Car lights dim when the bass hits? Unfortunately, the solution to this problem is not cheap or simple.

The electrical system in most cars is designed to supply power to all the factory-installed components and accessories. If you add power-hungry amplifiers, the car’s electrical system can not keep up with the current demands. The car’s components and your audio system are in a constant fight for the low power resources.


The first thing to do is to add a stiffening capacitor. A capacitor acts as a secondary battery that reacts very quickly to sudden current demands (i.e. music transients). A rule of thumb is to have 1 farad of capacitance for every 1000 watts of power. The sound system will sound a bit smoother and hopefully the light dimming problem will be either fixed or reduced. Your lights don’t necessarily have to be dimming for you to have to add a capacitor. Stiffening capacitors always help a sound system.

Capacitors should be installed as close to the amplifiers as possible, with the positive lead going right into the positive lead of the amp and the ground to a metal part of the car (not to the ground terminal of the amp).

When a capacitor is first installed, it has no voltage in it, so if it is hooked up to a battery, it will act as a short circuit and draw a LOT of current (not a good sight). Capacitors need to be “charged up” first. This is simply done by either putting a resistor (1k-ohm or more) or a test light between the positive post of the battery and the positive terminal of the cap (with ground hooked up to metal). After a few minutes, the capacitor will be charged up, and it can be connected. Every time the capacitor is “drained” i.e the lights were left on in the car and battery is dead, the capacitor should be DISCONNECTED, battery charged, and then capacitor has to be recharged and reconnected.

Capacitors might help, but they do not take care of the fact that your car’s system is underpowered. They are a “patch” for a more serious problem

High Output Alternators

Another upgrade is a high output alternator and maybe adding a secondary battery (for car off listening) to keep up with higher system’s demands. This should be done by someone who has a fairly good understanding of a car’s electrical system, since computers are designed to control the factory components.

Additional Batteries

Always keep in mind that when a car is running the batteries become loads that take power away from your amps. The advantage of secondary batteries is that when the car is off, you can listen to your stereo for longer periods of time.

Adding more batteries will not make your stereo perform any better when the car is running. In fact, a second battery will steal power from the electrical system when the car is running. A battery isolator should be used to avoid batteries draining each other.

Replacing factory batteries with high performance batteries will also improve power output. Companies such as Optima make batteries that take less power to charge and have a higher output, placing lower demands on an electrical system.