Trend-Based Evaluation of Battery Internal Resistance: Using Individual Values, Temperature and Cell Comparison Correctly

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When maintaining UPS systems, stationary battery systems, telecommunications installations or other DC power supplies, measuring battery internal resistance is one of the quickest ways to identify abnormal cells or battery blocks. However, a single resistance value is considerably less meaningful than is often assumed.

Whether, for example, 4.2 mΩ is good or bad for a particular battery block can hardly be assessed without additional information. Battery type, size, temperature, state of charge, age, measurement method and even the quality of the electrical contact influence the result. The measurement becomes considerably more meaningful when it is evaluated as a trend over time and simultaneously compared with the other cells or blocks within the same battery string.

A weakening cell often does not first become apparent through an unusual terminal voltage. Particularly in stationary battery systems under float charging, the voltages within a battery string can still appear relatively uniform even though the condition of individual blocks has already changed. Internal resistance, impedance or conductance can make such changes visible as additional diagnostic parameters.

For testing and monitoring corresponding battery systems, ICS Schneider offers battery testing equipment and battery testers. Additional equipment for electrical testing and diagnostics can be found under electrical measuring and testing equipment.

What does battery internal resistance mean?

A real battery is not an ideal voltage source. When current is drawn from or supplied to the battery, electrical and electrochemical losses occur within it. As a result, the terminal voltage changes depending on the load.

For a highly simplified model, a battery can be represented as an ideal voltage source with an internal resistance. A change in current produces a corresponding change in terminal voltage.

In simplified form:

ΔU = Ri · ΔI

or:

Ri = ΔU / ΔI

Where:

• R_i = determined internal resistance,
• ΔU = change in battery voltage,
• ΔI = change in current.

The actual behavior of an electrochemical battery is more complex, however. In addition to ohmic losses in electrodes, grids, terminal connectors and electrolyte, electrochemical processes at the electrode surfaces also play a role. The measurement result therefore depends on which test method is used and in which time-dependent or frequency-dependent range the measurement is performed.

A displayed internal resistance or impedance value should therefore always be considered together with the measurement method used. Values from different measuring instruments should not be compared directly without verification.

Distinguishing internal resistance, impedance and conductance

In battery diagnostics, the terms internal resistance, impedance and conductance are often used alongside each other. However, they do not necessarily describe exactly the same measured quantity.

Internal resistance

In a DC or load-step measurement, for example, the change in battery voltage resulting from a defined change in current is evaluated. A dynamic resistance can then be determined for the time range under consideration.

Impedance

In an AC measurement, the battery is subjected to a defined alternating-current signal. The impedance is determined from the voltage response and the applied current.

Because electrochemical processes respond differently depending on frequency, the determined value can change with the test frequency.

Conductance

For a purely ohmic resistance, electrical conductance is mathematically the reciprocal of resistance:

G = 1 / R

With a battery tester, however, this should not be interpreted to mean that every displayed conductance value is simply the reciprocal of any arbitrary internal resistance value. The respective testing and evaluation method is also decisive here.

Measured quantity	Basic principle	Important note
Internal resistance	Evaluation of changes in current and voltage	Consider the measurement method and time range
Impedance	Evaluation under AC excitation	Test frequency influences the result
Conductance	Characteristic value for electrical conductivity	Only directly compare values obtained using the same test method

How is battery internal resistance measured?

For condition diagnostics, the battery is subjected to a defined electrical excitation and its voltage response is evaluated. Depending on the measuring system, this can be achieved, for example, by means of a defined load change or an alternating-current signal.

The basic principle is to generate a known change in current or a defined test signal and record the resulting electrical response of the battery.

In a simplified load-step analysis, a higher internal resistance produces a greater voltage drop for the same change in current.

For long-term trend evaluation, the following points are particularly important:

• use the same measurement method wherever possible,
• maintain comparable measurement conditions,
• use the same measuring points,
• document temperature and operating condition,
• clearly assign measured values to the respective cell or block.

