The USABC peak-power test measures a cell’s 30-second discharge capability at defined depths of discharge (DOD). The cell is tested in 10% DOD increments from 0% to 90%, using a baseline discharge followed by a high-current pulse. At each DOD, resistance is calculated from the voltage and current changes during the pulse, then used with the minimum allowable voltage to estimate peak power.

Core takeaway: USABC peak power is derived by modeling the cell’s terminal voltage as an IR drop from an IR-free voltage. The allowable pulse current is the current that reduces the cell voltage to its minimum operating limit, and peak power is the product of that current and the limiting voltage.

How the USABC Test Is Conducted

Test across defined DOD points

The test evaluates the cell at 0%, 10%, 20%, and up to 90% DOD.

DOD is used here as the test coordinate: increasing DOD means that progressively more of the cell’s available discharge capacity has been removed.

Apply a baseline discharge

At each DOD point, the cell is discharged at a defined baseline current for 30 seconds.

This establishes the cell’s voltage and operating condition before the high-power pulse.

Apply the high-current pulse

The baseline current is then replaced by a high-current discharge pulse lasting 30 seconds.

The cell voltage and current are recorded during the pulse, particularly at approximately 1 second and 30 seconds after the pulse begins.

Continue to the next DOD point

After the pulse, the cell is discharged again at the baseline current until DOD has increased by another 10%.

This sequence is repeated until the required DOD range has been evaluated.

Voltage Limits That Constrain the Test

Maintain the minimum allowable voltage

The cell voltage must remain above the applicable baseline threshold during the test.

The threshold is the greater of:

• The manufacturer’s specified cut-off voltage.
• Two-thirds of the open-circuit voltage at 80% DOD, as specified by the test procedure.

This limit prevents the calculated power capability from being based on operation outside the cell’s usable voltage range.

Why the voltage limit matters

Peak power is not simply the maximum power the cell can produce before damage. It is the power available while respecting the cell’s defined voltage and operating constraints.

Exceeding those limits can cause excessive polarization, degradation, or irreversible damage to internal components.

How Internal Resistance Is Derived

Measure voltage and current changes

The pulse resistance is calculated from the changes in voltage and current between the 1-second and 30-second points of the high-current pulse:

[ R=\frac{\Delta U}{\Delta I} ]

where:

• (R) is the calculated internal resistance.
• (\Delta U) is the voltage variation during the selected pulse interval.
• (\Delta I) is the current variation over the same interval.

In expanded form:

[ R=\frac{U_{1s}-U_{30s}}{I_{30s}-I_{1s}} ]

for a conventional discharge-current sign convention. If discharge current is represented as negative, the signs change, but the reported resistance is normally expressed as a positive magnitude.

What this resistance represents

This is an effective pulse resistance, not necessarily a pure ohmic resistance measured independently of all electrochemical effects.

Because the calculation uses points separated by 29 seconds, the result can include contributions from ohmic resistance, charge-transfer effects, and other polarization processes that evolve during the pulse.

How IR-Free Voltage Is Derived

Remove the modeled resistive drop

The test defines an IR-free voltage as:

[ U_{\mathrm{IRFree}}=U-IR ]

where:

• (U_{\mathrm{IRFree}}) is the estimated voltage before the modeled resistive drop.
• (U) is the measured terminal voltage.
• (I) is the relevant current.
• (R) is the calculated pulse resistance.

This provides a simplified Thevenin-style representation of the cell: an IR-free voltage source in series with an effective resistance.

Interpret the result carefully

The IR-free voltage is a calculated modeling quantity. It should not automatically be interpreted as the cell’s true open-circuit voltage, because the measured voltage may also contain electrochemical polarization and dynamic recovery effects.

Its purpose is to support the peak-power calculation under the selected test conditions.

How Peak Power Is Derived

Determine the allowable pulse current

At the minimum operating voltage (U_{\min}), the simplified cell model is:

[ U_{\min}=U_{\mathrm{IRFree}}-IR ]

Solving for the maximum allowable discharge current gives:

[ I_{\max}=\frac{U_{\mathrm{IRFree}}-U_{\min}}{R} ]

This is the current at which the modeled terminal voltage reaches the minimum permitted value.

