What Is Thermal Insulation?

Battery fire

So that Aricell factory fire in Korea last year. 23 dead. If you work in batteries you probably followed it pretty closely.

Primary cells not Li-ion, but same basic problem-heat from one cell cooking the next one, no way to stop it once it starts. Whole warehouse gone in minutes.

Thermal insulation = stuff between cells to slow that down. That's really all it is.

Not gonna lie, the regulatory situation got way more serious after GM had to pull all those Bolts back. And Hyundai with the Konas. Procurement people who never asked about thermal management before suddenly want test reports, material specs, everything.

GB 38031 in China says you need 5 min before propagation hits adjacent cells. Europe has similar requirements now in ECE R100. Pain in the ass to certify but probably needed.

What insulation actually does

Doesn't stop thermal runaway from happening. Nothing does once a cell decides to go. BMS can catch some failure modes early but if internal short happens you're already in trouble.

What insulation actually does

Insulation just slows things down after. Buys time. Maybe the BMS cuts power before the next cell goes. Maybe someone hears the alarm and gets away from the forklift.

Also keeps heat from setting the pack housing on fire. Or the warehouse. Seen pictures of what happens when a forklift battery goes in a room full of cardboard. Not pretty.

Cold weather thing too-insulation keeps heat in when you actually want it warm. Can't charge Li-ion below freezing without causing damage. Plating on the anode, permanent capacity loss. Polinovel developed a self-heating system back in 2019 for exactly this. Mining sites up north, cold storage, etc. Heating element inside + insulation outside = pack stays warm overnight.

Materials

Aerogel

Everyone talks about aerogel. And yeah it's great-thermal conductivity crazy low, like under 0.02 W/m·K. Aspen Aerogels makes the good stuff.

Cost Factor

Cost is insane though. Like 5-10x what you'd pay for ceramic. Works for Tesla maybe. Doesn't work for industrial equipment where margins are tight.

Ceramic Fiber

Ceramic fiber is what most of us actually use. Aluminum silicate boards, mullite, whatever. Handles 1000°C+, way cheaper. Brittle so you gotta layer it with polymer. Morgan makes it, Unifrax, bunch of Chinese suppliers.

PCMs

PCMs were trendy for a while. Phase change materials-absorb heat when they melt. Sounds good until you realize real thermal events don't care about your design assumptions. Cell next to the PCM is already at 200°C and climbing. We stopped speccing these for anything serious.

Regular Foams

Regular foams like PU or EVA-fine for vibration damping and gap filling. Not for thermal barrier. Most of them catch fire at temps you'd see in runaway anyway.

Random design things

Busbars. Copper melts when cells get hot enough. If your high-voltage bus is too close to cell terminals you just created a short circuit path. Ceramic spacers help.

Module gaps. Everyone wants max energy density so they pack tight. Then one module goes and heat has nowhere to dissipate except into the next module. We leave 5-10mm gaps with ceramic felt between. Loses some density, gains a lot of safety margin.

Insulation thickness-diminishing returns past about 6mm. Biggest improvement between 3-6mm. After that you're mostly adding weight.