A battery technician takes a reading and notices that one block is higher than it was six months ago.

The immediate question is understandable:

How much higher is too high?

People often want one number. Twenty percent. Thirty percent. Fifty percent.

It would certainly make maintenance easier if there were a universal line: cross this value and replace the battery.

In practice, stationary batteries do not work quite that neatly.

Internal resistance—or the broader ohmic value reported by different test instruments—is useful because it can show that something inside the battery or its connection path has changed.

But the reading is a diagnostic clue.

It is not, by itself, a complete capacity test or an automatic replacement order.

That distinction matters in UPS systems, where a battery decision has to consider not only one measurement but the condition of the string that must support the critical load.

The Number Is Useful. The Threshold Is the Problem.

As lead-acid batteries age, changes inside the cell can increase their resistance to current flow.

That is why resistance, impedance and conductance measurements have become common maintenance tools for stationary batteries.

But different instruments do not necessarily measure exactly the same thing in exactly the same way.

Internal ohmic measurements are most useful when they are interpreted as part of a trend rather than treated as a standalone verdict.

That already tells us something important.

If Battery A reads higher than Battery B, that does not automatically mean Battery A is bad.

They may be different models, different ages, different constructions or measured under different conditions.

The more useful comparison is usually:

How has this battery changed relative to its own established baseline and to comparable units in the same system?

Start With Your Own Baseline

A resistance reading becomes much more useful once there is history behind it.

Imagine a new UPS battery string has been commissioned.

Instead of waiting several years and then taking the first measurement, establish a repeatable baseline while the battery condition is known and stable.

Then keep using a consistent measurement method.

Over time, you are no longer asking:

Is 4.8 milliohms good or bad?

You are asking:

Why did this unit move away from its previous behavior while the rest of the string remained relatively stable?

That is a much better maintenance question.

The trend often tells you more than the isolated number.

Before Blaming the Battery, Check the Measurement

A higher reading deserves attention.

It does not mean the first action should always be to remove the battery.

There are several things worth checking first.

Is the connection sound?

An ohmic measurement can be affected by more than the electrochemistry inside the cell.

Loose hardware, corrosion and interconnection problems can contribute resistance to the measured path.

That is why connection inspection belongs in the same conversation as battery condition.

Has the temperature changed?

Battery electrical characteristics are temperature-dependent.

If today's test was made under materially different conditions from the baseline, the comparison needs context.

The goal is not to compensate by guessing.

It is to make sure the measurements being compared were obtained under conditions that make the comparison meaningful.

Was the same method used?

Resistance, impedance and conductance testers may use different measurement techniques.

A number from one instrument should not automatically be treated as interchangeable with a number from another.

If the maintenance program changes equipment or method, document that change.

Otherwise the trend may partly reflect the test system rather than the battery.

One Battery Should Not Be Read in Isolation

A UPS battery string gives you another useful reference point: its neighbors.

Suppose most batteries in the same string have followed a fairly stable pattern, while one unit has begun moving noticeably faster.

That divergence deserves investigation.

Now imagine that every unit in the string is moving upward in roughly the same way.

That tells a different story.

The first case may point toward an individual unit, connection or local condition.

The second may suggest a broader aging or operating pattern.

This is one reason battery monitoring becomes more useful when it shows the whole string as a trend, rather than presenting dozens of unrelated numbers.

A dashboard full of measurements is not the goal.

The goal is to make abnormal behavior easier to recognize.

When a Rising Reading Deserves Action

There is a difference between:

something changed

and

this battery must be replaced immediately.

If one battery begins to move away from its established pattern, I would want to know a few things before making the replacement decision:

  • Is the reading repeatable?
  • Has the measurement method changed?
  • Are the terminals and interconnections in good condition?
  • Is the battery operating at a comparable temperature?
  • Is the same unit also showing abnormal voltage or temperature?
  • Is the trend accelerating?
  • How does it compare with similar units in the string?
  • What does the battery manufacturer's guidance say?
  • Has an appropriate capacity or discharge test been performed where required?

None of those questions is particularly complicated.

Together, however, they are much more useful than asking whether the resistance has crossed one generic percentage.

Why I Would Not Use a Universal 25%, 30% or 50% Rule

This is where battery discussions often become confusing.

Industry papers, service organizations and individual manufacturers have used different percentage changes as maintenance alerts or replacement indicators.

You will see numbers such as 25%, 30% and 50% in different contexts.

That does not make them universal thresholds.

A threshold may belong to:

  • a particular battery family
  • a particular test method
  • a manufacturer's recommendation
  • a monitoring system's alarm policy
  • a specific maintenance program

It should not automatically become a rule for every UPS battery.

Capacity Still Has the Last Word

Internal resistance is valuable because it can be measured without performing a full discharge every time.

That makes it practical for trending.

But resistance is not the same thing as available capacity.

A good maintenance program does not ask one measurement to do every job.

Ohmic measurements help identify change.

Voltage and temperature provide more context.

Inspection may uncover connection or physical problems.

Historical data shows whether the behavior is stable or accelerating.

And when the actual ability to support the required load has to be demonstrated, the applicable battery manufacturer's procedures and approved capacity-testing practice become important.

IEEE 1188 provides recommended practices covering maintenance, testing and replacement of stationary VRLA batteries.

Where Continuous Battery Monitoring Helps

Periodic testing gives you snapshots.

Continuous monitoring adds the time between them.

That difference can be useful when a battery does not fail in a neat, scheduled way.

Instead of looking at this:

January: normal

July: abnormal

the maintenance team can see how the value moved between January and July.

Was the change slow?

Did it accelerate?

Did temperature change at the same time?

Was there a discharge event?

Did only one block move, or did the entire string shift?

Those are much better questions for maintenance planning.

CONLUXS currently includes battery-monitoring systems within its EMS & Battery Monitoring Systems product portfolio, alongside its data-center and critical-power equipment.

The value of monitoring, however, should not be described as “the system decides when to throw away the battery.”

A better role is:

give the maintenance team enough consistent data to see which battery needs attention, and why.

A Better Replacement Rule

If I had to reduce the whole subject to one maintenance principle, it would be this:

Do not replace a stationary UPS battery because one resistance number crossed an internet threshold.

And don't ignore a meaningful change just because the battery has not crossed that threshold either.

Instead:

  • Establish a baseline.
  • Watch the trend.
  • Compare like with like.
  • Check the connections.
  • Consider operating conditions.
  • Compare the unit with the rest of the string.
  • Use the battery manufacturer's criteria.
  • And when actual available capacity has to be confirmed, use the appropriate approved test method.

That approach is less satisfying than one magic percentage.

It is also much closer to how a critical battery system should actually be maintained.

Reviewing a UPS Battery Monitoring System?

For a battery-monitoring or critical-power project, useful information to prepare includes:

  • battery chemistry and model
  • number of strings
  • batteries or cells per string
  • nominal system voltage
  • existing UPS arrangement
  • available baseline data
  • required monitoring points
  • communication requirements
  • alarm and integration requirements
  • maintenance and reporting expectations

CONLUXS can review the defined monitoring and electrical interfaces with the project team and discuss an appropriate equipment configuration.