Only then is it possible to distinguish whether the battery itself has actually changed or whether the measurement conditions were simply different.

Why a single measured value has limited significance

Assume that a test system determines an internal resistance of 5.1 mΩ for a battery block. Without a reference, this value alone provides little basis for a reliable assessment.

The measured value can be influenced by factors including:

• battery type and size,
• cell chemistry,
• temperature,
• state of charge,
• operating condition,
• age and service duration,
• previous charging or discharging operations,
• measurement method,
• measurement frequency or measurement duration,
• electrical contact quality.

A value of, for example, 5 mΩ may therefore be normal for one battery type and significantly elevated for another.

Three levels of comparison are particularly useful when assessing condition:

1. Comparison with the battery’s own initial value: How much has the same cell or block changed since commissioning?
2. Comparison within the battery string: How does the block compare with identical neighboring blocks?
3. Comparison with suitable reference or manufacturer values: Are reliable comparative data available for exactly this battery type and the measurement method being used?

The more of this information is available, the more reliable the evaluation becomes.

How does temperature affect the measured value?

Temperature influences the electrochemical processes within a battery and therefore also resistance or impedance. Measurement series should therefore be carried out under comparable temperature conditions wherever possible.

A battery block at a significantly lower temperature than the other blocks may, for example, show a different measured value without this necessarily indicating correspondingly greater aging.

Conversely, an unusually high temperature is itself an important diagnostic indication. It may indicate unfavorable environmental conditions, increased losses or problems at electrical connections.

For recurring measurements, at least the ambient temperature should therefore be recorded. In critical systems, recording the temperature of individual cells or blocks provides considerably more useful information.

Why different maintenance dates can be problematic

If a battery string is tested in winter at a significantly lower temperature and the subsequent measurement is carried out in summer at a higher temperature, the two measurement series are not automatically directly comparable.

A long-term trend therefore requires thermal conditions that are as similar as possible, or at least proper temperature documentation so that changes can be interpreted plausibly.

Taking state of charge and operating condition into account

In addition to temperature, the electrical operating condition also influences battery behavior. Recurring measurements should therefore be carried out in a clearly defined condition wherever possible.

For stationary battery systems, this may, for example, be a stable condition under float charging.

Measurements that are unsuitable for direct comparison include those performed:

• immediately after a heavy discharge,
• during the recovery phase following a load,
• directly after an intensive charging phase,
• with strongly fluctuating charging or load current,
• at significantly different states of charge.

Anyone wishing to compare measured values over several years should therefore document not only the measured quantity but also the operating condition at the time of measurement.

Defining the correct reference value

A suitable reference value is the basis of every trend evaluation. A so-called baseline measurement, carried out after installation and complete commissioning of a new battery set, is particularly valuable.

An initial value is documented for every individual cell or battery block.

Subsequent measurements can then be referenced directly to this value:

Change = current measured value − initial value

or as a relative change:

ΔRrel = ((Rcurrent − Rreference) / Rreference) · 100 %

If no measurement from the new condition is available, alternatives include:

• the earliest available maintenance measurement,
• manufacturer data for the respective battery type and suitable test method,
• statistical values from comparable cells within the same battery string.

The further the reference value differs from the actual conditions of the battery being evaluated, the more cautiously it should be interpreted.

Cell comparison within the battery string

Comparing identical cells or battery blocks within the same string is a particularly useful method for identifying outliers.

The blocks are typically:

• installed in the same battery system,
• operating under similar conditions,
• subject to the same charging regime,
• of comparable service age.

If a single block deviates significantly from the typical level of the string, this is often more noteworthy than an absolute measured value without comparison.

Example:

Block	Internal resistance	Initial assessment
Block 17	3.8 mΩ	Typical level of the string
Block 18	3.9 mΩ	Normal
Block 19	6.1 mΩ	Abnormal – follow-up check required
Block 20	3.7 mΩ	Normal

Block 19 should not automatically be replaced solely on the basis of this result. First, factors such as contact quality, temperature and repeatability of the measured value must be checked.

