[DonRotolo]

Ether is correct. The cold weather myth regarding lead acid batteries comes from the fact that a car's engine requires more energy to start in cold weather than in warm weather, because of the difference in lubrication viscosity and in the combustibility of fuel at low temperatures.

Since you need more energy at low temperatures, a slightly weak battery won't have enough energy to start the engine. This makes people believe the cold weather is bad for batteries, whereas hot weather is worse for battery life.