Thermal mass, lag and why systems overshoot
Why a thermal system reacts minutes after the heat load changes.
You'll learn
- Explain thermal mass, lag and why systems overshoot at engineering level
- Read the numbers that define the behaviour
- Apply the idea to a real vehicle
Overview
Electrical systems respond in milliseconds. Thermal systems respond in minutes, because heat has to physically travel through metal and fluid before a sensor sees it.
In depth
Lag builds in at every stage
Heat generated in a cell must first conduct through the can and cold plate, then convect into coolant, then travel through hose length to a sensor. Each stage adds delay, so a control system reacting only to the immediate reading will overshoot and hunt.
Formula
τ = m × c ÷ (U × A)
The thermal time constant grows with mass and specific heat, and shrinks with heat transfer coefficient and area — exactly mirroring the electrical RC time constant.
Example: A large aluminium cold plate might have a time constant of several minutes, meaning setpoint changes take that long to fully show up.
Key takeaways
Remember this
- Thermal systems lag electrical ones by orders of magnitude.
- Time constant scales with mass and specific heat, inversely with U·A.
- Predictive control compensates for lag that feedback alone cannot fix.
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