Charge/discharge curves and CCCV, reading a cell's voltage profile
The voltage-vs-capacity curve is the most-run battery measurement and a dense one: plateaus mark phase transitions, the taper marks the protocol switch, and the slow inward creep over cycles is degradation made visible.
A galvanostatic charge/discharge curve plots cell voltage against capacity (or time) at constant current. Voltage plateaus mark phase transitions in the electrode; their position and length identify the chemistry and the accessible capacity. The common CCCV protocol charges at constant current to a voltage limit, then holds that voltage while current tapers, squeezing in the last capacity. The gap between charge and discharge capacity is the coulombic inefficiency, and as the cell ages, discharge curves creep inward, visible capacity fade.
What the curve encodes
At constant current, the x-axis (capacity) is proportional to charge passed, so the curve's shape is the cell's voltage response to being filled or emptied at a fixed rate. Flat regions (plateaus) are two-phase coexistence, the potential barely moves while one phase converts to another. Sloping regions are single-phase (solid-solution) behavior. The plateau voltages and lengths are a fingerprint of the active material.
How to read it well
1 · Plateaus → phase behavior and capacity
Count and locate the plateaus: each corresponds to a phase transition, and the capacity spanned by a plateau is the charge stored in that transition. Shrinking plateaus over life mean active material is being lost.
2 · The CCCV taper → how much you're forcing
In CCCV, once the voltage limit is hit, current tapers at constant voltage. A long taper that contributes a lot of capacity signals polarization or rate limitation, the cell needs the voltage hold to finish charging.
3 · Coulombic efficiency → side reactions
Discharge capacity divided by charge capacity is coulombic efficiency. Persistently below 100% means charge is going somewhere irreversible, SEI growth, electrolyte oxidation, and is an early degradation signal.
4 · Inward creep → capacity fade
Overlay successive discharge curves: as they shift inward, accessible capacity is fading. Where the curve loses length (which plateau shrinks) points at the mechanism.
Common ways the curve misleads you
- Rate confusion. A curve at C/2 and one at 2C look different purely from polarization, compare like-for-like rates before reading degradation.
- Ignoring the CV taper capacity. Quoting only the constant-current capacity understates total stored charge when the taper contributes meaningfully.
- Temperature drift. Plateau voltages shift with temperature; an uncontrolled cell can fake a chemistry change.
- First-cycle losses. The large first-cycle inefficiency (formation/SEI) is normal, don't extrapolate it as the steady fade rate.
Where this gets slow by hand
A cycle-life test produces thousands of curves. Extracting capacity, coulombic efficiency, plateau positions, and the CV-taper fraction for each, then trending them and projecting to the 80% end-of-life threshold, is a large, repetitive analysis where the meaningful signal is a slow drift across hundreds of cycles no one reads one-by-one.
From a cycler export to a projected cycle life
Niobia ingests the raw cycler files and extracts capacity, coulombic efficiency, plateau positions, and the CV-taper fraction for every cycle, then trends them and projects cycle life to the 80% state-of-health threshold from the early curve. When capacity fades, it attributes the loss across mechanisms and ties the voltage-profile change to impedance growth from EIS and diffusion from GITT. The result is a projection and a cause, not a stack of curves.
Frequently asked
What do voltage plateaus in a charge/discharge curve mean?
A plateau is a region where voltage stays nearly constant while capacity changes, it corresponds to a two-phase transition in the electrode, where one phase converts to another at a fixed potential. The plateau voltage and length identify the active material and the capacity stored in that transition.
What is the CCCV charging protocol?
Constant-current, constant-voltage charging: the cell is charged at a fixed current until it reaches a voltage limit, then held at that voltage while the current tapers off. The constant-current phase delivers most of the capacity quickly; the constant-voltage taper squeezes in the remainder.
What is coulombic efficiency?
The ratio of discharge capacity to charge capacity in a cycle. Values persistently below 100% indicate irreversible side reactions such as SEI growth or electrolyte decomposition, making it an early indicator of degradation.