Mean or median?

For larger battery strings, a statistical comparison may also be useful. The median is less sensitive to individual strong outliers than a simple arithmetic mean.

Technical interpretation nevertheless remains decisive: a statistically abnormal block is initially a candidate for closer investigation and is not automatically a defective block.

Trend evaluation instead of fixed limits

The greatest strength of internal resistance or impedance measurement lies in monitoring changes over time.

For each block, for example, the relative change compared with an initial value can be calculated:

ΔRrel = ((Rcurrent − Rreference) / Rreference) · 100 %

If a block increases from 3.5 mΩ to 4.2 mΩ, the relative change is:

((4.2 − 3.5) / 3.5) · 100 % = 20 %

However, such a percentage change is not a universal replacement limit.

Meaningful warning or replacement limits depend on factors including:

• battery type and cell chemistry,
• manufacturer specifications,
• measurement method used,
• experience gained from comparable systems,
• age and operating conditions,
• criticality and redundancy of the power supply,
• additional parameters such as voltage and temperature,
• results of previous capacity tests.

Three trends are particularly meaningful

1. Deviation from the battery’s own initial value: How much has the same block changed since the first reference measurement?
2. Deviation from the battery string: Is the block developing differently from identical neighboring blocks?
3. Rate of change: Is the value increasing slowly over several years or significantly within a short maintenance interval?

The rate of change provides particularly useful additional information. A slightly elevated but stable value that has remained unchanged for a long time should be evaluated differently from a block whose resistance increases significantly within a short period.

Correctly assessing contact resistance and test leads

For resistances in the milliohm or even sub-milliohm range, even small transition resistances at contact points can have a noticeable influence on the measurement.

Possible sources of error include:

• oxidized or contaminated battery terminals,
• corrosion on connectors,
• loose battery interconnects,
• damaged or contaminated measuring contacts,
• different contact pressure,
• measurements taken at different positions.

Suitable measuring methods therefore use a four-wire or Kelvin measurement. The current path and voltage measurement are separated from each other.

This significantly reduces the influence of test leads and contact resistance on the voltage measurement.

Contact problem or weak battery?

If a single block suddenly shows an unusually high value, the measurement should first be reproduced.

1. Remove the measuring contacts.
2. Inspect the contact surface.
3. Reconnect at the defined measuring position.
4. Repeat the measurement.
5. If necessary, also inspect battery interconnects and terminal connections separately.

If the deviation disappears after establishing a clean contact, the cause was probably a contact or measurement problem. If the value remains reproducibly elevated, the battery block should be investigated further.

Test current and voltage drop

Resistance determination is fundamentally based on the fact that an electrical excitation produces a measurable voltage response.

For a simplified load-step measurement:

R = ΔU / ΔI

The smaller the generated voltage change is relative to resolution, noise and interference, the more demanding reproducible measurement becomes.

Professional battery testing and monitoring systems therefore use defined test methods in which excitation and voltage measurement are matched to each other.

For trend measurements in particular:

Measured values should only be compared directly if the measurement principle and test conditions are sufficiently consistent.

Changing the measuring instrument or test method can produce a sudden step in the historical data even though the battery itself has barely changed.

Typical fault patterns in battery measurements

Observation	Possible cause	Recommended check
One block has significantly higher resistance than the rest of the string	Aging, contact problem or different temperature	Repeat measurement and check temperature and connections
All blocks show changed values compared with the previous maintenance interval	Different temperature, different measurement method or different operating condition	Compare measurement conditions and test equipment used
Measured value changes significantly when the contacts are applied repeatedly	Contact resistance or unsuitable measuring position	Check contact surfaces and measuring connections
Internal resistance increases continuously over several maintenance intervals	Progressive aging	Continue monitoring the trend and consider scheduling a capacity test
Internal resistance increases significantly within a short period	Accelerated aging or local cell problem possible	Check voltage, temperature, operating conditions and capacity
Block voltage appears normal, but resistance or impedance is elevated	Deterioration may still be only slightly visible in voltage under float charging	Investigate trend and behavior under load
Internal resistance appears normal, but capacity is nevertheless reduced	Internal resistance does not clearly represent all aging mechanisms	Perform a defined discharge or capacity test
A single battery connector becomes unusually warm	Increased transition resistance possible	Check connection and tightening; if necessary, perform a separate low-resistance measurement

Systematic test procedure for a battery string

For recurring maintenance, the measurement procedure should be standardized as far as possible. Only in this way can measurement series still be compared meaningfully after several years.

