Also, even if they're separate, AC and heating can use a lot of electricity and normal ICE cars usually don't have big batteries. Especially in the winter you'd

Also, even if they're separate, AC and heating can use a lot of electricity and normal ICE cars usually don't have big batteries. Especially in the winter you'd also need a solid reserve, because the car might be already be discharged due to cold and need more power to get going. Throw in some short range trips and an engine stop automatic and you'll quickly end up with an empty battery, which will reflect very bad on the manufacturer. You could of course add a second battery, but that would add a lot of weight and complexity for a not very noticeable benefit.