The battery warning light in your vehicle turns on when you turn the ignition key to the "on" position. As soon as you start the engine, the light goes off and remains off until you stop the engine again -- unless there's a problem. The light connects, via low voltage wiring, to the charging system in your car, so there can be several causes for a battery light to flicker.
A fault with the alternator is probably the main reason for a battery light to flicker. It means your battery has taken over to keep the electronics working. As soon as the output from the alternator drops to a certain fairly low level, the battery light flickers. As simple method to check if it is the alternator is to park the vehicle and let the engine idle, and the light gets a little brighter. Turn on the headlights and the engine slows slightly and the battery light gets even brighter. Increase engine speed and the battery light starts to flicker.
The voltage regulator in your vehicle ensures that voltage remains stable, usually about 13 to 14 volts. It's fitted between the alternator and the rest of the electrical equipment, including the battery. If the voltage regulator fails, voltage can't be controlled. If the voltage regulator lets the voltage drop too much, then your battery doesn't get charged and your electrical equipment won't function correctly. Low voltage gets registered by a sensor and is relayed to the battery light which can make it flicker, if the voltage is hovering just under the minimum amount. The lower the voltage the more the light illuminates.
Low Battery Power
Another cause for a battery light to flicker is when your battery is nearly discharged, possibly caused by a malfunctioning alternator. The battery is one of the components to which your battery warning light is wired. It detects a low voltage electricity supply because the force of electricity through the main cables in the vehicle has dropped to an unacceptable level. This voltage drop triggers the light to start illuminating. At first it flickers, but if the battery continues to discharge, then it can illuminate permanently.
Poor Battery Connections
You have probably noticed the size of the main cables connected to your battery. This is because they need to carry substantial current, especially when you start your car. If the battery cables are not connected tightly, arcing occurs between the battery cable and the battery terminal, and corrosion can set in. Eventually the connection starts to break down and voltage drops off. The effect is similar to a low battery. It triggers the sensor and the battery light begins to flicker. It's always worth checking the connection if you get this problem.