Compare like-for-like methods and conditions. Use resistance for electrical response and screening; measure delivered Ah and Wh separately for capacity and usable energy.
Why do two testers disagree?
In a DIY Solar Forum discussion, owners reported different resistance readings from handheld meters, capacity testers and battery-management systems. The question was whether those numbers identified poor cells. The comparison starts with each instrument’s method and the agreed acceptance criteria. Read the originating discussion.
First ask what the instrument actually measures. An AC reading at a specified frequency and a resistance calculated during a DC load change are different measurements. A number without its method, connection points and operating state is incomplete evidence.
Three tests, three different questions.
| Measurement | Useful question | Not established by it alone |
|---|---|---|
| AC internal resistance | Does the reading differ from comparable reference cells? | Delivered capacity or safe maximum current |
| DC pulse response | How much does voltage change during this load event? | Behavior at every pulse length, temperature or charge state |
| Discharge Ah and Wh | How much charge and energy reach the defined cutoff? | Capability during a different, higher-power duty |
AC resistance is a defined measurement.
HIOKI describes AC-IR testing using a small alternating signal. Frequency matters; 1 kHz is common, not a universal substitute for every method. Four-terminal measurement separates the current and voltage connections to reduce the contribution of test leads and contacts. Record the instrument, frequency and connection arrangement. HIOKI: internal resistance testing.
A DC pulse describes a load event.
A pulse test relates the voltage change to a current step. State when the voltage is sampled: the response evolves after the step, so a short observation and a longer pulse must not be compared as if their definitions were identical. HIOKI explains why AC and DC methods involve different parts of the cell response. HIOKI: DC-IR and AC-IR methods.
Capacity needs a discharge measurement.
Ah accumulates current over time; Wh accumulates voltage multiplied by current over time. An electronic load or battery cycler can provide that record. Keysight documents separate Ah and Wh accumulators in its electronic-load manual. A charger display showing charge returned to a battery is not automatically the same measurement as energy delivered to equipment. Keysight: Ah and Wh measurement, page 162.
A small number can matter at pack level.
Current step: 10 A to 60 A
Voltage drop: 0.50 V
Apparent resistance: 0.50 / (60 - 10) = 0.010 Ω = 10 mΩThis result belongs to the stated sampling interval and measurement points. It is not a 1 kHz AC-IR value and does not establish an allowable discharge current.
At the complete battery terminals, the path can include protection circuitry and connections as well as cells. HIOKI explicitly notes protection-circuit resistance in pack measurements. Do not reject a cell by comparing a pack-terminal result against a bare-cell specification. HIOKI: measuring protected lithium-ion batteries.
10 A for 2 hours = 20 Ah
Measured average voltage of 24 V at that constant current: 480 WhThe figures are hypothetical. In a real test, integrate the recorded voltage and current and state the cutoff. Nominal voltage multiplied by rated Ah is not a substitute for that measurement.
Ask for a report you can compare.
For an OEM acceptance discussion, request these fields together. Adapt this checklist to the chemistry, battery configuration and acceptance criteria.
- Identity: cell or pack reference, sample identifiers, revision, age and relevant prior cycling.
- Starting conditions: charge procedure, rest period, state-of-charge basis and temperature.
- Method: AC frequency or DC pulse amplitude, duration and voltage-sampling times.
- Boundary: bare-cell terminals, pack output or equipment connector; include fixture and sensing points.
- Discharge record: current profile, cutoff, delivered Ah and Wh, temperature record and stop reason.
- Decision: agreed acceptance criteria, deviations and the next engineering action.
Use the result for the decision it supports.
A practical comparison has three separate conclusions: whether resistance is consistent under the chosen method; whether the battery delivers the required usable energy; and whether the complete assembly supports the equipment’s load profile. Passing one does not settle the others.
Battery testing needs appropriately rated equipment, protection and a controlled procedure. Do not short a battery, bypass protection or improvise a high-current test from a forum reply.
Sources and calculation notes.
The reader question comes from DIY Solar Forum. Measurement explanations draw on the linked HIOKI and Keysight documentation. Calculations use hypothetical values; the checklist is an editorial review aid.