Internal Resistance Testing and Maintenance for Lead-Acid Batteries

What internal resistance testing is really for

From an operator's point of view, battery testing exists to answer two questions: what is the state of health (SOH), and what is the state of charge (SOC). SOH decides whether a battery should be replaced; SOC decides whether it needs charging. That distinction is the core of practical battery maintenance.

For more than a decade the industry has looked for a test method that works in the field rather than in a laboratory. Internal resistance testing is the one that has become standard in telecom, UPS and solar installations worldwide.

1. Understand the reference value, not an absolute one

A given battery model leaves the factory with a characteristic initial internal resistance. Measure it with a battery state tester when the battery is new, and record it on a label fixed to the battery.

• If a reading is more than 40% above the recorded initial value, the battery has failed or is about to fail.
• In practice, batteries that have actually failed usually measure more than twice their initial resistance.
• Compare like with like: the reading is affected by temperature, state of charge and age at the moment of measurement.

2. Separate inherent capacity, retained capacity and state of charge

• Inherent capacity is the maximum energy the battery can store — its design capability.
• Retained capacity is what the battery can actually deliver under current conditions.
• State of charge describes how much energy the battery has actually received.

Loss of inherent capacity and chronic undercharging both reduce retained capacity. Retained capacity is the number that matters in operation, because it is what the load actually sees.

3. Judge SOC from terminal voltage, not from float voltage

Retained capacity depends on discharge rate and temperature, so it is best treated as an indicative figure. A practical shortcut is to estimate state of charge from the open-circuit terminal voltage after the battery has rested, then correct for temperature.

Read the open-circuit voltage, not the float voltage measured while the charger is connected. Float voltage stays artificially high and hides a weak battery.

In a substation or telecom base station running near 25 °C under normal float charging, state of charge should sit close to 100%. A battery that never reaches it is being undercharged.

4. Turn the readings into a maintenance routine

1. Record the initial internal resistance of every new battery.
2. Re-measure every 3–6 months in standby service, monthly in cycling service.
3. Log readings per string; the trend matters more than any single value.
4. Replace a unit once it exceeds 40% above its own baseline, then re-match the string.
5. Keep charging voltage temperature-compensated, and keep ambient temperature near 25 °C.

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_Xbatt supplies AGM, GEL, deep-cycle, front-terminal and tubular lead-acid ranges. If you need help setting up an acceptance or maintenance test procedure for your market, contact our engineering team._