QUESTION: I've noticed my headlights change brightness along with the music when I turn my system up pretty high. I'm using a 300 watt amplifier that is mounted in the trunk. Do I need a bigger alternator, a new battery or maybe larger cables to the amp?

ANSWER: It sounds like your amplifier is asking for a bit more electrical current, at times, than the vehicle has to offer. During deep bass notes the current requirement sharply rises, momentarily pulling the vehicle's system voltage down. Let's take a look at your vehicle's electrical system, and how it can be beefed up.

Your battery serves several purposes. It stores energy for engine-off accessory usage, starting the engine, and while driving, it acts as an electrical shock absorber, helping to stabilize system voltage. Should the alternator fail to function properly, the battery can also provide needed energy to limp you home. Each vehicle has a recommended battery capacity, which is rated in CCA (cold cranking amps). It's generally possible — and not a bad idea — to install a higher-CCA battery.

Your alternator is a mechanically driven generator. Depending on its size, which is determined by engine displacement and vehicle equipment, output can range from 35 to 100 amps. All alternators, large or small, require a tight drive belt and good electrical connections in order to produce their rated output. During idle or low engine speed, alternators produce only about half of what they can at higher engine speed. A higher-rated alternator may be available for your vehicle — it would be used on the deluxe model, and another possibility is an aftermarket replacement, modified to produce greater output.

I'd start by having the battery and alternator checked for proper performance. If they're OK but you wish to upgrade, a larger alternator would be the first thing to try. While large amplifier power cables are important, its sounds like yours are doing the job well, as they're enabling the amp to noticeably manipulate the vehicle's system voltage.

The most cost-effective solution is to add a large capacitor, perhaps 1-2 farad, near the amplifier. Capacitors store energy, and can provide the brief bursts of energy your amplifier needs, working in parallel with the vehicle's electrical system. Capacitors charge up when voltage is plentiful and discharge when it isn't, acting like an electrical shock absorber.

(Brad Bergholdt is an automotive technology instructor at Evergreen Valley College in San Jose. E-mail under-the-hoodearthlink.net. He cannot make personal replies.)