Ok slightly off topic but relevant to the battery.
I did Technical Architecture at University instead of opting for an Honours Degree in Electrical or Mechanical engineering...
So thermal design & heat transmission etc is part of the remit...
Heat will flow in an object/solid/liquid/gas until the temperature is the same (equilibrium). The transfer of heat will be by either conduction, convection or radiation.
So battery in engine bay, engine emits (radiates) heat to heat up the air around it to reach the same temp as the engine. The air in the engine bay will then heat up any object in the engine bay, by convection. Anything that is in direct touch/contact with the engine will heat up by conduction.
Radiation, most people just think of sunlight & thus an engine bay is not in direct sunlight so this does not apply. Wrong! Sunlight is short wave length....as very high temps, lower temps including red heat from metal are long wavelength radiation & are what is called "far infra-red" which is why we can see this "invisible" heat from a hot engine in a thermal imaging camera! So the engine will radiate heat to heat up any item in the engine bay, even if in dark garage!
There is then the whole "thermal resistance" of the materials to consider. i.e. does the material present resistance to the flow of heat from one side of the material to the other side?
A good example of this is the bulkhead in your engine bay, black felt to provide sound insulation & some thermal properties when cold. However it has a silver coloured corrugated aluminium foil on top to stop this black felt from gaining heat from the turbo etc...& this is in a dark engine bay..so no visible radiation...so why??
Basically shiny metallic surfaces have a low emissivity in both the visible heat radiation & the invisible far infrared heat spectrums...& they can work in both direction..ie wrap the battery, & it can stop the battery emitting heat...& also stop the battery gaining heat from the surrounding air.
Low emissivity materials are concerned with the heat absorbed & radiated by the material & this can have NOTHING to do with sunlight or visible radiation! which is why you have aluminium foils inside building walls etc. as "low-e"..same applies to engine bays... wrap the items you want to protect from heat in polished silver or gold foils...not a black fabric...
I could write a whole thesis on this if you want but a good webpage on wiki here:-
https://en.wikipedia.org/wiki/Thermal_radiation
& the specifics on Low emissivity materials are here-
https://en.wikipedia.org/wiki/Low_emissivity