Deceptive Lights: The Battery Light

Dash lights that mysteriously and randomly illuminate mean things. What they mean is almost never what you might assume and without a little bit of knowledge about what the intention of the engineer was that designed this light you may never be able to make a correct guess.

The battery light is one of these deceptive lights. This is the light that when illuminated, looks like a little car battery with two terminals with a plus and minus sign on each side. We call this light the battery light because it looks like a battery, but if you look up the real name in the owners manual it’s probably referred to as something like the “Charging System Indicator Light” or some such thing.

This light does not indicate that your battery is bad, nor does it really indicate anything related to battery condition or function. This light will illuminate when the vehicle’s charging system output drops below a predetermined voltage, usually somewhere in the neighborhood of 11 to 12 volts.

When you first turn the key on to start your car this light comes on because the charging system isn’t doing anything. The alternator will not put out any electricity until the engine is running, thus the light stays on. Once the engine starts the battery light should go out because the alternator is now doing its job, and charging system voltage is at normal levels.

Older cars used a circuit of opposing voltages to make this light come on. This means that voltage from the battery and voltage from the alternator meet from two directions at this light in the dash. As long as the alternator is working correctly neither one of the opposing voltages can flow through the light and cause it to illuminate. If the alternator output falls to something too low, the voltage from the battery will overcome the alternator output, voltage will flow through the light, and it will illuminate. Newer vehicles have computers that monitor alternator output and if it drops too low, the computer turns the light on.

If this light comes on while driving what is the best thing to do? Hopefully you can either get the car home, or to the mechanic in a timely manner. If this light comes on it means that charging system voltage is very low, perhaps nonexistent, and the only thing keeping electricity flowing to vital engine control systems is the battery. When the engine is running electricity is needed to power the fuel pump, produce a spark for combustion, and run the computer that controls everything. Once the battery is drained because the charging system isn’t functional the engine control systems shut down and the engine stalls.

The alternator is where all voltage in the electrical system
Originates when the engine is running.

So if this light comes on while driving you should do everything you can to conserve the electricity in the battery and get to someplace where you can have the car looked at by a mechanic. Shut off the radio and any other electrical devices that are not vital, rear defrost, air conditioning, blower fans. Turn off any unnecessary lights, but if it’s dark outside don’t turn all your lights off. A fully charged battery may last for about 75 miles of freeway driving if all unnecessary electric devices are shut off.

If your vehicle does stall it probably won’t crank when you try to restart. Don’t go and install new battery. If you have some jumper cables you can charge your battery from the charging system of another car. With the jumper cables hooked up between a good vehicle and a dead vehicle, let the good vehicle run for 20 or 30 minutes. This will put enough electricity back into the dead battery to start that vehicle and drive it for another 20 or 30 miles. This process can be repeated if as many times as needed to make it too safety, but every time you run a battery dead it shortens the life span of the battery.

So if you see the dreaded battery light, don’t plan on driving the vehicle anywhere except to the repair shop. Chances are you will need a new alternator, but it’s also possible that your alternator drive belt is very loose or has come off. Neither one of these things is the battery, but that’s what the light looks like. Don’t be deceived by this deceptive light.