Progressive Dynamics - max current before overload

I don't know about the PD 9145, but battery chargers in general are current sources, not voltage sources. They output a fixed current. As the voltage rises, they'll sense that and then begin to taper off the current until it's fully charged. They may then switch to a constant voltage mode to maintain a float voltage.

So with more batteries, the current would stay fixed at 45 amps, but it would take longer for batteries to charge.