1. Identify the battery system: Clearly document location, battery string, battery type and block number.
2. Check operating condition: Is the battery in a condition comparable with previous measurements?
3. Record temperature: Document ambient temperature and, where necessary, block temperatures.
4. Perform visual inspection: Check housings, terminals, connectors, corrosion, deformation and other abnormalities.
5. Record voltages: Document cell or block voltages.
6. Determine internal resistance or impedance: Use the specified measuring position and the same measurement method.
7. Repeat measurements on outliers: Rule out contact problems and operator errors.
8. Perform string comparison: Compare the measured value with identical blocks within the same battery.
9. Evaluate the trend: Compare the current value with historical values for the same block.
10. Include temperature and voltage: Never evaluate resistance in isolation.
11. Mark abnormal blocks: Define further monitoring or more detailed testing.
12. Test capacity if required: Verify actual performance using a defined discharge test.
13. Document results: Store measured values, conditions and maintenance decisions in a traceable manner.

Why internal resistance does not replace a capacity test

One of the most important limitations of internal resistance or impedance measurement is that it does not directly determine the battery’s actual available capacity.

Internal resistance, impedance and conductance are excellent diagnostic and trend parameters. They make it possible to detect changes and abnormal blocks quickly.

For a UPS or emergency power system, however, the decisive question is:

Can the battery still supply the required load for the required duration?

This question is answered much more directly by a defined discharge test.

Test method	Strength	Limitation
Internal resistance / impedance	Rapid condition and trend assessment of individual cells or blocks	Does not directly determine the actual available capacity
Voltage measurement	Simple and quick to perform	Weak blocks may appear relatively normal under float charging
Discharge / capacity test	Tests actual performance under a defined load	More time-consuming and involves actual discharge

The methods therefore complement each other: regular resistance, impedance and voltage measurements enable efficient trend monitoring. A discharge test is used to confirm the battery’s actual performance.

Documentation and replacement strategy

Trend analysis only works if every measured value can be clearly assigned to the correct battery block.

At minimum, the following should be documented:

• system and location,
• battery string,
• cell or block number,
• battery type,
• installation date or age,
• measurement date,
• voltage,
• internal resistance, impedance or conductance,
• temperature,
• operating and charging condition,
• measuring system and test method used,
• special observations.

Do not store absolute values only

For a meaningful maintenance strategy, the measurement data should also clearly show the change over time.

Useful parameters include:

• current measured value,
• original reference value,
• relative change since the reference measurement,
• median or mean value of the battery string,
• deviation of the individual block from the string level,
• change since the previous maintenance interval,
• rate of change over several test intervals.

A replacement decision can therefore be justified much more effectively than by using a single fixed milliohm limit.

Individual block or complete battery set?

Whether only one block or the complete battery set should be replaced depends on battery type, system concept, age, manufacturer specifications and the criticality of the power supply.

With an already significantly aged battery set, replacing a single abnormal block may solve the immediate problem. However, the remaining blocks may also already be in an advanced stage of aging.

The combined trend of all blocks therefore helps distinguish between a local outlier and general aging of the battery set.

Practical example: abnormal block in a UPS battery

A UPS system has a battery string consisting of several identical 12 V blocks. The battery is tested regularly.

In the current measurement, most internal resistance values are between 3.6 and 4.1 mΩ. Block 23, however, shows 5.4 mΩ.

A single value of 5.4 mΩ would initially be difficult to assess without further information. The available measurement history, however, provides additional indications.

