BMS Diagnostics Maintenance & Safety Standards 2 — Questions and Answers
Question 1: Which diagnostic parameter best indicates the beginning of lithium plating in a Li-ion cell during charging?
- Rising internal resistance at high SOC
- Voltage plateau below 3.0V during charge
- Coulombic efficiency dropping below 99.5% (Correct answer)
- Cell temperature exceeding 45°C
Correct answer: Coulombic efficiency dropping below 99.5%
Coulombic efficiency (charge-out/charge-in) dropping below ~99.5% is an early indicator of lithium plating, as plated lithium is not fully recovered on discharge.
Question 2: A BMS reports a persistent cell voltage divergence that increases during discharge but resets after a rest period. The most likely cause is:
- Faulty voltage sense wire with high contact resistance
- Differing self-discharge rates between cells
- Incorrect SOC calibration offset
- Cell-to-cell capacity imbalance (Correct answer)
Correct answer: Cell-to-cell capacity imbalance
Capacity imbalance causes weaker cells to deplete faster under load, creating divergence that recovers at rest as OCV equalizes.
Question 3: When performing insulation resistance testing on a high-voltage battery pack, what minimum acceptable IR value is typically required per IEC 60664?
- 100 Ω/V
- 500 Ω/V
- 1,000 Ω/V (Correct answer)
- 10,000 Ω/V
Correct answer: 1,000 Ω/V
IEC 60664 and most EV safety standards require a minimum insulation resistance of 1,000 Ω/V (100 Ω/V for AC systems) relative to system voltage.
Question 4: A technician uses an EIS (Electrochemical Impedance Spectroscopy) tool and finds the Warburg impedance component has significantly increased. This most likely indicates:
- Increased contact resistance at the cell terminals
- Electrolyte depletion or lithium-ion diffusion limitation (Correct answer)
- SEI layer growth on the anode
- Separator micropore blockage
Correct answer: Electrolyte depletion or lithium-ion diffusion limitation
The Warburg element in an EIS Nyquist plot represents solid-state diffusion; a large Warburg tail indicates slowed Li-ion transport due to electrolyte depletion or concentration polarization.
Question 5: OSHA 1910.303 requires that electrical equipment in battery rooms must be listed and labeled. What is the primary purpose of this requirement?
- To ensure equipment is energy-efficient
- To confirm equipment has been evaluated for safety by a recognized testing laboratory (Correct answer)
- To verify equipment meets FCC electromagnetic compatibility standards
- To document the equipment's service history
Correct answer: To confirm equipment has been evaluated for safety by a recognized testing laboratory
OSHA requires listing/labeling to confirm that a Nationally Recognized Testing Laboratory (NRTL) has evaluated the equipment for electrical safety hazards.
Question 6: During a capacity test, a battery pack delivers only 78% of rated capacity at 0.5C but 91% at 0.2C. The technician should conclude:
- The battery has a calibration error in the BMS
- High internal resistance is causing significant voltage drop at higher rates (Correct answer)
- The cells have excessive self-discharge
- The battery's BMS is limiting current output incorrectly
Correct answer: High internal resistance is causing significant voltage drop at higher rates
Rate-dependent capacity reduction is characteristic of increased internal resistance; higher current causes greater IR voltage drop, triggering the lower-voltage cutoff earlier.
Question 7: When replacing a failed cell in a battery module, the replacement cell's SOC should be matched to the existing pack within:
- ±20% SOC
- ±10% SOC
- ±5% SOC
- ±2% SOC (Correct answer)
Correct answer: ±2% SOC
Replacement cells should be within ±2% SOC of the pack to prevent large initial balancing currents and thermal stress during the first charge-discharge cycle.
BMS Diagnostics Maintenance & Safety Standards 2
Which diagnostic parameter best indicates the beginning of lithium plating in a Li-ion cell during charging?