Bernie

I totally disagree that the cap is useless!!
Engineering 101 or not, the cap made a big difference in my setup!
I had wired up some stuff temporarily before the main install and my voltage gauge was jumping up and down to the beat of the music.
The battery is an almost brand new Interstate Battery and my alternator at least appears to be in perfect working order.
Since I finished the install with the cap in place, my voltage meter on the dash doesn't move at all, no matter how hard I drive the system and I have lots of clean voltage on hand for any big bottom transients. Before the cap, my bottom end would give up.
It may be the amplifiers, I don't know? I have a Aura amp for the bass shakers and a Beyma HC2300 amp running the rest of the system. The Beyma is a MOSFET amp and I would put that up against any amplifier - Rockford Fosgate included!!
The Aura amp may be a little on the cheesy side but I can still notice a big difference...

Scientific or not, I know what I hear and see. The 1 Farad cap in my system "IS" making a difference.........

V-Fib

Interesting thread. My son says he needs a capacitor for his stereo, and I've been dragging my feet. He's running three 12 inch alpine subs and 6 other speakers. He's got 2 amplifiers, one bridged and only to the subs(1000+w), the other for the 6 other speakers (600W). He kept blowing an 80 amp fuse to the amp/subs, we replaced with a 100 amp and no problem since. We did have to mount a couple of computer 2 inch cooling fans (radio shack) for the larger amp because it would shut down due to heat build up. Both amps are directly connected to the battery w/4ga. He has a Kenwood head unit. Does he need a capacitor(s), or would Dad just be wasting money?

Anthony Tate
79/928 Silver Metallic
(and I was happy with a Craig Power Play 8 track)

BrianG

And here I thought a rechargable battery was just a low-loss, slow discharge capacitor.............. <img border="0" alt="[hiha]" title="" src="graemlins/roflmao.gif" />

BC

Thats what I THOUGHT The ground dist. was, but wasn't sure.

Thanks,

deltaP

Once again, the capacitor is a bandade for improperly designed electronics. An amplifier should have the proper capacitance built in to handle transients. It's called a filter capacitor. It is fundamentally important to the operation of the power supply.

Notice Anthonys post. 100 amp fuse is fine for all that hardware. The battery is capable of an excess of 400 or 500 amps minimum. that's a 400% overhead. So what gives?

If you draw a schematic of the system, you can only draw a couple of conclusions. Resistance in the supply wire or resistance in the terminals (including ground) or battery itself. And again, improperly designed electronics.

The fact that you have a new battery doesn't mean much. Batteries have different internal resistances. I'd be willing to bet that the optima battery would eliminate the need for a capacitor. I had a perfectally good battery in my car and the lights would dim when the subs would hit. I put in an Optima and no more...

The capacitor is a "fix" for one of the preceding transparent problems. Most likely, it is simply acting in place of some missing capacitors in the amplifier that were left out to maximize profit (who's going to know?). I view most amplifier builders as snake-oil salesmen. They constantly overrate their equipment and are usually put more development into the outside of the amplifier than the inside.

Also, Anthony, don't forget that the alternator can only put out around 100 amps (anybody know off hand what the 928's alternator is capable of?). If the stereo pulls more than that, the stereo output will be attenuated because of the drop in voltage.

In previous vehicles, I had to have custom built alternators installed so that the car wouldn't suddenly die when driving around town due to a completely dead battery.

bcdavis

Well, I have an Optima battery, fairly new. And a fairly new alternator.
But I do indeed notice changes in voltage related to RPM with other components. I see my lights dim slightly, etc, when the car is at low RPM. So at the core of the issue, is a slight loss of power at low RPM. It is not drastic, but with high powered amps, it may be too much power for the alternator to handle. So my idea was that if I get a huge Capacitor, it will charge up, and act like a backup battery, and feed the amps when the alternator is turning slowly. And I think it is doing it's job just fine for the amps. They are full power, all the time. And yet I still get a "dimming" of the audio signal, at low RPM. This leads me to believe that my head unit itself is running low on power at low RPM, since it has no battery backup, like the amps do. So yes, it is a band-aid, to put caps on the amps and the head unit. Ideally, I would have an alternator with high enough output to handle it. But when I have the AC on, the fan on, the wipers on, the headlights on, and the stereo bumping, that 928 Alternator is not cutting it. I think it would be fine if I were not running so much stereo stuff. So I guess my choices are to get a custom alternator, or use some sort of cap for my head unit, or perhaps a newer one, that draws less power, and has a built in capacitor in it's power supply...

deltaP

You could also spin your alternator a bit faster. It probably isn't charging at full voltage when at the lower RPM's just like the power steering isn't as great when trying to park. Put on a smaller alternator pulley so it spins a little faster and gets the voltage back up over 14 and you will probably be fine.

