Which of the following statements about the relationship between internal resistance and thermal runaway is true?

• A. High internal resistance can cause runaway
• B. Moderate resistance causes runaway
• C. Low internal resistance can allow large current between cells causing overheating
• D. Internal resistance has no effect

Answer: (C)

When a fault creates a path between cells, what matters most is how much heat is generated inside that path. Heat produced inside a conductor follows P = I^2R, so if the internal resistance is low, the fault can push a large current through that path. That large current, even with a small resistance, generates a lot of heat inside the cells. If the pack can’t dissipate this heat quickly enough, the temperature rises. As temperature climbs, the chemical reactions inside the cells can release more heat, accelerating further temperature increase and potentially leading to thermal runaway. That’s why low internal resistance is a key risk factor for overheating and runaway.

Higher internal resistance would limit the current during a fault, reducing heat generation and the likelihood of runaway from a short between cells. Internal resistance does affect heating, so saying it has no effect isn’t correct.