Idle-Stop not working ... cold weather or fault ?

[Skyrider]

Cars should have thermostatically controlled louvres on the radiator grille to help get the engine compartment up to a suitable working temperature as quickly as possible.

Do you remember the days when cars had external muffs to keep the radiator warm when driving in sub-zero temperatures?

Yes, but my cars have all had thermostats.  :-)

I think John might be onto something, a cold battery may give the same symptoms as a less than fully charged one. Maybe there is a bit of both going on. A cold 75% charged battery might start to be protected by the cars systems. The same may be true for a hot battery, soon after I bought my car it auto started after I had been stopped in traffic for about five minutes and gave a low battery warning. I remember it was a hot day.

[John Ratsey]

Do you remember the days when cars had external muffs to keep the radiator warm when driving in sub-zero temperatures?

I remember hanging a plastic sack between the radiator grille and the radiator of my A35 during the winter months, if that counts. It was easily accessible. Anyway, getting back on topic, the measurement of battery temperature might explain why the idle stop tends to be more reluctant to work during cold weather.

[Skyrider]

I now suspect that the battery management is an infinitely variable blend of battery charge level, charge rate, load, and temperature. Good luck with expecting consistent or predictable performance from your cars electrical systems.  :-)