Does leaving a 6v sub-c pack battery charged over 12 hours do any harm to the battery?
Hi WinterX,
That strongly depends on the kind of battery and charger that you are using.
As far as I know a Ni-CD battery can’t overload but there are better batteries with more power and a longer life on the marked.
If you are using a Ni-MH battery you will need a special Ni-MH charger which protects the battery from overloading. If you use a Ni-CD charger which doesn’t limit the charging current your batteries can be seriously damaged or explode! 
Li-on and Lipo batteries also need a special charger but they don’t come in a 6V version so I think that you don’t got one of those. 
I hope this information helps.
Xan
Thank you…
i am using the Ni-MH battery with the Universal Smart Charger…for 2.4v - 7.2v NiCd/ Ni-MH Battery Pack…
No those chargers will switch to a trickle charge when it,s done with the quick charge. You can leave it connected as long as you want.