If the voltage is actually dropping because of the lower RPM's, a capacitor will not help. The capacitor only charges to the supply voltage and no more. The capacitors used will discharge in a matter of milliseconds. It's not like it's going to help for 10 or 15 seconds even.

It would be very strange for a head unit to not be getting enough power. There are hundreds of amps sitting in the battery ready to help and the deck might need 3 or 4. It could be, again, the voltage drop due to the instantaneous demands of the amps. A second battery is always an option, too. A 12 volt ni-cad wired in parallel by the head unit (something like an RC car uses) contains a tremendous amount of energy for reserve and has very high current capabilities. Just a thought.

The best solution is to never stop driving fast...

Bernie

I would also like to add that my capacitor (1 Farad) was purchased off ebay with clamps and terminal connectors + fuses + holders for 45.00 and 10.00 for shipping.
What does an Optima battery cost?

BrianG

BC:
Also, get a good DVOM to your battery and to your alternator. Good alternator values with low battery values indicate a conduction problem, and that cable is LONG!! and the grounds are troublesome. <img border="0" alt="[grrrrrrr]" title="" src="graemlins/cussing.gif" />

The 928's charging voltage is said to be dependant upon the "excitor resistor" in the instrument pod. Apparently, the heat cycles can degrade this resistor, and that results in a lower voltage output. Check that it's between 13.8 and 14.2 with no major electrical equipment on. It should be that at idle or real close to idle.

I have been playing with cars for an aweful long time and this "external excitor resistor" has got to be one of the dumbest ideas I've seen in a long time. The new GM alternators have 100A+ capacity and one-wire connections. This looks like an obvious "upgrade" project to me!!

deltaP

bc,

the capacitor kind of acts like a battery like you say, but not really. The amount of energy storage, even in a huge capacitor, is minimal. Maybe enough to help you for a few milliseconds. Even if it were a second or two, that wouldn't be enough.

I would find it strange that your head unit would be running out of power, but you never know.

To maintain power, you need to maintain voltage. The best way to do that would be as I mentioned with a small ni-cad or dry-cell battery near the head unit, isolated from the rest of the car's electrical system with a diode. This is because, imagine you have a giant capacitor right next to the head unit. When the amps thump and the voltage drops, they are going to pull voltage from wherever they can. That capacitor is a prime candidate. So the amps are pulling power away from everything, including capicitors anywhere in the car.

So adding a capacitor by itself probably won't help any starvation issues at the head. You will need to isolate them from the voltage-sucking amplifiers with a diode.

BrianG

Chris, I just don't get it.

Are you suggesting that all those extreme-performance sound systems in those show cars are using poorly designed amps, and that the huge Caps often specified by the manufacturer ought to be replaced by more/bigger batteries? Somehow there is some incongruity here.......

deltaP

Those guys do run multiple batteries. Often times, they are plugged into a battery charger while at the show to keep the voltage up.

Most amplifiers designed today are very poorly designed IMHO.

But my opinions usually differ vastly with the mainstream, that's why I drive a 928 and not a Camaro or Civic.

I believe that a loud stereo isn't as impressive as one that accurately reproduces the original music. Sound pressure in my systems is only allowed after accuracy.

Most of those show guys have never heard a live orchestra, never played an instrument, don't know what sound stage is, don't know what imaging is, don't know how skin effect in a conductor effects sound, etc, etc...

The last car show I went to, some guy was bragging about his system and all his trophies, so I took a listen. The vocals were coming from the back seat! How many times do you sit backwards at a concert? This is how observant most of these guys are. They'll razzle-dazzle you with a lot of sound pressure, but there's no magic.

Anyways, since they took the music and chopped it up into a billion little ones and zeros it has never been the same.

BC

Brian - is this "exciter" easy to get to behind the pod? Can it be tested independantly?

I'm pretty sure I simply have a crap-a$$ alternator on the 81, but before I install the system I am removing from the 89, I want to make sure its not going to kill the car somewhere I don't want to be.

bcdavis

Yeah... I had the "exciter" resistor changed a while back.

I agree, it is a REALLY stupid idea.

My alternator is outputting quite well, and the battery is good. So I may look into the connection wire to the battery and ground. That seems like it may be part of the issue. I need to run around with a volt-meter, and actually see if there is a big drop in voltage at the battery, or at the amps, or at the head unit. I need to just check the power at these places, and see where the problem lies...

BrianG

and don't forget the "Wally admonition".... clean up them ground points!! <img border="0" alt="[jumper]" title="" src="graemlins/jumper.gif" />

Actually, it's a far more important issue than I would ever have thought. This car is so "electric dependant" that it might qualify for "hybred status"!! <img border="0" alt="[ouch]" title="" src="graemlins/c.gif" />