Will accidentally charging a 12v battery with 6v charger do any lasting damage? Is it a bad idea to switch over to charging it with a 12v charger once it's been exposed to the 6v?
I don't think there's any harm in doing this, only that it will take twice the time. Usually, charging a dead 12-volt battery using a 12-volt charge takes about 12 to 18 hours, but using a 6-volt charge could take 24 to 36 hours.
It won't do any damage to anything, I don't see how it could, but it won't fully charge your battery. Your charger might explode though because you're asking it to do too much. All the while your battery is sitting there saying "This tickles". LOL