What is the lowest voltage a 12V lead-acid battery in a car could be?
Inside a car is a particularly brutal environment.

It depends whether you're making a one-off for your own joy, or manufacturing something to sell.

There are two specifications, working, and not being damaged.

For a one off, working from 11 to 14v would be good enough. To sell, you would need to work down to 8v for ancillary functions, and to 5v for engine critical functions.

To avoid damage, it would be a nice idea (for a one-off) you must (if you're selling) be able to tolerate sustained reverse 12v (if somebody tries to jump start with a reversed battery), sustained 24v (if a garage jump starts with a lorry battery) and transient 160v (load dump when the last heavy load is removed from a charging system). 

You might want to think about the highest temperatures you can meet on a sunny day in the cabin, or in the engine compartment.