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Choose the method that answers the engineering question

A DC current step observes how terminal voltage changes when the load changes. The apparent resistance depends on when the voltage is sampled because the battery response includes more than an ideal resistor. A very early observation and a later observation can capture different contributions. Specify both the pre-step state and the delay used for the second reading.

An AC measurement applies a changing stimulus and observes its response at a stated frequency or across frequencies. Impedance can have magnitude and phase, and a reported resistance-like number depends on the instrument's method. A single-frequency result is not a substitute for a complete frequency response. Keep the method label with the result.

For production comparison, select a repeatable procedure and a reference population relevant to the product. A numerical limit borrowed from another cell format, chemistry or test condition can reject good parts or accept unsuitable ones. Official instrument guidance, such as Tektronix's discussion of DCIR and AC methods, helps identify these measurement distinctions without providing a universal battery acceptance limit.

Separate the cell response from the connection

Use a measurement arrangement that makes the voltage boundary explicit. Current-carrying contacts and wires develop their own voltage drops. A separate sense connection at the intended terminals can reduce the inclusion of those drops, but contact placement, fixture repeatability and instrument configuration still matter.

Record whether the measurement covers a single cell, a module, a complete pack or a path that includes switching and protection electronics. These objects contain different interconnections. A result at an external connector should not automatically be described as the internal resistance of one cell. If that path includes a BMS switch or fuse, preserve that fact in the record.

Inspect contact condition and repeatability when results change unexpectedly. Repeating a test with the same poor connection can produce a consistent but misleading number. The fixture's maintenance and sense-lead arrangement belong to the procedure, along with instrument calibration and measurement range.

Two-point discharge resistance

Calculate a two-point DC discharge result

For this worksheet, discharge current is positive and the second current must exceed the first. Apparent resistance is (V1鈭扸2)/(I2鈭扞1). A first reading of 4.05 V at 1 A followed by 3.95 V at 3 A gives a 100 mV change over a 2 A current change, or 50 m惟. The declared sample delay is carried into the output as essential context.

The worksheet rejects a nonincreasing current or a nondecreasing voltage for this specific model. Such data can indicate a sign, timing, connection or state issue; the tool does not silently take an absolute value to make them appear valid. Other test conventions are possible, but they need their own explicitly defined equation and procedure.

No current pulse, state-of-charge range or timing in the example is a recommended test for an unknown battery. The calculation uses already recorded measurements. Select actual test conditions from the cell, equipment and product requirements, using a procedure appropriate to the energy and operating limits involved.

Keep timing, temperature and history with the number

The same battery can respond differently at different states of charge and temperature. Recent charging, discharging and rest history can also influence the observation. A comparison intended to identify a change should hold those conditions consistent or explain how their influence is handled.

Sampling time requires a precise reference. State whether the delay begins at a commanded current change or at an observed current transition, and whether voltage and current measurements are synchronized. Instrument settling and sampling filters can change the effective observation window. Store the waveform or acquisition settings when the application needs more than two summary points.

Do not compare a nominal one-second result with a ten-second result as if they were identical measurements. Likewise, an AC result without its frequency is incomplete. A carefully named column in a production record prevents later analysis from merging incompatible values into one apparent trend.

Interpret resistance alongside other evidence

Resistance can help describe voltage sag and power behavior under a relevant condition, but it is not a direct capacity measurement. A battery can exhibit changes in capacity and resistance that do not follow one universal relationship. Remaining runtime also depends on the load, voltage limits and the battery's actual condition.

Use the result to investigate a defined problem: unexpected voltage drop, a connection change, a production outlier or a trend under a controlled method. Check whether the measurement path and test conditions support that interpretation. Avoid presenting a single reading as a complete health percentage without a validated model and reference.

If the measurement electronics are being developed as a product, review ADC accuracy, current-source behavior, synchronization, range and protection independently. The digital thermometer circuit guide offers a related example of separating resolution from installed measurement uncertainty. More digits in a result do not reduce uncertainty in the inputs.

Specify a reproducible board or fixture test

For manufacturing, release the stimulus definition, connections, instrument configuration, timing, acceptance rule and record format together. If the fixture uses replaceable contacts, include their maintenance and verification requirements. Define how repeated tests are recorded so that an unfavorable first result is not erased by an untracked retry.

When a PCB assembly contains current-sense circuitry, an electrical substitution or known stimulus may verify part of the circuit without exercising a real battery. Keep that board-level test distinct from the cell or pack test. The two tasks need different equipment, risk controls and acceptance evidence.

For an inquiry, identify the measurement boundary and whether you need the sensing PCB manufactured, a fixture assembled or a defined test included. Provide schematic, BOM, quantity and the method you intend to use. The downloaded calculation report records the entered arithmetic and timing; it does not send an inquiry or establish that the physical measurement was performed correctly.

Keep a measurement result separate from a protection decision

A battery measurement has a location, timestamp, operating state and uncertainty. A limit has an authoritative source and an assigned response. Connect those two records explicitly before implementing a protection function. A monitor reading that appears plausible is not proof that every cell connection, temperature input or current path is valid.

Keep the cell manufacturer's operating requirements with the system design. Identify which circuitry remains active when the host is unavailable and which recovery actions require independent conditions. Configuration and calibration must be versioned as carefully as the PCB. When requesting assembly, state whether the work stops at the populated board or includes programming, calibrated measurements and a defined system test. A worksheet supports this discussion; it cannot authorize pack operation or turn an unknown requirement into a default threshold.