If connected to a running vehicle, however, then you would see an alternator voltage of 13.9 to 14.8 volts.
a 6 volt lantern battery would have an actual voltage range of ?? to ~6.3 volts.
All-electric vehicles have a battery voltage of ~400 to 800 volts, which will probably be outside the range of this project, unless they contain a 12 volt "cigarette lighter" outlet via step-down electronics.
two voltage converters
The theory here is that 12 volts is SUCH a common voltage to use in vehicles that you probably have some electronics for it already — you probably have a phone modern fast-charger in your car, and you may even have a portable power station that charges from 12 volts.
(note that it's 12 v nominal, 11.6 to 14.8 v actual)
Obviously this would be more efficient. However, it would need to integrate the latest fast-charge standards, which means it could lag behind a bit, if it sits in the emergency kit for too long.