You don't have to get to worried about this. Alternators in today's current vehicles put out about 14 volt when charging. They HAVE to have a greater voltage than the battery they are charging, otherwise the battery would try to discharge through the alternator. Since voltage is potential energy, and to add to a store of potential energy, you have to have a greater supply than what is in the energy store - a battery can only be charged by a voltage source that is greater than the rating of the battery.
The thing you really need to make sure of is to limit the current going into the battery, and you need to make sure that you don't continue to charge the battery when it is fully charged (don't leave the charger on it).
