Battery and UPS Systems Testing Flashcards

During a conductance test on a VRLA battery string, one cell measures 25% below the manufacturer's baseline conductance value while the string voltage under load remains within 2% of nominal. What is the MOST appropriate immediate action?

Answer: Flag the cell for accelerated monitoring and schedule replacement at the next maintenance window, as conductance degradation typically precedes capacity loss by 3–6 months

A single cell at 25% below baseline conductance indicates incipient failure but does not yet demand emergency replacement if string voltage remains acceptable. NETA and IEEE 1188 guidelines recommend flagging such cells for increased monitoring intervals (monthly rather than quarterly) and planning replacement within the maintenance cycle. Conductance loss is a leading indicator—capacity failure typically follows within months. Replacing the entire string prematurely is cost-prohibitive and unnecessary. An equalizing charge cannot restore a cell that has structurally degraded grid or plate material.

A technician performing a load bank discharge test on a 480V, 3-phase UPS observes that the DC bus voltage drops from 480V to 432V at 50% rated load after just 8 minutes, but the manufacturer's discharge curve predicts this voltage at 85% rated load after 45 minutes. Which failure mode BEST explains this discrepancy?

Answer: The battery string has significant capacity loss, likely due to sulfation or dry-out in multiple cells, reducing effective Ah capacity

The discharge curve deviation—reaching a given voltage milestone far too early and at lower load—is the classic signature of severely reduced battery capacity. Sulfation in flooded cells or electrolyte dry-out in VRLA cells reduces the available Ah far below nameplate rating. A miscalibrated load bank (A) would show consistent deviation at all loads, not the curve shape described. Inverter inefficiency (C) would affect AC output quality and heat, not the DC bus discharge rate independently. Float voltage masking (D) could affect initial SoC but would not produce an 8-minute to 45-minute time compression.

When performing inter-cell connection resistance measurements on a large flooded lead-acid battery bank, a technician finds that one inter-tier cable connection reads 175 µΩ while all others measure between 20–45 µΩ. The NETA MTS acceptance criterion flags connections above 150% of the lowest measured value. What is the expected temperature rise of the 175 µΩ connection compared to a 35 µΩ baseline connection at 1000A discharge current?

Answer: The 175 µΩ connection will dissipate approximately 5× more power than the 35 µΩ baseline, resulting in roughly 5× greater temperature rise above ambient

At constant current (series string), power dissipated is P = I²R. The ratio of resistances is 175/35 = 5×, so power dissipated is also 5× greater (since I is constant). Temperature rise above ambient is proportional to power dissipation (assuming similar thermal mass and environment), so the faulty connection runs approximately 5× hotter. Answer A conflates the ratio with the result but states it correctly. Answer B incorrectly claims 25× by squaring the resistance ratio—that would apply if voltage were constant (P = V²/R), not current. Answer C is wrong because equal current does not mean equal power—P = I²R depends on R. The correct framing is P_ratio = R_ratio = 5, so ~5× temperature rise.

A UPS system uses a double-conversion online topology with a 15-minute battery backup rating at full load. During acceptance testing, the technician discovers that the static bypass switch transfers to bypass in 4ms when the inverter is deliberately faulted. IEEE 446 and NETA MTS require that critical loads tolerate no more than a ½-cycle (8.33ms at 60Hz) transfer gap. However, the site's IT equipment power supplies are rated to ride through only 2ms of voltage interruption per ITIC curve Class 1 requirements. What corrective action is MOST appropriate?

Answer: Upgrade the static bypass switch to a faster SCR-based design rated for sub-2ms transfer

The conflict here is between the UPS's 4ms bypass transfer and the IT equipment's 2ms ride-through rating per the ITIC curve. Although 4ms meets IEEE 446 requirements for most critical loads, it violates the specific equipment's power quality requirement. The correct solution is to upgrade the static switch to a faster SCR-based design capable of sub-2ms transfer—modern solid-state bypass switches can achieve <1ms. A ferroresonant transformer (C) adds complexity, cost, and can introduce voltage regulation issues. Delaying fault detection (D) risks extended exposure to an inverter fault. Simply accepting the 4ms transfer (A) ignores the documented equipment sensitivity and creates a latent reliability risk.

A technician is evaluating a 10-year-old nickel-cadmium (NiCd) aircraft ground power UPS battery bank. Capacity testing shows 92% of rated capacity, but individual cell voltage measurements reveal a 'memory effect' pattern: 8 of 24 cells show a characteristic plateau at 1.0V during discharge before recovering to 1.15V. The NETA-certified technician should PRIMARILY recommend:

Answer: A controlled deep-discharge/recharge reconditioning cycle per the manufacturer's procedure to break up the cadmium hydroxide crystalline deposits causing the plateau

NiCd memory effect is caused by cadmium hydroxide crystal formation on the negative plate when cells are repeatedly partially discharged and recharged. Unlike common misconception, true NiCd memory effect IS reversible through controlled deep-discharge reconditioning cycles (typically discharge to 1.0V/cell, rest, then full recharge, repeated 2–3 times per manufacturer procedure). At 92% capacity with reversible memory effect, replacement is premature and costly. Increasing float voltage (C) has no effect on memory effect and can cause overcharge damage. Bypass shunts (D) reduce string capacity and create voltage imbalance. The memory effect plateau is a recognized, correctable condition in NiCd batteries through proper reconditioning.

During commissioning of a new 750kVA UPS installation, a NETA technician performs a harmonic analysis on the input current and measures a Total Harmonic Distortion (THDi) of 28% at 100% load. The facility's power purchase agreement requires THDi ≤ 8% at the point of common coupling (PCC). The UPS specification sheet lists input THDi of '< 5% with optional input filter.' The input filter was NOT included in the purchase order. What is the CORRECT finding to document in the NETA commissioning report?

Answer: Fail — the measured THDi of 28% violates the facility's PCC requirement; the optional input filter or active front-end must be retrofitted before energizing critical loads

A NETA commissioning report must document compliance with the facility's power quality contractual requirements, not just equipment specifications. The power purchase agreement's ≤8% THDi at the PCC is a binding requirement regardless of whether the UPS input filter was omitted from the purchase order. The 28% measured value exceeds this by 3.5×. The correct finding is a commissioning FAIL with a specific deficiency requiring correction before operational acceptance. NETA has no 'deferred deficiency protocol' for contractually binding power quality violations. The 6-pulse rectifier explanation (D) is factually accurate but irrelevant—the limit governs, not the technology's typical behavior. The commissioning engineer must flag the missing filter as a deficiency to be resolved.