Measurement time	Block 23	Typical level of the string
Reference measurement	3.5 mΩ	3.3 to 3.7 mΩ
Later maintenance	3.9 mΩ	3.4 to 3.8 mΩ
Follow-up measurement	4.4 mΩ	3.5 to 3.9 mΩ
Current measurement	5.4 mΩ	3.6 to 4.1 mΩ

Two developments therefore stand out:

• The block is increasingly above the typical level of the battery string.
• Its resistance is increasing faster than that of the other blocks.

The measurement is first repeated. Contact quality and temperature are checked. The elevated value remains reproducible.

The block voltage still appears relatively normal under normal charging conditions. Due to the clear trend, however, the block is marked for further testing.

During a subsequent defined load or discharge test, the block shows a greater voltage drop than the other blocks.

Result: The decisive factor was not the isolated resistance value, but the combination of historical trend, comparison within the battery string and behavior under load.

Suitable battery testing and monitoring systems

Depending on the maintenance concept, different measurement methods are useful for evaluating the condition of stationary battery systems. For trend-based internal resistance monitoring, a system capable of recording measured values regularly and reproducibly is particularly useful. For actual verification of battery capacity, however, a controlled discharge test is required.

CM 9010 – stationary battery monitoring with internal resistance determination

The CM 9010 battery monitoring system is designed for stationary battery installations and has a modular design. Among other functions, the system enables active determination of internal resistance.

This measuring principle is particularly well suited to trend-based condition assessment: instead of examining individual battery blocks only during occasional maintenance visits, relevant battery parameters can be monitored systematically and changes can be detected at an early stage.

Particularly in critical applications such as UPS, emergency power or other stationary DC power supplies, continuous or automated monitoring can be useful if purely periodic testing results in excessively long intervals between measurements.

Further information can be found under CM 9010 battery monitoring at ICS Schneider.

BTS 200 MKII – verifying capacity using a discharge test

Internal resistance is an important diagnostic parameter, but it does not directly answer the question of the battery’s actual available capacity.

The BTS 200 MKII Battery Test System is designed for defined discharge cycles on high-performance batteries. The system records parameters including battery voltage, discharge current and the course of the discharge.

With external loads, discharge currents of up to 1300 A can be achieved. Up to nine external loads can be combined with the system.

The BTS 200 MKII therefore provides a useful complement to trend-based condition monitoring: abnormalities identified in internal resistance, voltage or temperature trends can, where necessary, be investigated further by means of a controlled discharge test.

Further information can be found under BTS 200 MKII Battery Test System at ICS Schneider.

Trend measurement and capacity testing serve different purposes

Task	Suitable approach
Detect changes in internal resistance	Recurring or continuous resistance monitoring
Identify abnormal cells or blocks	Trend and string comparison
Evaluate temperature and voltage deviations	Combined evaluation of several battery parameters
Verify actual available battery capacity	Defined discharge test

A meaningful maintenance strategy therefore combines several levels: condition parameters are monitored regularly, abnormal changes are investigated specifically and actual battery performance is verified by means of a discharge test when required.

An overview of corresponding equipment can be found under battery testing equipment and battery testers at ICS Schneider.

Conclusion

Measuring battery internal resistance is an effective tool for assessing the condition of battery systems – provided the value is interpreted in the correct context.

A single resistance, impedance or conductance value has only limited significance. Much more informative are the historical trend, temperature, operating condition and comparison with identical cells within the same battery string.

Blocks whose measured values change faster than those of the rest of the battery are particularly noteworthy. Before a replacement decision is made, however, contact quality, temperature and measurement conditions should be ruled out as possible causes.

Trend-based internal resistance evaluation is therefore highly suitable for the early detection of abnormal cells. However, it does not replace a capacity test. For critical battery systems, the combination of regular condition monitoring, consistent measurement documentation and discharge testing as required provides particularly meaningful information.

FAQ: Measuring and evaluating battery internal resistance

What does the internal resistance of a battery indicate?

