Coulombic Efficiency (CE)
Definition: The ratio of discharge Amp-hours to charge Amp-hours in the same cycle, expressed as a percentage.
CE = (Discharge Ah ÷ Charge Ah) × 100%
What it measures: The fraction of charge delivered back out of the cell for every unit of charge put in. Any charge not returned represents irreversible losses — SEI layer formation, lithium plating, electrolyte decomposition, or parasitic side reactions.
Typical values: Mature Li-ion cells in routine cycling: 99.5–99.9% per cycle. Formation cycles: 80–95% as SEI forms. First-cycle CE below 80% typically indicates an electrolyte or anode compatibility issue.
Why it matters: CE is the most sensitive early-cycle indicator of cell health and chemistry stability. At high cycle counts, even a 0.01% deviation per cycle compounds into measurable capacity loss. CE trajectory in the first 50 cycles has been demonstrated to predict long-term cycle life with high accuracy, making it a powerful screening metric in cell development programs.
Energy Efficiency (EE)
Definition: The ratio of discharge Watt-hours to charge Watt-hours in the same cycle, expressed as a percentage.
EE = (Discharge Wh ÷ Charge Wh) × 100%
What it measures: The fraction of energy returned per unit of energy input. EE is always lower than CE because it incorporates voltage hysteresis — the charge voltage is always higher than the discharge voltage. The gap between CE and EE reflects the average polarisation (internal resistance × current) during charge and discharge.
Typical values: Lithium-ion at moderate C-rates: 95–99%. Lead-acid: 70–85%. The EE difference between charge and discharge rates is significant — high-rate operation increases polarisation and reduces EE substantially.
Why it matters: For grid storage and EV applications, EE directly translates to operating cost. A 1% drop in round-trip EE across a 100 MWh installation represents significant annual energy loss, making EE the primary commercial metric for stationary storage bankability analysis.
In MIMS: Wh accumulates in the test record alongside Ah. Per-cycle Wh values are available directly from cycle-based chart analysis and end-of-cycle ASCII export files.
Capacity Retention and Capacity Fade
Definition: Capacity retention is the discharge capacity at cycle N expressed as a percentage of the reference capacity — typically cycle 1 or first RPT. Capacity fade is the complement: (100% − retention).
Retention(N) = (Discharge Ah(N) ÷ Discharge Ah(ref)) × 100%
What it measures: How much of the original usable capacity the cell retains after N cycles. The shape of the retention curve — linear, knee-point, sudden drop — reveals the dominant degradation mechanism and remaining useful life.
End-of-life convention: Most specifications define end-of-life at 80% capacity retention. Some aerospace and medical programs use 70% or other thresholds; always confirm with the applicable standard or customer specification.
RPT integration: In long cycle life programs, capacity retention is typically measured at standardised Reference Performance Test (RPT) intervals rather than from every routine cycle. The RPT protocol — fixed rate, temperature, and cutoffs — ensures that the capacity measurement is comparable across the full program duration regardless of any temperature or protocol drift.
DC Internal Resistance (DCIR) and ESR
Definition: DC internal resistance is calculated from the instantaneous voltage response to a step change in current.
DCIR = ΔV ÷ ΔI
Where ΔV is the instantaneous voltage change at the step transition and ΔI is the current step magnitude. ESR (equivalent series resistance) is the same quantity — the terms are used interchangeably in most battery testing contexts, though ESR technically refers to the resistive component at a specific AC frequency.
Measurement requirements: Accurate DCIR measurement requires capturing the voltage at the moment of the current step — before capacitive and diffusion processes have time to respond. This requires high-frequency data logging (≥10 Hz, ideally ≥100 Hz) at the step transition. At lower logging rates, the measured ΔV includes relaxation contributions and underestimates the true ohmic resistance.
What it tracks: DCIR rises as a cell ages — SEI thickening, electrolyte decomposition, and contact resistance all contribute. DCIR growth rate correlates closely with capacity fade rate and is a sensitive leading indicator of end-of-life in many chemistries.
In MIMS: ESR is calculated and logged automatically at each step transition and is available in both the real-time channel display and the ASCII export record.
Round-Trip Wh Efficiency for Grid Storage
Round-trip energy efficiency is the same as cycle energy efficiency — discharge Wh divided by charge Wh — but in grid storage contexts it carries additional commercial and contractual significance. For a grid storage system, RTE is typically specified at the system level and must be demonstrated over thousands of cycles at realistic partial state-of-charge conditions.
Partial SoC implementation: Grid storage cells often cycle within a narrow SoC window (e.g., 20–80%) rather than full charge and discharge. In MIMS, this is implemented using LHCAhr or LHCWHr end conditions — stopping charge or discharge when a specified fraction of the previous half-cycle's accumulated capacity has been delivered, rather than at a fixed voltage cutoff.
Metric Quick Reference
| Metric | Unit | Primary Use |
|---|---|---|
| Coulombic Efficiency | % | Chemistry stability; early lifetime prediction; formation QC |
| Energy Efficiency | % | Operating cost; system-level performance; polarisation tracking |
| Capacity Retention | % of reference | Cycle life; warranty demonstration; end-of-life determination |
| DCIR / ESR | mΩ | Power capability; degradation tracking; end-of-life indicator |
| Round-Trip Wh Efficiency | % | Grid storage bankability; system operating cost; contract compliance |
Frequently Asked Questions
Why is coulombic efficiency a better early-cycle predictor than capacity?
Capacity changes very slowly in the early cycles of a healthy cell — a cell might retain 99.5% capacity after 50 cycles even if it will fail at 300 cycles. CE reflects the rate of parasitic side reactions with much higher sensitivity. A CE of 99.80% versus 99.90% per cycle looks small but corresponds to a large difference in total lithium consumed by side reactions over 1,000 cycles. CE trajectory reveals degradation trends that are invisible in the capacity curve until the cell approaches end-of-life.
How do I measure DCIR accurately on a Maccor system?
DCIR is calculated from the voltage change at step transitions — the transition from rest to discharge, or charge to rest, for example. To capture the ohmic voltage drop accurately before diffusion and capacitance effects obscure it, the data logging rate at those steps must be ≥10 Hz (0.1 s per record) and ideally ≥100 Hz (10 ms) for small-format cells. Configure a Step Time report type with the appropriate interval on the steps where DCIR measurement is required; ESR is then automatically calculated and logged by MIMS at each step boundary.
Should capacity retention be measured at every cycle or only at RPTs?
For long cycle life programs, measuring at every routine cycle is impractical — the routine cycle protocol often differs from the standardised capacity measurement protocol. RPT insertion at regular intervals — typically every 100–500 cycles depending on the program — provides a consistent capacity baseline measured under identical conditions each time, ensuring that capacity fade values are comparable across the full program duration.