Calculate power at the voltage limit

Peak discharge power is then:

[ P_{\mathrm{peak}}=U_{\min}I_{\max} ]

Substituting the current expression:

[ P_{\mathrm{peak}}

\frac{U_{\min}\left(U_{\mathrm{IRFree}}-U_{\min}\right)}{R} ]

The primary reference expresses this using a negative discharge-current convention:

[ P_{\mathrm{peak}}

\frac{-U_{\min}\left(U_{\mathrm{IRFree}}-U_{\min}\right)}{R} ]

The negative sign is a bookkeeping consequence of defining discharge current as negative. The reported power capability is normally stated as a positive magnitude.

Why resistance strongly affects power

The equation shows that, with voltage conditions held comparable, peak power is approximately inversely proportional to resistance:

[ P_{\mathrm{peak}}\propto \frac{1}{R} ]

A lower-resistance cell can deliver more current before reaching (U_{\min}). A higher-resistance cell experiences a larger voltage drop during the same pulse and therefore reaches the voltage limit sooner.

What the Test Result Represents

Produce a DOD-dependent power profile

The test generates a peak-power value at each DOD point.

The result is therefore not one universal cell rating. It is a map of power capability versus DOD, and it can be repeated at different temperatures, aging conditions, or test currents.

Support pack design

The results help engineers determine how many cells are required in series and parallel to meet a system’s power requirement while staying within voltage and current limits.

Cell-level resistance and voltage data are especially important because pack interconnects, busbars, cooling conditions, contact resistance, and cell imbalance add further constraints.

Support R&D verification

Repeating the test during development or cycle-life testing can reveal changes in:

• Pulse resistance.
• Voltage response.
• Available peak power.
• Performance across DOD and temperature.

Resistance growth generally reduces pulse-power capability, although power changes also depend on the cell’s voltage and polarization behavior.

Understanding the Trade-offs

Resistance is not the only performance variable

Although lower resistance generally improves pulse power, peak power also depends on IR-free voltage and the minimum operating voltage.

A cell with low resistance but insufficient usable voltage may not outperform a higher-voltage cell under a particular operating condition.

The resistance value depends on the test definition

Resistance varies with temperature, DOD, pulse duration, current level, rest history, and the time points selected for the calculation.

Consequently, a resistance value from this USABC procedure should not be compared directly with a value obtained from a different pulse duration or measurement method without accounting for those differences.

Cell power is not pack power

A cell-level peak-power result does not automatically equal the available power from a battery pack.

Pack-level limits may arise from series-cell voltage spread, parallel-group imbalance, thermal limits, current-collector and busbar resistance, battery-management-system constraints, and inverter limits.

Avoid treating peak power as continuous power

The USABC result describes a defined 30-second pulse capability under specified conditions.

It should not be used as a continuous-power rating unless a separate thermal and electrical analysis supports that conclusion.

Respect voltage and current limits

Testing beyond the specified voltage or power limits can accelerate degradation or cause severe damage to the electrodes, separator, electrolyte, current collectors, or other cell components.

Battery test equipment must therefore control current, voltage, timing, temperature, and safety cutoffs precisely.

Making the Right Choice for Your Goal

The most useful interpretation depends on what you need the result to support.

• If your primary focus is cell comparison: Compare cells only when DOD, temperature, pulse duration, current convention, voltage limits, and test history are equivalent.
• If your primary focus is pack sizing: Use the DOD-specific power and resistance results together with series/parallel configuration, thermal limits, imbalance, and pack-level parasitic resistance.
• If your primary focus is aging analysis: Track resistance growth and peak-power decline at consistent DOD and temperature conditions over cycle life.
• If your primary focus is test validation: Verify the current and voltage traces, confirm the 1-second and 30-second sampling points, and document the sign convention used in the equations.

A sound USABC peak-power evaluation combines disciplined pulse testing with clear resistance, voltage-limit, and sign-convention definitions.

Summary Table:

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