UPS Capacitor & Fan Maintenance Guide

Electrolytic capacitors are aluminium cylinders filled with a liquid or gel electrolyte. They perform critical roles across every stage of a UPS: filtering input rectifier ripple, stabilizing the DC bus, supporting the inverter output, and dampening transient spikes on the bypass path. Because they operate under continuous electrical and thermal stress, they are the most common cause of age-related UPS failure.

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The electrolyte inside every aluminium electrolytic capacitor slowly evaporates through the rubber seal over time. This process is accelerated by heat, ripple current, and voltage stress. As the electrolyte depletes, capacitance falls, equivalent series resistance (ESR) rises, and the capacitor can no longer perform its filtering or energy-storage functions. A capacitor degraded in this way causes increased voltage ripple on the DC bus, output waveform distortion, and in severe cases, catastrophic failure, rupturing the top vent or, in rare instances, thermal ignition.

Temperature is the Primary Enemy: Every 18°F rise in core temperature approximately halves capacitor life. A cap rated at 2,000 hours at 221°F will deliver around 16,000 hours at 149°F — but only 4,000 hours at 185°F. Internal UPS temperatures regularly exceed ambient by 36–72°F.

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Ripple Current: High-frequency ripple from rectifiers and inverters generates heat inside the capacitor body. Caps operating above their rated ripple current rating age disproportionately faster, regardless of temperature.

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Voltage Stress: Operating near the rated voltage ceiling, or exposing capacitors to frequent transient spikes, accelerates dielectric breakdown and electrolyte degradation. Size your UPS to avoid sustained high-load operation.

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Calendar Aging: Chemical degradation occurs regardless of use. A UPS stored without power still has ageing capacitors. Long decommission periods without periodic reforming can cause electrolyte to crystalize.

• Bulging or domed top: Internal gas pressure has built up due to electrolyte decomposition — replace immediately

• Electrolyte leakage: Brown, oily residue on the PCB around the capacitor base indicates seal failure

• Discolored or scorched PCB: Heat from a failing capacitor can discolour surrounding board material

• Cracked vent: Safety vent at the top of the capacitor has opened, releasing electrolyte vapor

• UPS output distortion: Connected equipment experiences unexplained crashes or power anomalies

• Unusual buzzing or hum: Resonance changes as capacitor internals degrade can alter acoustic signature

• Overheating UPS chassis: Degraded filter capacitors generate more heat, raising internal temperatures

• UPS fault codes: DC bus voltage faults, output voltage faults, or charger faults can all result from capacitor failure

Proactive replacement is recommended at 5–8 years service life. Even without visible symptoms; electrolyte depletion is invisible until failure.

UPS systems use one or more DC brushless fans to remove heat from power transistors, transformers, inductors, and battery compartments. Without adequate cooling, internal temperatures climb rapidly, shortening the life of every component in the unit and dramatically increasing the risk of thermal shutdown or fire. Fans are simple, mechanical components — but their failure triggers a cascade of secondary damage that is far more costly to repair.

• Bearing seizure from lack of lubrication: Sleeve bearings dry out over time, especially in hot environments

• Dust accumulation on blades: Sauses imbalance and vibration that accelerates bearing wear

• Motor coil degradation: Insulation breakdown in motor windings from heat cycling

• Power supply/controller failure: Voltage regulator or PWM controller failure means fan receives no/incorrect voltage

• Humidity and condensation: Water ingress corrodes bearings and shorts motor windings

• Chemical/airborne contaminants: Salt, acids, or cleaning solvents corrode components

• Continuous maximum speed operation: Running constantly at 100% speed rather than variable speed reduces bearing life

• Audible grinding or rattling: Bearing wear is progressing; fan will seize without replacement

• Intermittent operation: Fan stops and starts unpredictably, indicating electrical or mechanical intermittent fault

• Reduced airflow (not due to dust): Fan running but delivering less airflow than normal, blade damage or motor degradation

• Elevated UPS chassis temperature: Exterior of UPS noticeably hotter than usual under the same load conditions

• UPS thermal shutdown: UPS switches to bypass or shuts down due to over temperature,  a failed fan is a primary suspect

• Fan fault alarm: UPS management software or front panel indicates fan failure status

• Vibration increase: Imbalanced fan blades cause vibration transmitted through the UPS chassis

• Visible blade damage: Chipped, cracked, or warped fan blades found during visual inspection

Proactive replacement is recommended at 5–8 years service life. This is typically completed alongside the capacitor replacement.