[Ronbo]

Another helpfull thing is to wire capacitors across your relay / switch contacts. Use a Ceramic disk or Mica capacitor of .1 uf value or higher.

The capacitor will absorb some of the initial arcing that occurs because of high in-rush current that inductive loads (solenoids, motors) produce. Light bulbs like your headlights also produce in-rush during heat up of the filiment but it's not as harsh as inductive loads.

To keep spikes off the electrical system you can also place a reverse polarity diode across the coil to absorb the inductive kick produced on de-energizing. Collapsing magnetic fields produce a reverse polarity spike. (this is how your ignition coil works) Any of you that have stereos may notice the "pops" when certain items are switched off.

With more electronic systems becoming common place even on vintage cars, the electrical system needs to have better filtering than just the battery. You'll find decoupling caps scattered throughout the electrical system of most modern cars to remove high frequency noise that can effect various electronic controls like anti-lock brakes, engine computer, suspention controls, ect.

A modern riding lawnmower has more electronics than the original Cobras had.