O.K. I am ready to start wiring my 1915 Roadster. I have all new wiring and a wiring diagram. I am installing 12 volt bulbs and want to run the headlights off the battery. My question is, can I just put the wire that is normally attached to the Mag terminal on the coil box that feeds the lights and attach it to the battery side ? Or is there more to it than that?
Yes, that will work but you need to rewire your headlights too. If wired in series (original), they will have to wired in parallel. That is, a hot lead to each lamp.
Here's a quickie drawing of the change for the Headlights. This will change the Series wiring to parallel so that each headlight gets 12v.
I just leave the headlights wired as is and use two 6V bulbs. 6+6 = 12. I've carried a spare 6V headlamp bulb under the seat for 15 years but never had a bulb fail yet.
If I understand your question, the short answer is yes. I use a 12v Battery hidden under the rear seat with a wire to the BAT post on the coilbox to provide power while I crank. Another wire runs from same terminal to one screw of the headlight switch. I just charge the battery every so often.
Better headlights can be made simply by being careful of the filament shape since the tall teepee shape will allow the beam to be focused using the adjustment screw while filaments that are simply horizontal "bar" type from one side to the other will not focus. You can simplify the job if you have not bought your headlight plugs yet by getting single center contact type plugs. You can then use a common 1195 bulb which typically has the correct filament shape if made correctly and also has the ground side of the filament connected to the bulb barrel like any other bulb. That eliminates the ground wire. Now you simply hook both wires to the single connection on the plug and run one of them through the tube to the other side and use the same connections on that side with single center contact plug. The whole wiring will look stock and the 1195 is a pretty powerful 12V headlight bulb. You can get an 1196 that is the same bulb but with 2 connections on it with neither side of the filament connected to ground. That makes a good headlight bulb too but for either the 1195 or 1196 you will need a decent sized 12V battery since these draw about 3 amps each.