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.