Charging 48V low voltage to turn on charger

[FredA]

My cart batteries will not start the charger due to low voltage. I was on a long trip to New England and the batteries discharged while in our garage. I will be using a 12V automotive battery charger. >>> 2002 Club Car 48 Volt <<<
I have read to charge batteries one at a time for 5-6 minutes. Does that mean disconnecting all batteries or just connect automotive battery charger cables to one battery at a time while connected to each other? I have also read that I can charge two batteries (total 16 volts) using the 12V charger. The same question about connected or unconnected. The batteries are only three years old so I do not want to damage them even more than the low voltage has already. Thanks for any help and clarity you can provide, Fred 1-17-19