phase 1 (CC): constant current forces voltage to rise from ~3.0V to 4.2V/cell. Phase 2 (CV): voltage is held at 4.2V/cell while current tapers. When current drops to ~5% of 1C, the cell is fully charged.
What is a BMS (Battery Management System)?
A BMS protects LiPo cells from overcharge (>4.2V), over-discharge (<3.0V), overcurrent/shorts, and high temperatures. Multi-cell BMS boards also balance all cells to the same voltage during charging.
What does C-rating mean on a LiPo battery?
C-rating × capacity = max discharge current. A 2000mAh 25C pack can deliver 50A continuously. For charging, 1C = capacity in Ah of charge current (1 hour to full at 1C).
Why do LiPo batteries need cell balancing?
Cells in a pack drift apart over cycles. Without balancing, one cell may overcharge (4.2V+) while others are still at 3.8V. Balance chargers equalize each cell individually through the JST balance connector.
What are safe LiPo storage and handling practices?
Store at 3.7–3.85V per cell in fireproof bags at room temperature. Never charge unattended, never discharge below 3.0V/cell, discard puffed batteries, and never charge below 0°C.
📚 References & Sources
Related Resources
Why Lithium Batteries Explode
Understand thermal runaway and the chemistry of LiPo failures.
How Power Supplies Work
AC-DC power supply design that feeds LiPo chargers.
How Transistors Work
MOSFET switches used inside BMS protection circuits.
Field Oriented Control
Advanced motor control that demands precise LiPo power management.