Missing parts?

I have a 1999 GEM E582 48v that has an external charger that plugs in under the passenger seat. There are 4 12v batteries but there are 2 empty battery trays and several unused harneses. I have a schematic for the 4 12v batteries but they all show an on board charger. Is this correct?

Dear Friends,
Do I have a story to tell you!

I have the exact same issue but ive seen that they are actually supposed to take 6 8v Batteries.

And ive heard that you can run 4 12volts but the dont get much range to them