EVT EVT Thermal Management & Cooling Systems 2 — Questions and Answers
Question 1: What is 'thermal runaway' in a lithium-ion battery cell?
- Gradual capacity fade from repeated charge cycles
- Self-sustaining exothermic reaction where rising temperature accelerates further heat generation, potentially leading to fire (Correct answer)
- Reversible loss of range in cold weather
- Electrolyte evaporation at high state of charge
Correct answer: Self-sustaining exothermic reaction where rising temperature accelerates further heat generation, potentially leading to fire
Thermal runaway is a positive feedback loop where heat causes more reactions that produce more heat, often resulting in cell venting, fire, or explosion if not contained.
Question 2: Which refrigerant is most commonly used in modern EV heat pump and battery cooling systems in the US?
- R-22
- R-134a
- R-1234yf (Correct answer)
- R-410A
Correct answer: R-1234yf
R-1234yf (HFO) has a global warming potential (GWP) of <1 compared to R-134a's GWP of 1,430, making it the current industry standard for automotive A/C and heat pump systems.
Question 3: A heat pump system in an EV can provide cabin heating more efficiently than a resistive heater. What is the typical Coefficient of Performance (COP) advantage?
- Heat pump COP ≈ 1.0 vs. resistive heater COP = 0.5
- Heat pump COP ≈ 2–3 vs. resistive heater COP = 1.0 (Correct answer)
- Both have identical COP of 1.0
- Heat pump COP ≈ 0.5 vs. resistive heater COP = 2.0
Correct answer: Heat pump COP ≈ 2–3 vs. resistive heater COP = 1.0
A heat pump moves heat from outside air or waste sources into the cabin with a COP of 2–3, meaning 2–3 kWh of heat per 1 kWh of electrical input, while a resistive heater is always COP=1.
Question 4: When inspecting an EV battery cooling plate, a technician finds a greenish-white deposit near a fitting. What does this likely indicate?
- Normal oxidation of aluminum — no action required
- Coolant leak causing corrosion at the fitting (Correct answer)
- Refrigerant oil residue from a prior repair
- Thermal paste migration from adjacent cells
Correct answer: Coolant leak causing corrosion at the fitting
Greenish-white deposits at coolant fittings are a classic sign of glycol coolant leaks reacting with aluminum, indicating a seal or fitting failure requiring immediate repair.
Question 5: What is the function of the coolant expansion tank (reservoir) in an EV battery thermal management system?
- Stores reserve coolant and accommodates volume changes due to thermal expansion (Correct answer)
- Mixes refrigerant with coolant for heat exchange
- Provides pressurized coolant to the high-voltage pump
- Separates air from refrigerant in the AC circuit
Correct answer: Stores reserve coolant and accommodates volume changes due to thermal expansion
The expansion tank acts as a buffer to accommodate coolant volume changes as temperature varies, and as a reservoir to maintain system fill level.
Question 6: A BMS flags a 'cell temperature delta high' fault with a 12 °C spread across the pack. What is the MOST likely root cause?
- One cell has a higher state of charge than others
- Blocked or restricted coolant flow causing uneven cooling of the pack (Correct answer)
- High ambient humidity affecting sensor accuracy
- Normal variation during a Level 1 charge session
Correct answer: Blocked or restricted coolant flow causing uneven cooling of the pack
A large temperature delta across cells typically indicates uneven coolant flow, often from a partial blockage, air pocket, or failing pump causing poor thermal uniformity.
EVT EVT Thermal Management & Cooling Systems 2
What is 'thermal runaway' in a lithium-ion battery cell?