QuestionQUESTION: I am wanting to install 2 12 inch db drive plw series subs and their 1200 watt amp and 2 sets of component speakers and a 400 - 500 watt amp to power the components. I was told that I needed a new alternator and a second battery to power all of that. I have a 2002 ford ranger and my alternator puts out about 14 volts. Do I need to go with a new alternator and a second battery for a 1700 watt system?
Thanks, Brandon
ANSWER: Hi Brandon,
I can tell you that, unless you plan to use the system extensively without the engine running, then you don't need to add a second battery for your audio system. As for the alternator upgrade, it really depends on how you plan to use it.
The critical spec for the alternator isn't the voltage output; 14.4 volts is standard. The issue is the alternator's current capacity. Depending on your truck's engine, you might have a stock alternator rated for 95 amps or less. There are quite a few replacement alternators available with 130-amp ratings. Other high-output alternators with even higher ratings are available, but are likely to cost quite a bit more.
Keep in mind that even if you have 1700 watts of power available, you won't be using all that power except for a small fraction of the playing time. This has to do with the way music works: there are plenty of quiet spots along with the loud spots. Obviously, your amplifiers won't have to produce as much power for the quiet parts of the music as they will at the musical peaks. The difference between the maximum amplifier output, compared to the average output, is called the "crest factor". For most music, the crest factor is about 20%. That means if you're playing the music loudly enough that the speakers are getting 1000 watts on the peaks, then you might be using 200 watts of power on average.
This varies, of course, depending on the type of music--some music has more quiet spots than others. Also, it depends on how loud you intend to play the system, most of the time: are you going to be using the system at its maximum useful volume, for the most part; or just at a comfortable listening level?
If your goal is just a good-sounding system, for daily-driver use, and you're not planning to make enemies in the houses as you drive by, then you may find that your stock alternator is perfectly ample for your needs. On the other hand, if you're planning to try getting the maximum volume out of your system, then you might need an alternator upgrade. Remember that if you upgrade the alternator, then you'll also need to upgrade the size of your charging system wires (commonly known as the "big 3"). In fact, it's not a bad idea to upgrade these wires even if you don't replace the alternator right away.
Why not a second battery? When your engine is running, all electrical power must be generated by the alternator. Power will flow from the highest voltage point in the circuit: that's why an alternator produces 14.4 volts, even though a fully charged battery can only maintain a voltage of 12.8 volts. In other words, if the engine is running and the alternator is operating properly, then no power will flow from ANY battery in the system--the battery is just a load on the alternator at that point. It's only when the alternator is overloaded, and the voltage drops below 12.8 volts, that the battery produces any power. At all other times, the battery is a consumer of energy from the alternator: it uses up a certain amount of current just to maintain a charge. Adding a second battery won't take any strain off the alternator. It will increase the strain.
A second battery makes sense in two cases: first of all, in a competition dB system, the alternator is expected to be overloaded for a short period. In this case the extra current available from a multi-battery system can help the dB score: it might not make an audible difference, but it will show on the SPL meter. A competitor in an SPL event will accept the disadvantage of additional batteries (an increased load on the alternator) for the short-term advantage in an event. Also, a second battery with an isolator can be useful if you plan to use the system without the engine running. In this case the second battery can power the audio system without draining the primary battery, which is needed to start the engine.
Hope this helps!
Brian
---------- FOLLOW-UP ----------
QUESTION: This has been a big help. Thank you so much. What alternator would you recommend that I install? I will not be playing my system for any long period of time without the engine running.
Thanks again,
Brandon
AnswerHi Brandon,
Sorry for the very late reply to your follow-up.
I probably can't give you a recommendation on an alternator; for one thing, I'd need to know more about your truck's engine. Also, I personally haven't done much with alternator upgrades. You might get better information on a Ranger owner's forum. However, I can give you a broad idea of the types of specs you might look for.
First, let's estimate how much extra current you might need. This is going to end up as a very rough estimate, but it should give you a place to start.
We'll take the total RMS power output of your audio system: about 1600 watts; and from there, determine how much extra current you need to support the power output. Now, if you expect to play bass test tones at maximum volume most of the time, then we'd probably need to support that entire 1600-watt output. However, it's much more likely that you're just planning to play music. In this case, we can apply the musical "crest factor" and estimate that if you're playing a musical program at maximum volume, you may be using about 400 watts on average.
So how many amps are required for a 400-watt power output? We can calculate this using Watt's Law: Current is equal to power divided by voltage. In theory, your vehicle's electrical system can produce about 14.4 volts, but for a system under load, it's probably more realistic to use 13.5 volts for our calculations. That gives us a current requirement of about 30 amps. That means if your amplifiers are producing 400 watts of output on average, they'd require an average input of no less than 30 amps of current. However, car audio amplifiers are not 100% efficient. That means some of the power drawn from the truck's electrical system will just be wasted as heat. Let's assume your amplifier system is about 60% efficient, because it makes the math easier and is probably fairly realistic. In this case, your system will consume about 50 amps of current from the electrical system to maintain an average output of 400 watts.
From what I can tell, your truck is probably equipped with a stock alternator rated at 95 amps (although I think there were higher-output models available as an option). You might want to call a local dealer with your VIN, and find out the specs of your factory-equipped alternator. However, 95 amps gives us a good idea of the maximum current requirements of the truck. Usually, that's a worst-case scenario: the factory alternator has to be designed to support all the electrical systems in use at the same time. You won't normally be playing your audio system at maximum volume, while using the headlights, windshield wipers, maximum blower fan, and rear defroster (if equipped). However, it could certainly happen, and it doesn't hurt to pick an alternator that has enough reserve current to support all those devices.
From what I can tell, you'd probably do best with an alternator rated for 150 amps or higher. However, I've seen quite a few alternators for your truck that are rated at 130 amps (although most seem to be for the 4-liter engine). If you have a 130-amp stock alternator, or upgrade to a 130-amp alternator, it will probably be sufficient for normal use. If you want to be sure of having an adequate current supply at all times, then look for alternators designed for 150-200 amps. If you pick one, I'd suggest contacting the retailer or manufacturer directly, and make sure it's the right one for your particular truck (and check for additional parts that might be required for installation).
I hope this helps, and I apologize again for the late response.
Brian
