[Piwoslaw]

You can have start-stop with HVAC fan on a single 12V battery.

My Peugeot 308 with e-HDi start-stop engine has only 1 battery. It is a normal 12V, 70Ah, 700A battery, dimensions same as for regular car battery. And the ventilation fan still works when the engine turns off. It will turn the engine back on after a while (30-60sec), either to circulate the coolant (heating) or run the compressor (A/C).

The only requirement is that the battery needs to be AGM technology for the Start-Stop feature to work consistantly. When I bought the car it had a lead acid battery, and the start-stop activated only 1-2 times, at the end of a very long road trip, otherwise the engine would stay on. When I invested in an AGM (roughly twice the price of comparable lead acid), suddenly the feature started working. Though now, when the battery is 3 years old, it hardly works during winter, even though I trickle charge once or twice per week.

So, it's doable, but with limitations. I'm sure it puts a large strain on the battery, tho, shortening it's useable life.

I have kept the old battery as a spare, which I've contemplated connecting parallel, but not sure about the consequences of connecting lead acid to AGM?