Section 4 · Topic 13
The CC/CV Algorithm and Charge Modalities
Lithium-ion cells cannot be charged with a simple constant voltage or constant current alone. The industry standard is the two-phase CC/CV algorithm — and what it forbids matters as much as what it does.
The three phases
In constant current, the charger applies a fixed current and the cell voltage rises steadily, restoring roughly 60–70% of capacity while the cell is empty enough to accept current without approaching its limit. In constant voltage, the charger holds the cell at its ceiling — near 4.2 V for NMC and LCO — and the current tapers naturally as the cell fills, which prevents overshoot. In cutoff, once the tapering current falls to a defined threshold of ~C/10, the charge ends and the current is cut completely.
The danger of trickle charging
Holding current into a full lithium-ion cell — the continuous trickle that lead-acid tolerates — is the failure case. The cell can intercalate no more lithium, so the forced current instead drives electrolyte oxidation at the cathode and continued lithium plating at the anode, generating heat and flammable gas, swelling the cell, and progressing toward runaway.
The damage from over-charging a full cell is not electrolysis of water — the electrolyte is non-aqueous. The mechanisms are cathode-side oxidation of the electrolyte and continued metallic-lithium plating at the anode. A lithium-ion cell is charged to a finish and disconnected, never floated at voltage.
Multi-stage fast charging
Fast charging works within this framework by reshaping where the current sits, not by abandoning the voltage limit.
These systems often charge at a very high C-rate during the initial CC phase when the cell is empty, but aggressively taper the current much earlier in the CV phase to prevent lithium plating as the cell reaches 80% SoC. The last increment of charge speed is traded for protection against the plating that high current would otherwise force near full charge.
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