The measured internal resistance provides information about the electrical and – depending on the test method – electrochemical losses within a battery. An increasing value can indicate a change in condition or aging. However, the development over time is more meaningful than an individual value.

Does high battery internal resistance always indicate a defective battery?

No. An elevated measured value can also be caused, for example, by temperature differences, unsuitable measurement conditions or contact problems. Abnormal values should therefore first be reproduced and evaluated together with additional battery parameters.

What battery internal resistance is still considered good?

There is no universal milliohm limit for all batteries. The typical value depends, among other factors, on battery type, size, cell chemistry, temperature and measurement method. Particularly useful references are the battery’s own initial value, suitable manufacturer specifications and comparison with identical blocks in the same battery string.

Why is the trend more important than a single measured value?

The trend shows how the same battery block develops over months or years. This makes it possible to identify gradual changes even when the current individual value does not yet appear clearly abnormal when viewed in isolation.

How should a reference value for battery internal resistance be defined?

Ideally, a reference measurement should be carried out after installation and commissioning of a new battery set. If this is not available, the earliest available measurement or a suitable comparison within a string of identical batteries can be used. Manufacturer values are only directly applicable if the measurement method and conditions are sufficiently comparable.

Why must temperature be documented during an internal resistance measurement?

Temperature affects the electrochemical behavior of the battery and therefore also resistance or impedance. Measurements performed at significantly different temperatures cannot therefore be compared directly without further consideration.

What is the difference between internal resistance and impedance?

Internal resistance can, for example, be determined from a change in current and voltage under a defined load. In an impedance measurement, the battery’s response to an alternating-current signal is evaluated. Since batteries are electrochemical systems, the measured value may depend on the frequency used.

What does battery conductance mean?

Conductance fundamentally describes electrical conductivity. For an ideal ohmic resistance, it is the reciprocal of resistance. With battery testers, however, the displayed characteristic value depends on the respective measurement method. Values from different instruments should therefore not be compared directly solely on the basis of their designation.

Why are Kelvin or four-wire connections useful for low resistance measurements?

At very low resistance values, test lead resistance and contact resistance can represent a considerable proportion of the result. In a four-wire measurement, the current path and voltage measurement are separated, significantly reducing the influence of the test leads.

Can a battery have a normal voltage and still be weak?

Yes. Particularly under float charging, the voltage of a weakened battery block may initially appear relatively normal. Under load or during a resistance or impedance measurement, however, differences compared with the other blocks may already become apparent.

How can a weak cell be identified in a battery string?

The combination of cell or block voltage, internal resistance or impedance, temperature and historical trend is particularly informative. A block that reproducibly deviates significantly from the level of identical neighboring blocks should be investigated more closely.

Why should the same measurement method be used wherever possible?

Different measuring systems may use different test currents, frequencies, time ranges and evaluation algorithms. This can result in different numerical values on the same battery block. A consistent test method is therefore particularly important for long-term trend analysis.

How often should battery internal resistance be measured?

The appropriate interval depends on battery type, system criticality, operating conditions, maintenance strategy and manufacturer specifications. For trend analysis, the most important factors are that test intervals are defined and measurement conditions are sufficiently reproducible.

Does internal resistance measurement replace a capacity test?

No. Internal resistance or impedance measurement is highly suitable for diagnostics and trend monitoring. However, it does not directly determine the battery’s actual available capacity. A defined discharge test is required to verify this.

When should an abnormal battery block be replaced?

A replacement decision should not be based solely on a single resistance value. A combined evaluation of trend, string comparison, voltage, temperature, age, operating conditions, behavior under load and, where necessary, the result of a capacity test is more appropriate.

What is the advantage of permanent battery monitoring?

With permanent monitoring, relevant battery parameters are not evaluated only during individual maintenance visits. This allows changes occurring between maintenance intervals to be detected earlier and historical developments to be evaluated more systematically.

When is a discharge test useful?

A discharge test is useful when the battery’s actual available performance or capacity must be verified. It can also be used to investigate abnormal results from resistance, voltage or trend monitoring in greater detail.