[Ducati750cc]

How does a battery warning light work, very simply.


It serves two functions, the main one being to allow a small current to the alt to start charging at low revs, the second is that it acts as a charge warning light.



Engine stationary, ignition on, the battery + current say 12v flows through the bulb and to bat - 0v through the alt, via the brushes and slip rings, a difference of 12v from one side of the bulb filament to the other, so it lights up.


Start the engine and the alt starts to charge at say 14v, this voltage is now present at both the battery + , ignition switch and ignition switch side of the bulb and also at the alternator side of the bulb, 14v at one side of the filament and 14v at the other side of the filament, the bulb won't light.


So what can cause the bulb to come on and, or flicker ?


Dead simple in theory, anything in any circuit from battery +, to ing. switch, to bulb, to wiring, to multi-plugs to alternator internals, to bat -, where there is is a poor resistive or loose connection and causes a volt drop across that connection of say 1v or more.


We than have a difference in voltage between either side of the filament and depending on what the difference or cause is, it will cause the filament to glow, flicker or stay on even though the alt is charging perfectly.


So in theory, somewhere, there is simply something causing a volt drop on one side of the bulb or the other, the practical side, finding the fault isn't that simple though.


There will be many an alt. replaced unnecessarily due to the above.