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I have a 2015 Golf whose battery needed replacement. I noticed that the battery tray was designed to accommodate three different lengths of battery, with the factory battery being the shortest (and smallest) possible. So, in hopes of having the replacement battery last longer, I got a larger battery.

Problem: the factory battery had a sort of a box wrapped around it, made of a folded sheet of black fibrous material. The material was puffed up in thickness, as if for thermal insulation:

enter image description here

Of course, the original box won't fit the new, larger battery; here it is wrapped as well as I can: enter image description here

It don't make sense that the box is for electrical insulation, as protecting more than just the terminals isn't useful. It also doesn't make sense it's for thermal insulation, as no matter what the battery will eventually get to the temperature of the surrounding engine compartment. Maybe it's "just for pretty"?

So, what is this box for, and do I need to replace it?

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Insulation works to keep things hot or cold, while the battery will always achieve thermal equilibrium anyway.

But it does help keep the battery cool for a while as under bonnet temperatures can easily exceed 70 deg C.

In winter it helps the battery stay a bit warmer between uses as a battery looses a lot of performance as the temperature drops.

You can replace it or not as you wish - it is not super critical - other users just “forget” to refit sometimes - it will still start...

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