Introduction

Uninterruptible Power Supplies (UPS) are the silent sentinels of our data centers and critical infrastructures. However, their beating heart — the sealed lead-acid (VRLA) battery bank — has a limited lifespan. Its degradation is inevitable, but not unpredictable.

Waiting for a UPS to emit an annoying alarm beep or, worse, to fail during a blackout is not a maintenance strategy. It’s a calculated risk no serious professional should take.

In this article, we examine the first technical and measurable signs that signal the beginning of the end for a VRLA battery. Acting on these indicators allows you to plan replacements, prevent costly downtime, and ensure the integrity of the protected load.

1. Increase in Internal Impedance (or AC Resistance)

This is the most revealing parameter — and often the first to deteriorate.

What it is: Internal impedance is the opposition the battery presents to the flow of alternating current. In a new battery, it is very low.
Why it increases: As the battery ages, plate sulfation occurs: lead sulfate crystals (a normal byproduct of discharge) become large, hard, and no longer fully reconvertible during charging. These crystals insulate the active material, reducing the reactive surface area and drastically increasing internal resistance.
How it’s measured: With a dedicated battery impedance tester, which provides a value in milliohms (mΩ). This measurement cannot be obtained with a simple multimeter.
Warning sign: An impedance increase greater than 20–30% compared to the nominal value of a new battery clearly indicates advanced degradation, even if the open-circuit voltage still appears normal.

2. Capacity Drop (Ah) During Controlled Discharge Tests

The rated capacity, expressed in ampere-hours (Ah), is the very reason the battery exists.

What it is: The amount of energy the battery can deliver at a specific current until it reaches its cutoff voltage.
Why it decreases: Sulfation and positive grid corrosion — natural processes accelerated by high temperatures — progressively reduce the amount of active material available for electrochemical reactions.
How it’s measured: By performing a controlled discharge test. Using a dedicated resistive load bank, the battery is discharged at a preset current (e.g., C/8, meaning rated capacity divided by 8 hours) while measuring the time to reach the end-of-discharge voltage.
Warning sign: When the measured capacity drops below 80% of the rated capacity, the battery has exceeded its useful life and is at risk of imminent failure under real load conditions.

3. Rise in Temperature During Rest or Float Charge

Heat is the number one enemy of VRLA batteries.

What it is: An abnormal battery temperature, especially during float charging.
Why it increases: High internal impedance or the presence of micro–short circuits caused by dendrites result in part of the charging energy being dissipated as heat instead of being converted into chemical energy. This triggers a vicious cycle: heat accelerates grid corrosion and electrolyte evaporation, further worsening battery health.
How it’s measured: With an infrared thermal camera, you can detect “hot spots” on a single cell or battery within a bank. A simple contact thermometer can provide a general reading.
Warning sign: Surface temperatures consistently 5–10°C higher than ambient temperature during float operation, or significant temperature differences between cells in the same bank.

4. Inability to Maintain Float Voltage

The UPS electronics constantly work to maintain a stable float voltage (typically 13.5–13.8 V for 12 V batteries).

What it is: A battery that struggles to stabilize at the float voltage even when connected to the UPS charger.
Why it happens: Batteries with high self-discharge or internal shorted cells continuously “absorb” current without ever reaching a stable charge state. This forces the charger to operate at full capacity, often without success.
How it’s measured: By monitoring float charging current and battery voltage using a power analyzer or datalogging multimeter. If after a full charge cycle the float current doesn’t drop to a few milliamperes and voltage fluctuates or fails to stabilize, the battery is compromised.
Warning sign: An abnormally high and persistent float current that does not decrease 24–48 hours after charging completion.

5. Physical Deformation (Swelling) of the Battery Case

A clear and dangerous visual indicator.

What it is: Swelling of the battery’s plastic casing.
Why it happens: Caused by excessive internal pressure, often due to inefficient gas recombination or, more commonly, overcharging that triggers water electrolysis and the generation of oxygen and hydrogen faster than the system can recombine them.
How it’s detected: Through regular visual inspection.
Warning sign: Any deformation, even minor. A swollen battery is a safety hazard due to the risk of case rupture and acid leakage, and must be replaced immediately.

Conclusions and Best Practices

Battery failures are not sudden events but gradual processes.
The key to preventing them lies in a predictive maintenance plan that goes beyond simple calendar-based replacement.

Investing in advanced monitoring tools and periodic inspections carried out by qualified personnel is not a cost — it’s the only real insurance for the operational continuity of your infrastructure.

Don’t wait for a blackout to put your batteries to the test. Contact ON LITE’s specialized technicians for a complete check-up of your UPS system.