In order to hook up a 48 volt to a 12-volt reducer, you will need to purchase a voltage reducer. Once you have the voltage reducer, you will need to connect the positive terminal of the 48 volt battery to the positive terminal of the voltage reducer. Then, you will need to connect the negative terminal of the 48 volt battery to the negative terminal of the voltage reducer. Finally, you will need to connect the positive terminal of the 12 volt battery to the positive output terminal of the voltage reducer, and then connect the negative terminal of the 12 volt battery to ground.