The recent thread on capacitors got me thinking about power again...

Long duration issue:
Alternator. It's known that the S2000's alternator is capable of 105 amps. I'm wondering if anyone has done any sort of measurement of the car's "constant" drain while in use. Consider a worst case scenario: high beams on, heater on full blast, lots of turning (electric steering, remember), along with the ignition and normal electrical systems. How much is left over for the stereo?

I ask because my amps in combination should be capable of an honest 1000W RMS (70-80 amps, depending on voltage), even if my ears can't take it. I'm wondering if that'll put me in steady discharge at max volume in the worst case scenario. It hasn't been a problem yet because none of my subs have been able to take more than 1/4 the power of my sub amp, but when the new sub gets here that'll change. My guess is "probably", but I won't really know till it's all together. Maybe it's time for a voltmeter?

Short duration issue:
Batteries, capacitors. I've asked a couple different people whose opinion I hold in high esteem on the necessity of capacitors in a high power stereo system, but get varying answers. One argues that the cap is there to provide extra juice for heavy bass hits, that it will charge during transients when there isn't as large a current draw, and that "all the pros use them." This person hasn't ever used a cap himself but has good knowledge of electronics and car stereo.
Another argues that they're pretty much a waste. After having experimented personally with many different setups of varying capacitance, he found they did almost no good, and that any money spent on a cap would have been better used towards a better battery, or more of them. This person's understanding of electronics and car stereo I hold in even higher esteem (he even drives a fully electric vehicle, though that's not where he did his testing) but as with most eccentric genius-type people, his view is sometimes completely nuts.
More to the point, has anyone ever heard a system, plugged in a capacitor, and have it make a qualitative (sounds better) or quantitative (higher SPL reading) improvement in the sound? Right now I'm thinking I'll hook up in parallel some gel-cell UPS batteries I've got lying around, and hope that does the trick.

Thanks for reading, if you've made it this far...

Jason

 

Yes add a cap! I have a poor opinion of the stock battery. You're combo of amps WILL drain it fast. At least 1 farad is recommend for 1000 RMS. Go with a 1.5. They're not that costly.

 

You will also find good performance increase with a different battery... not only is our stock battery small and light but it was designed to only one thing: start the car.

Your high-powered stereo is connected to the battery which in turn is charged by the alternator. So basically, your system is powered by the battery. For fast power delivery during loud transient musical passages, your battery must discharge power quickly. Most auto batteries have too high internal resistance for extremely quick power delivery... because to start the car all that is required is a large sustained current supply (CCA).

A better battery (gel cell designed for car audio) will provide lower internal resistance, longer power reserve while vehicle is off, and most importantly will most likely provide a great number of completely recoverable deep cycle discharges: you are always told never to run down a lead-acid auto battery... because they never completely charge back to their original charged state. Every complete discharge and recharge returns the battery to a slightly lower level of capacity/charge. A deep cycle gel cell will be able to recover from complete discharges far more times than a standard lead-acid battery... sure there is a limit.

I changed to a gel cell... I noticed slightly tighter bass hits. Nothing dramatic but noticeable. I must admit that the gel cell definitely adds a lot of weight: about double or slightly more than double the original weight... and I had to modify the battery tray it sits on.

The next battle after changing the battery is making sure the stock alternator can sufficiently recharge it... which was already mentioned...

MY .02

 

[QUOTE]Originally posted by shaner
[B]You will also find good performance increase with a different battery... not only is our stock battery small and light but it was designed to only one thing: start the car.

Your high-powered stereo is connected to the battery which in turn is charged by the alternator.

 

Shaner pointed out the big issue here...batteries are not designed for fast transients, whereas caps can handle those with reasonable ease. I doubt the companies who make "audio quality" caps list the actual specs, but if possible, look for one with as low internal resistance as possible, a high temp range if you plan on placing it in the engine compartment, and a long life at that temp is used that way.

 

Warning - opinions, and some facts, ahead. My intention is to have some polite discussion about this topic. I am often accused of being too blunt and/or accusatorial (sp?), so I will apologize in advance if I offend anyone. That is not my intention. My intention is an enthusiastic exchange of information.

[QUOTE]Originally posted by Shaner
[B]A deep cycle gel cell will be able to recover from complete discharges far more times than a standard lead-acid battery... sure there is a limit.

 

I've been WAY too tired to come up with any intelligent arguments for discussion, but I'll put in half a cent worth tonight.

I would retry your old experiment with an input frequency of 50 Hz instead of 1000 Hz. I can't say for sure right now, but off-the-cuff common sense tells me you'll see some real difference at high power and low frequencies.

