EVT Battery Technology & Energy Storage 5 ā Questions and Answers
Question 1: A technician performing a battery health check finds the pack capacity is 74% of original. Under US EPA and typical warranty standards, what does this usually indicate?
- Normal; the pack has years of service remaining with no concern
- The battery is at or near the warranty replacement threshold, typically set at 70ā80% (Correct answer)
- Immediate replacement is required by federal law
- The BMS needs recalibration before capacity can be accurately assessed
Correct answer: The battery is at or near the warranty replacement threshold, typically set at 70ā80%
Most EV manufacturers set battery warranty replacement thresholds between 70ā80% of original capacity, so 74% is at or near that threshold and warrants further inspection.
Question 2: What role does the cathode material play in determining a lithium-ion battery's nominal voltage?
- The cathode determines internal resistance only; anode sets voltage
- The electrochemical potential difference between cathode and anode materials sets the cell voltage (Correct answer)
- Voltage is determined by electrolyte concentration, not electrode materials
- Only the separator material affects cell voltage
Correct answer: The electrochemical potential difference between cathode and anode materials sets the cell voltage
Cell voltage is determined by the difference in electrochemical potential (Gibbs free energy) between cathode and anode materials; different cathode chemistries produce different nominal voltages.
Question 3: When storing an EV battery pack long-term, what state of charge (SOC) is recommended to minimize degradation?
- 0% SOC to prevent overcharge
- 100% SOC to maintain cell formation
- Approximately 25ā50% SOC (Correct answer)
- Exactly 75% SOC for optimal electrolyte distribution
Correct answer: Approximately 25ā50% SOC
Storing lithium-ion batteries at 25ā50% SOC minimizes cathode and anode stress, reduces SEI growth rate, and slows calendar aging compared to storage at extremes.
Question 4: Which failure mode is indicated when a lithium-ion cell shows high self-discharge rate without mechanical damage?
- Separator integrity loss or internal micro-short circuit (Correct answer)
- Cathode delamination from current collector
- Lithium plating on the cathode
- Electrolyte over-concentration of LiPF6 salt
Correct answer: Separator integrity loss or internal micro-short circuit
A high self-discharge rate without visible damage typically indicates micro-short circuits caused by lithium dendrites or separator degradation allowing electron leakage between electrodes.
Question 5: How does regenerative braking affect the state of charge and temperature of an EV battery pack?
- It discharges the battery and lowers temperature
- It charges the battery and may increase temperature due to charge current (Correct answer)
- It has no effect on SOC; it only recovers kinetic energy as heat
- It charges the battery at constant voltage with no heat generation
Correct answer: It charges the battery and may increase temperature due to charge current
Regenerative braking converts kinetic energy into electrical energy, increasing battery SOC while also generating heat due to charging current flowing through the pack's internal resistance.
Question 6: What is the primary difference between 'calendar aging' and 'cycle aging' in battery degradation?
- Calendar aging only occurs below 0Ā°C; cycle aging occurs at all temperatures
- Calendar aging is time and temperature-dependent degradation at rest; cycle aging results from repeated charge/discharge use (Correct answer)
- They are interchangeable terms for the same process
- Cycle aging only affects the anode; calendar aging only affects the cathode
Correct answer: Calendar aging is time and temperature-dependent degradation at rest; cycle aging results from repeated charge/discharge use
Calendar aging degrades the battery simply by sitting over time (accelerated by heat and high SOC), while cycle aging accumulates damage through the mechanical and chemical stresses of charging and discharging.
Question 7: An EV's BMS activates a 'high voltage interlock loop' (HVIL) fault and disconnects the pack contactors. What does this indicate?
- The battery has reached 100% SOC and is preventing overcharge
- A break in the high-voltage connector safety loop has been detected, indicating an open circuit or connector issue (Correct answer)
- Cell temperature has exceeded 60Ā°C threshold
- The vehicle is in park and HVIL is a normal shutdown function
Correct answer: A break in the high-voltage connector safety loop has been detected, indicating an open circuit or connector issue
The HVIL is a series safety circuit through HV connectors; a fault means the loop is broken, indicating an open HV connector, damaged wiring, or a cover removed from a HV component.
EVT Battery Technology & Energy Storage 5
A technician performing a battery health check finds the pack capacity is 74% of original.
Under US EPA and typical warranty standards, what does this usually indicate?