Re: grounding my project
#1 - for the objects in the metal box. make sure the objects are insulated before placing it in the box. then connect the ground from each object to the box.
If you have a voltmeter, measure the potential of the ground of each object relative to the box. if it says zero, you are in good shape.
#2 - for the lead acid battery, or the objects not in the box. Measure the potential between the ground of the non-box object and the metal case.
if the pd is almost zero, you are fine.
if something comes up, you have a floating ground. then you would worry about grounding issues. you could consider connecting the non-object ground and the metal case to the spokes or a metallic object on your bike.