Why would you beef up the smaller cap inside the amp with a larger one inside the amp? For the same reason you would place an even LARGER cap outside (the darn thing would never fit inside the case)...to provide that extra bit of juice for the low-frequency pulls.

To look at another potential reason, let's say we're on the edge of what the alternator can provide. The capcitor charges up between large current pulls, and discharges when the extra juice is needed, many times per second. The cap will recharge faster than the battery between current pulls. If it was just the battery alone, it may not charge up fast enough between pulls and slowly drag the amount of juice left in it down.

Does any of that sound reasonable?

 

The above test was flawed. You were testing for an increase in peak power, which will only occur if the circut to the amplifier didn't have sufficient power. Try the test with a 10 amp source into a 40amp draw and then measure peak. Add a charged cap and you should move your peak up a few db. Assuming you don't fry the amplifier with a major sag.

Better yet, install a 1000watts (At 100% efficiency = approx 70amps sustained draw @14.4volts) worth of equipment into the car, turn on the music, and watch the headlights blink. Then install your caps and try the same thing. The other drivers will no longer have a strobing light show to enjoy. This is a no-brainer, install a few capacitors.

On a side note: Since your car needs electrical power to operate, charge the battery, and run the stereo, I would recommend purchasing a beefier alternator. Be sure to install a voltimeter along with the system. With the voltimeter you have a least some warning if you are stressing you electrical system. Especially if the circut drops into the 12 volt range. If you are in the 12 volt range the alternator is no longer charging the battery and you may wonder why you car doesn't start the next morning.

BTW - the stock battery isn't sufficient to run your stereo by itself for more than 30-60 seconds at the volume discussed above. Do not run a stereo of this caliber unless the car is running as well.

 

Well, once again, the opinions of persons I hold in high esteem are differing.

In the long-term issue, if I am capable of drawing more power than the alternator can provide, I'll have to find a rough "equilibrium" volume level to keep below of most of the time. Don't feel like spending megabucks on an upgraded alternator, and my ears will thank me anyway.

For the short-term issue, I'll try a little test. Tools to include a signal generator, o-scope, voltmeter, 800W RMS amp, and a sub that can handle it.
Test will be bursted and constant tone, probably 50hz.
Baseline will be to examine waveforms on output terminals of the amp, without any external capacitors. Then for comparison I'll use about .28f worth of capacitors (3 .093f caps bought at an electronics surplus store for $12 total).
I'm guessing for the constant tone they won't help, but I'll test anyway.
For the bursted tone, if I am able to achieve a higher non-clipping output level with the capacitors (or if things just seem "happier" in the system), the test will continue to try to find the duration that the caps need to charge. For instance, if they need 1 second to charge, that's not useful because the next bass hit would already have arrived in a musical situation. My electronics knowledge is all of a few semesters of physics in college so there may be a formula for figuring cap charge time I've forgotten...
The test would probably be more meaningful with a more typical audio cap situation (1-2f) but any measurable difference could probably be extrapolated from my .28. To charge the battery during the test, I'll use a plug-in battery charger in rapid-charge mode, and if necessary have the car running.

Any thoughts on what more I could do, or what I could do differently to make this a better test? If all goes well I'll be doing the experiments this weekend.

Thanks for all the great info guys!

Jason

 

I would agree that the capacitor will charge faster than the battery. But then it will discharge faster than the battery too, so the peaks better be short enough to not discharge the cap below your optimal supply voltage. And being right next to the amp will help too, if there's too much impedance in the supply lline.

Your description sounds like the o-scope test I talked about should show voltage dips at the amp during the peaks, and voltage rise between peaks (assuming a system with no extra caps, and enough current drain to exceed the power supply output). So the scope should show an AC voltage corresponding to the "missing" power. If there is no appreciable AC, then there's no problem. I think I'll try to find a local car with a huge amp and try this. The test I did years ago probably never overloaded the power supply.

I guess I have trouble imagining an audio system with a continious power output of 700 watts, then peaks above that. (based on crude estimates of 50% amp efficiency, and an amp input current sufficient to exceed the alternator output). Do people really listen to music at that level continuously? Do they have separate air conditioners to cool the amps? 700 watts of heat is quite a bit of heat. That's half the heat of your typical portable electric heater on maximum output. Holy Heatsink, Batman.

One more question, do you know how long a 1 farad cap will suppply 100 amps before the voltage drops 10%? It's been 30 years since I had high school electronics, and the math is long gone.