Welly wrote:I wouldn't have thought it would do much after a couple of hours it would be stone cold again, are you sure the insulation kits are not for reducing heat gains from the engine bay? my Volvo has a cold air duct connected
to the battery box so presumably the battery is better being kept cool

In summer use id agree mate,
frost reduces the performace of batterys so ive always presumed the jackets used on vaux cars where to
aid the battery thru winter, dunno tho, icouid be wrong
just read this off tinterweb,
The automotive battery is a very interesting device on an automobile. There is no other part that is more neglected, and then expected to perform at peak capability. On top of this, the automotive battery responds to certain outside elements depending on the climate in which it is installed.
For example, living in the Northwest, we are subjected to a moist and mild climate. This affects battery life in many different ways. For instance, moisture contributes to corrosion on the posts of a battery. This can drastically affect the starting capability of the battery, due to the electrical current having resistance to overcome before reaching its destination, which is usually the starter motor. This mild climate we live in also produces cooler weather. Cooler weather can cause starting problems due to a battery's inability to respond by warming itself up. At 32 degrees Fahrenheit, a battery loses up to 50% of its starting ability. Insulation blankets are available, but most of the buying public does not invest in such preventative maintenance items.
When the season changes in the Northwest, warmer weather appears. Although more of an ideal climate, when heat is excessive, it produces resistance. This is a battery's number one enemy! Too much resistance and a battery simply will not be able to release its charge to the starting components of an automobile. The same insulation blankets that keep a battery warm in cool weather also reduce heat when the weather is warmer. Please invest in one to maximize the life of your battery!