Battery and UPS Systems Testing Flashcards

During a conductance test on a VRLA battery string, one cell registers a conductance value 35% below the manufacturer's baseline. The adjacent cells are within 5% of baseline. According to IEEE 1188, what is the MOST appropriate next step before recommending replacement?

Answer: Perform a discharge capacity test at the 8-hour rate to confirm actual capacity loss

IEEE 1188 recommends that conductance readings deviating significantly from baseline be confirmed with a capacity (discharge) test, as conductance is a screening tool rather than a definitive capacity measurement. A single anomalous reading warrants verification before replacement, since temporary conditions (temperature, surface charge) can skew conductance values. The discharge test at the rated hour-rate provides definitive capacity data.

A double-conversion online UPS is being tested under a 100% resistive load. The input kVA reading is significantly higher than the output kW reading even though the output power factor is unity. Which of the following BEST explains this condition?

Answer: The rectifier draws non-sinusoidal current from the utility, resulting in a low input displacement power factor and high THDi

Double-conversion UPS rectifiers (especially older SCR-based and modern IGBT 6-pulse designs) draw non-sinusoidal current from the source. This high total harmonic distortion of input current (THDi) causes the true input power factor to be significantly less than unity even when the output load power factor is 1.0 (pure resistive). The ratio of input kVA to output kW therefore exceeds 1, reflecting the UPS's own input power quality characteristics — not a fault condition.

When performing an impedance spectroscopy test on a flooded lead-acid cell, the technician notices a depressed semicircle in the Nyquist plot at mid-frequencies. This feature is MOST indicative of:

Answer: Non-uniform current distribution due to porosity distribution in the active material

A depressed (flattened) semicircle in a Nyquist impedance plot indicates a distribution of time constants rather than a single RC process — the hallmark of non-uniform current distribution through porous electrode active material. This is modeled using a Constant Phase Element (CPE) instead of an ideal capacitor. Sulfation shifts the semicircle diameter (charge-transfer resistance) but does not necessarily depress it; stratification appears at very low frequencies as a Warburg diffusion tail; separator effects manifest at high-frequency intercepts.

A technician is load-testing a UPS system rated 500 kVA / 450 kW. The test load bank is set to draw 360 kW at 0.8 power factor lagging. The UPS passes the test, but the facility engineer questions whether the test was valid. The engineer is CORRECT to question the test because:

Answer: The test load exceeds the UPS kVA rating when reactive power is included

360 kW at 0.8 power factor lagging requires 360 / 0.8 = 450 kVA of apparent power — equal to the UPS's full kVA rating. However, the UPS is rated 500 kVA, so the kVA loading is only 90%. More critically, a UPS rated 500 kVA / 450 kW at a specified power factor implies the inverter's current rating is the binding constraint. At 0.8 PF lagging with 360 kW, the apparent power is 450 kVA, which is within rating, but if the facility engineer expected a test at 450 kW (rated real power), the test only validates 80% of real power capacity. The core validity concern is that 450 kVA at 0.8 PF = 360 kW, not 450 kW — so the full real power capability was NOT tested.

During acceptance testing of a new UPS installation, the NETA technician measures the transfer time from normal to bypass mode during a simulated inverter fault. The measured transfer time is 6 ms. The facility runs sensitive process control equipment specified to tolerate no more than 4 ms interruption. Which action is MOST appropriate?

Answer: Flag the result as a deficiency and require the UPS manufacturer to adjust or verify the static switch transfer time specification

The load equipment's specified immunity is 4 ms; the measured transfer time is 6 ms — a clear incompatibility. The correct NETA acceptance action is to document and flag this as a deficiency requiring resolution from the equipment supplier. Averaging test results across load levels is not a valid acceptance methodology. Adding external ride-through capacitance is a workaround that bypasses the underlying deficiency and is not a technician's unilateral recommendation during acceptance. The ITIC curve is a voltage sag immunity standard for equipment, not a justification for exceeding equipment-specific transfer time requirements.

A VRLA AGM battery string on float service has been in service for 7 years. Thermal imaging reveals two cells with case temperatures 8°C above ambient while adjacent cells are 2°C above ambient. The float voltage across the hot cells is 0.05 V lower than adjacent cells. This combination of findings MOST likely indicates:

Answer: Thermal runaway in an early stage, driven by elevated local self-discharge and reduced oxygen recombination efficiency

The combination of elevated cell temperature AND lower float voltage on the same cells is the classic early signature of thermal runaway initiation in VRLA AGM cells. As internal resistance drops (due to electrolyte dry-out or separator degradation), the cells absorb more float current, generate more heat, which further lowers resistance — a positive feedback loop. The lower float voltage occurs because the cells are consuming more current (voltage drop across increased internal resistance) while dissipating power as heat rather than storing charge. Loose connectors would show high, not low, voltage across the cell. If adjacent cells were overcharging the weak cells, the hot cells would show elevated voltage, not reduced voltage.