What primary factors decrease the Wh energy efficiency of a lithium-ion cell during battery test procedures?

The primary factors that reduce a lithium-ion cell’s Wh efficiency are internal resistance, polarization, high current, low temperature, and aging. These factors increase the voltage difference between the cell’s open-circuit voltage and its operating voltage during charge and discharge. The resulting overpotential losses consume energy internally, so the energy delivered during discharge is lower than the energy required to charge the cell.

Wh efficiency decreases when resistive and polarization losses increase. High current, low temperature, increased internal impedance, and cell aging all enlarge the voltage losses that occur during testing.

How Wh Efficiency Is Lost During Testing

Energy Efficiency Depends on Voltage and Current

Wh efficiency compares the energy released during discharge with the energy supplied during charge:

[ \eta_{Wh}=\frac{E_{\text{discharge}}}{E_{\text{charge}}}\times100% ]

Because electrical energy is the time integral of voltage multiplied by current, losses in operating voltage directly reduce measured efficiency.

During discharge, the terminal voltage is lower than the open-circuit voltage. During charging, the terminal voltage is higher than the open-circuit voltage. The cell therefore releases less energy than it receives.

Overpotential Creates the Main Loss

A useful representation of terminal voltage is:

[ U_O=U_{\text{OCV}}-U_R-U_P ]

where (U_R) is the internal ohmic voltage drop and (U_P) is the polarization voltage.

During charging, these losses work in the opposite direction: the required charging voltage rises above the cell’s equilibrium voltage. The larger the combined overpotential, the lower the Wh efficiency.

Primary Factors That Decrease Wh Efficiency

Internal Ohmic Resistance

Internal resistance produces heat according to:

[ P_{\Omega}=I^2R_{\Omega} ]

As resistance increases, more of the input energy is dissipated internally instead of being stored or delivered as useful electrical energy.

Resistance can increase because of cell design, electrode and electrolyte properties, poor current collection, manufacturing defects, low temperature, or aging. The effect becomes especially significant at higher currents because resistive power rises with the square of current.

Polarization Resistance

Polarization represents additional voltage loss caused by electrochemical and mass-transport limitations. It includes effects such as charge-transfer resistance and concentration gradients within the electrodes and electrolyte.

Higher polarization increases the difference between the equilibrium voltage and the measured terminal voltage. It also reduces voltage utilization during both charge and discharge, lowering the measured Wh efficiency.

High Charge and Discharge Current

Higher current increases both ohmic and polarization losses. Even if the nominal capacity remains similar, the cell requires more charging energy and delivers less useful discharge energy at elevated current.

High current can also cause the discharge voltage to reach its cutoff threshold earlier. This may reduce the measured usable capacity because some chemically available energy cannot be extracted at the selected test rate.

Low Ambient Temperature

Low temperature generally increases electrolyte resistance and slows electrochemical reaction and ion-transport kinetics. Consequently, both (R_{\Omega}) and (R_P) tend to increase.

The resulting voltage drop is larger under load, and the cell may reach its discharge cutoff voltage prematurely. The test can therefore report lower discharge energy even when the cell still contains substantial chemical energy.

Cell Aging

Aging increases impedance and reduces the cell’s ability to transfer charge efficiently. One important mechanism is growth of the solid electrolyte interphase, or SEI, which increases resistance at the electrode interface.

Aging can also cause loss of active lithium, degradation of active-material bonding sites, and structural damage in the electrodes. These changes reduce capacity while increasing internal impedance, so both available discharge energy and Wh efficiency decline.

Test Cutoff Voltage

Cutoff voltage does not create the underlying electrochemical loss, but it determines how much of that loss appears in the measured result. When resistance and polarization are high, the terminal voltage may reach the cutoff limit before the cell’s equilibrium voltage indicates that the stored energy is exhausted.

This is why a high-current or cold-temperature test can show lower discharge energy than a low-current, temperature-controlled test using the same cell.

Operational History and State of Charge

Available discharge capacity and voltage efficiency vary with the cell’s operational history and state of charge. Previous cycling, rest time, temperature exposure, and recent charge or discharge rate can influence polarization and apparent resistance.

These effects make Wh efficiency a test-condition-dependent measurement rather than a single immutable property of the cell.

Understanding the Trade-offs

Higher Current Improves Test Speed but Reduces Efficiency

High-current testing shortens charge and discharge times and reveals power capability. However, it increases (I^2R) losses and polarization, so the measured Wh efficiency will usually be lower.

A test intended to compare intrinsic energy efficiency should therefore use a controlled and consistently reported current rate.

Low Temperature Reveals Weaknesses but Alters the Result

Cold testing is useful for evaluating real operating limits and thermal sensitivity. It is not directly comparable to room-temperature efficiency testing unless temperature is treated as an explicit test variable.

Without temperature control, changes in resistance and polarization can be incorrectly attributed to cell chemistry or fabrication quality.

Apparent Capacity Loss May Be Rate-Dependent

A cell can reach its discharge cutoff early because of voltage drop rather than because all active material has been consumed. This distinction matters when interpreting lower discharge energy.

Separating true capacity loss from rate-induced voltage limitation requires measurements across multiple current levels, temperatures, and rest conditions.

Efficiency Measurements Require Accurate Instrumentation

Wh efficiency is calculated by integrating measured voltage and current over time. Sensor offset, sampling errors, current-control instability, and inconsistent cutoff conditions can distort the result.

Battery test systems should therefore control current and temperature while recording synchronized voltage and current data. Resistance and polarization parameters can then be identified separately from total energy loss.

How to Apply This to Your Test Procedure

The most useful test design depends on whether the goal is efficiency comparison, power characterization, or aging diagnosis.

• If your primary focus is intrinsic Wh efficiency: Use controlled temperature, consistent charge and discharge rates, stable cutoff voltages, and synchronized integration of voltage and current.
• If your primary focus is high-power behavior: Test across multiple current levels and report the resulting resistance, polarization, cutoff-limited capacity, and Wh efficiency.
• If your primary focus is low-temperature performance: Measure (R_{\Omega}), (R_P), usable discharge energy, and cutoff behavior at each defined temperature.
• If your primary focus is aging diagnosis: Track efficiency, capacity, and impedance over cycle life to distinguish SEI growth, active-lithium loss, and other degradation mechanisms.
• If your primary focus is manufacturing optimization: Use resistance and efficiency results to evaluate electrode compaction, slurry formulation, current collection, and cell assembly quality.

Wh efficiency is reduced primarily when operating current, temperature, impedance, polarization, or aging causes a larger share of the cell’s energy to be lost internally.

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