I’m also skeptical about those battery blankets
That said, the car did leave the production line with it, so if I can manage to replace the battery without discarding the blanket, I’ll do so
But there have been times when no matter how careful you are, they just fall apart when you touch them
I’m no physicist, nor am I a mechanical engineer. However, car manufacturers don’t spend more per vehicle than necessary, especially for items that cannot be noticed by a perspective buyer. That said, I believe those battery blankets work.
I will try and explain my guess as to how they work. I agree with the idea that insulation neither adds heat or cold to what it’s insulating. It slows the transfer of heat. People still buy furnaces and air conditioners after insulating their homes, but almost everybody pays for insulation.
I’m thinking that while the car is running down the road and air is flowing the under hood temperature is acceptable to the battery’s design. Also, while idling, even with air conditioning on with high ambient temperatures, cooling fans help keep some air moving and maintain a reasonable temperature.
It’s when the car is shut off after driving (sudden stoppage of any air flow) that the under hood temperature begins to rise too high, albeit for a relatively short time. Since the insulation jacket works for a relatively short period of time, it works to protect the battery.
Just as a frozen battery takes a while to thaw, a reasonable temperature battery must also take some time to overheat . Before that happens the car’s mechanicals have cooled somewhat.
Any takers on my theory?
CSA
A battery takes up valuable space in the engine compartment. Our Aurora had the battery under the seat because there just wasn’t room for it under the hood with the V8. My 59 VW had the battery under the seat too. Guess they needed all the cargo area they could get.
I disagree that a properly insulated battery will warm to the the average temperature of the engine bay. The engine bay is not a closed system. Heat can radiate out of the bottom of the engine bay to the road. There is also air flow into the engine bay through the radiator and possibly from underneath. If this was a closed system, I would agree with you, but it is not a closed system.
Heat sinking of components under the hood occurs once the engine is shut off, and there’s no air movement.
Tester
I’m unfamiliar with the OEM battery blankets you guys are referring to (NOT to saturated glass mat batteries, which are something totally different) but have to wonder where these vehicles’ batteries are located relative to their exhaust systems. The statements that heat internal to the engine and its systems radiates out the surfaces when the engine is shut down and raises underhood temperatures is right-on, and CSA’s comment that peripheral underhood components act like heat sinks is correct. Exhaust manifolds get very hot, and if a battery’s proximity to an exhaust manifold is too close, it may need a blanket to protect it. Most vehicles today have heat shields over the manifolds, but on these vehicles that might not be enough.
In any event, if the manufacturer provided a blanket I’d be inclined to replace it. Even if it’s crumbling and needs a new one. I figure it must be there for a reason.
I’ve had battery insulators on several vehicles (Chrysler and GM). None of them had a battery near the engine (large vehicles with large engine compartments), let alone exhaust system.
I am always careful to reinstall the insulation when replacing the battery. As I stated earlier, it’s my thinking that the manufacturer wouldn’t go through the expense of adding this feature if they didn’t think it was helpful.
CSA
My Interstate battery salesman says Interstate are the best.
My Exide salesman says his are the best.
I have a Duralast in my personal truck, the wife’s car, and the church van.
Are you kidding me? They are the best! 
CSA