When the battery is full and connected to the charger you will see 14.2 volts if the battery is healthy. After the battery is off of the charger the voltage will drop to 12.6 to 13 volts. This is the voltage of a fully charged 12 volt battery at rest.

I would go with the adjustable LM2588.

With the adjustable the voltage can be set pretty much where you want it.