One of the most common questions when upgrading your vehicle’s audio system with the addition of aftermarket amplifiers and subwoofers is, “Will I need to add a capacitor or second battery to my vehicle?” If you need to add a second battery, then you have to decide if you should add a power cell or if you should pick up a second battery for $60 at your local discount store. Finally, which of these methods will remedy your headlight dimming problem? These are good questions that hopefully I can help you understand, even if you do not understand the physics behind Ohm’s Law.

Your common starting battery is referred to as a “wet cell” battery. Wet cell batteries contain metal plates submerged in a bath of lead acid that, when energized, creates electrons that produce electricity to power your vehicle. Have you ever noticed the corrosion that builds up on the terminals on your starting battery? This is caused by the acid spilling out of the top of the battery. Not only is this dangerous, but it reduces the battery’s performance. Wet cell is a much older technology but it has lower production costs, so car companies include them “factory standard” with your vehicle. When it comes to adding a second battery, I should point out that it is not recommended to mount a Wet cell battery inside the interior of your car, for obvious reasons. No matter what vehicle you drive, somehow I don’t think corrosion and acid would go well with the interior.

Not to be confused with dry cell batteries, power cells use a newer AGM (Absorption Glass Matt) technology which make the cell lighter and enables them to produce more electricity than your average wet cell battery. They are also safer than the average wet cell battery. AGM power cells use the same metal plate construction as the wet cell batteries; however, the lead acid is absorbed in the glass matt so that the acid cannot spill out. The glass mat also immobilizes the acid so that there is faster reaction between the acid and the plate material. The lower resistance and faster acid migration of AGM technology allows higher rates of amperage during normal charging and discharging. As such, AGM power cells cause less strain on your alternator than your factory battery and they produce more power for your system. Best of all, you can mount these power cells anywhere and in any position—because the acid is absorbed in the glass matt, they will not spill.

What size power cell should you use?

Capacitor Wiring

I’m going to use the example of a 1000W RMS system. Certain factors, such as heat and board configuration, prevent amplifiers from operating at 100% efficiency. Let’s say that our amp is 70% efficient (note: the best amps are about 80% efficient). To achieve 1000 watts continuous we really want to achieve 1000W/.70= 1428.57 peak wattage. Wattage divide by voltage gives us the amp rating we will need. 14.4V represents the peak voltage of your standard dry-cell battery so, 1428.57W/14.4V=99.21A. We can say, To produce 1000W of continuous power, the vehicle will have to supply the aftermarket amplifiers with 99.21A of power.

So, let’s figure out if you need a new battery. First, check the size of your current alternator. Usually, 30-40% of the amperage of your alternator is used to power the electronics (power windows, cabin lights) in your vehicle. If your vehicle has a 80 amp alternator, and 35% of the alternators output is being used by the electronics in your car, that would leave 65% for aftermarket accessories or, 80 x .65 = 52 amps for your system. Second, look at the Amp Hour(AH) rating on your current battery. To achieve 1000 watts RMS you will need a battery with about 50AH rating. If your factory battery does not have that rating, and most likely it won’t, you will want to upgrade your battery. For this example I’d recommend replacing your starting battery with a Kinetik HC1200, which has a rating of 50AH. Between the alternator and the battery I estimate about 102A available for my system.

Now that you know why you will want a power cell rather than a battery, let’s explore the advantages and disadvantages of a capacitor. Unlike a battery or power cell, capacitors do not create electricity; they store electricity. A stiffening capacitor will maintain voltage supplied to your amplifier. A stiffening cap will provide the immediate power your amp needs on those dragged out bass notes, which will improve the sound of your system. A car audio capacitor will help limit your headlight dimming because the amp is absorbing the voltage through the capacitor rather than your main battery, but it will not completely cure the problem.

What size of a capacitor should you use?

Capacitors are recommended at a rate of 1 farad per 500 watts of RMS power. Multiple 1 farad capacitors can be wired together—similar to a battery pack you’ll find in a remote control car. Although this might take up more space and cost more than buying a 5 farad capacitor, it allows for quicker charging and more storage than having one larger cap. More storage means a better supply to your amp, which will allow it to run cooler and be more accurate.

Secondary Battery and Capacitor Trunk Install

Remember, these specs are in an ideal environment. Most batteries operate in a 12V-14.4V range, so values do fluctuate. If you can’t produce the amperage you need by simply upgrading your factory battery, than you will want to add a second Power-Cell in your trunk. Thepower cell in the trunk will need to connect to your starting battery but should be isolated (using a battery isolator) from the main starting battery. If you do not isolate the cell, it will drain your starting battery to recharge itself when the vehicle is not running. Unfortunately, if this happens you will be able to listen to your system but you won’t be able to start your car. Capacitors on the other hand, work in-tandem with your starting battery and will run in-line on your amplifier’s power cable, which should already be fused. It is not necessary to isolate them from your main battery, because they automatically shut-off when they do not sense a change in voltage.

What can you do to eliminate your dimming headlights?

If your midsize car is designed “factory standard” with an 80-90A alternator, you shouldn’t have a problem adding a 700 Watt RMS system to your car without replacing the battery or alternator. A properly functioning charging system should not experience dimming headlights. If you are experiencing dimming headlights, before adding a power cell or capacitor to your system, have a professional automotive parts store check your battery and alternator to ensure they are in good working order. Changing from a wet cell battery to a power cell with AGM technology will limit the stress on your alternator and can remove the dimming light effect, as long as your new system does not have too much of a draw on your alternator. The automotive parts store will be able to tell you if your alternator is failing and can sell you a larger high output alternator if necessary. Another easy upgrade that might eliminate your headlight dimming is to upgrade your “Big 3.” By “Big 3” we are referring to upgrading the wiring running from alternator positive to battery positive, battery negative to chassis, and engine ground to chassis. Upgrading the “Big 3” will have a great effect on the performance of your electrical system.

There is a lot to consider when deciding how large of a system you want to install in your vehicle. What is important is remembering Capacitors and Power-cells are not the same. A Power-cell, is going to create the amperage you need to properly power your amplifier(s). A Capacitor is going to help regulate the voltage so that your amplifier(s) are constantly being provided with about 14.4V, which will help your amps run more efficiently.
Many vehicles can handle a 500-1000 watt system without needing any major upgrades. Still, the addition of a cap is always a plus because it helps improve the performance of your sub and amp, and upgrading your stock battery to a power cell will increase the safety and performance of the charging system in your car. A quick inspection of your vehicle’s battery and alternator will help you find out what you need to make your vehicle “safe and sound.”

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