It is possible to charge 48V batteries with a 12-volt adapter depending on your situation. However, this process will take much longer than if you were charging a single battery at the same time. Using a 12-volt charger for one battery can extend the life of the other two. You can also charge more than one battery in a series, reducing the overall charging time. You should make sure to connect one 12-volt charger across the battery banks in series.