not 100% sure....but if you have both batteries hooked up to the charger then you are feeding current on both sides of the vsr which is why it is not likeing it. turn your switch to off thus disconnecting the vsr and then you can charge maintain the batteries. Or just hook the charger up to the stereo battery and once it is up to charge then the vsr will kick in and charge the motor battery i am assuming it is a dual sensing vsr .