EVT Battery Technology & Energy Storage 3 — Questions and Answers
Question 1: A technician measures the insulation resistance of an EV high-voltage battery pack and gets a reading of 200 kΩ. What does this indicate?
- The pack is within safe operating limits above 100 Ω/V
- The insulation is degraded and the vehicle should not be energized (Correct answer)
- Normal reading; no action needed
- The BMS is limiting output voltage
Correct answer: The insulation is degraded and the vehicle should not be energized
EV high-voltage systems typically require insulation resistance above 100 Ω/V (often 500 kΩ or more for 400V systems); 200 kΩ on a 400V pack falls below safe thresholds.
Question 2: What is the primary purpose of cell balancing in a multi-cell battery pack?
- To equalize temperature across all cells
- To ensure all cells reach the same state of charge to maximize usable capacity (Correct answer)
- To reduce total internal resistance of the pack
- To prevent lithium plating during regenerative braking
Correct answer: To ensure all cells reach the same state of charge to maximize usable capacity
Cell balancing ensures every cell in the pack operates at the same SOC, preventing weak cells from limiting overall pack capacity and extending pack life.
Question 3: Which battery chemistry offers the highest energy density by weight, making it preferred for long-range BEVs?
- Lithium iron phosphate (LFP)
- Nickel manganese cobalt (NMC) (Correct answer)
- Nickel metal hydride (NiMH)
- Lead-acid AGM
Correct answer: Nickel manganese cobalt (NMC)
NMC chemistry achieves higher gravimetric energy density than LFP or NiMH, enabling greater range for a given battery weight in long-range BEV applications.
Question 4: During thermal runaway, in what order do the following events typically occur?
- Venting → fire → cell rupture → heat buildup
- Heat buildup → venting → cell rupture → fire (Correct answer)
- Cell rupture → heat buildup → venting → fire
- Fire → heat buildup → venting → cell rupture
Correct answer: Heat buildup → venting → cell rupture → fire
Thermal runaway follows the sequence of heat buildup triggering electrolyte decomposition, followed by gas venting, then mechanical rupture, and finally ignition.
Question 5: What is 'capacity fade' in the context of EV battery aging?
- A sudden drop in voltage under high load
- A gradual reduction in the total energy a battery can store over its lifetime (Correct answer)
- The decrease in peak power output at low temperatures
- Loss of BMS calibration accuracy over time
Correct answer: A gradual reduction in the total energy a battery can store over its lifetime
Capacity fade refers to the irreversible reduction in maximum storable energy that accumulates with cycling and calendar aging due to active material loss and electrolyte degradation.
Question 6: A battery pack uses passive balancing. What happens to excess energy from high-SOC cells during balancing?
- It is transferred to low-SOC cells through a DC-DC converter
- It is dissipated as heat through resistors (Correct answer)
- It is fed back to the vehicle's 12V auxiliary battery
- It is stored in capacitors for later use
Correct answer: It is dissipated as heat through resistors
Passive balancing bleeds excess charge from higher-SOC cells by dissipating energy as heat through bleed resistors, equalizing SOC without energy transfer.
Question 7: What is the significance of the 'knee point' in a battery's capacity vs. cycle-count graph?
- It marks the point where the battery reaches peak capacity
- It indicates where capacity fade accelerates rapidly, signaling end of useful life (Correct answer)
- It shows the optimal operating temperature range
- It represents the transition from constant current to constant voltage charging
Correct answer: It indicates where capacity fade accelerates rapidly, signaling end of useful life
The knee point marks where the gradual linear capacity fade curve bends sharply downward, indicating accelerated degradation and approaching end-of-life.
EVT Battery Technology & Energy Storage 3
A technician measures the insulation resistance of an EV high-voltage battery pack and gets a reading of 200 kΩ.
What does this indicate?