Two of the most common battery health checks — internal resistance measurement and capacity testing — are often assumed to tell you the same basic thing. In practice they measure genuinely different failure modes, and relying on only one can miss the specific problem a battery is developing.
What Internal Resistance Actually Measures
Internal resistance testing — the approach used by instruments like the MEWOI Battery Internal Resistance Tester and MEWOI Battery Elements Tester — measures the opposition to current flow within the cell itself, without requiring a full discharge cycle. Rising internal resistance is one of the earliest and most reliable indicators of battery ageing.
This makes internal resistance testing well suited to fast field assessment, early-warning detection, and individual cell/module testing within a string — essential because a single weak cell in a series string can bring down the whole pack's usable capacity even when other cells test fine individually.
What Capacity Testing Actually Measures
Capacity testing — full discharge to determine actual usable Ah or Wh — directly measures what internal resistance can only estimate. The MEWOI Multi-Function Lithium Battery Capacity Tester performs this directly, measuring capacity, internal resistance, cell voltage balance, and charge/discharge curves together in one test sequence.
Why Relying on Only One Method Creates Blind Spots
Internal resistance alone can miss capacity-specific degradation. A cell can retain relatively low internal resistance while still having lost meaningful capacity through mechanisms that don't primarily manifest as increased resistance.
Capacity testing alone is slow and doesn't isolate individual weak cells efficiently. For that, cell-level internal resistance and balance testing is the more efficient diagnostic path.
Matching the Test to the Situation
| Situation | Recommended approach |
|---|---|
| Routine field/workshop health check | Internal resistance testing |
| Warranty claim or second-life grading | Full capacity testing |
| Multi-cell pack showing lower capacity | Cell-level internal resistance/balance testing |
| Automotive/vehicle battery check | Internal resistance + CCA-style testing |
| Lithium pack requiring balance verification | Multi-function capacity tester |
Practical Testing Checklist
- Use internal resistance testing as the first-pass diagnostic for routine health checks
- Escalate to full capacity testing when a definitive usable-capacity figure is required
- For multi-cell packs, test at the cell/module level to isolate weak cells
- Don't rely on internal resistance alone where actual remaining capacity is safety-critical
- Record baseline figures early so later readings can be compared against the battery's own history
Finch Electronic Solutions — Melbourne, Australia
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