The most common reason for an alternator not charging a battery is because of a bad alternator or battery. You can easily test the voltage by connecting a voltmeter to your battery while the engine is off.Can an alternator charge a deep cycle battery?
Deep cycle batteries should not be charged off a car alternator. While you can do it, if you take proper precautions you wouldn't damage the alternator, it might damage the battery. No one can tell you the proper way to charge an unnamed battery without having more info than you put into your question.How do you charge a RV battery?
Attach the RV battery charger to the battery. Connect the positive side to the red connection first and then attach the negative side of the battery with the black connection or clamp the black connection to a metal piece to ground it. Plug in the charger and turn it on.