How low?
I'm running four D3100's in back, D3400 up front a Mechman elite 370, powering an NS1 @.65 and I stay in the highs 13s full tilt. Lowest I've seen is 13.4ish.
probably not enough runs/proper grounds. just my 2 cents.
Possibly, I have six cca runs and two OFC runs front to back. The rear batts have busbars on them.