Re: Cap or Second Battery?

Mark, can I quote your explanation of caps vs. batteries (and more) and use
it on www.maxima.org? Several threads have been posted there recently
regarding this subject and I found your "write up" extremely well written.
I'd use your entire post, which is included below, if you say it's okay.

Tony


--
2001 Nissan Maxima SE Anniversary Edition
Eclipse CD8454 Head Unit, Phoenix Gold ZX475ti, ZX450 and ZX500 Amplifiers,
Phoenix Gold EQ-232 30-Band EQ, Dynaudio System 360 Tri-Amped In Front and
Focal 130HCs For Rear Fill, 2 Soundstream EXACT10s In Aperiodic Enclosure

2001 Chevy S10 ZR2
Pioneer DEH-P9600MP Head Unit, Phoenix Gold Ti500.4 Amp, Focal 165HC
Speakers & Image Dynamics ID8 D4 v.3 Sub


"MZ" <myfirstname@xxxxxxxxxxxxxxxxxxxxxxx> wrote in message
news:muudnRm3wbUhsnnfRVn-2Q@xxxxxxxxxxxxxxx
>> Dimming headlights is one problem. Associated is the problem that when
>> my subs hit the amps that drive my components are obviously starved for
>> power. .... So what is the differnce between a cap and a battery?
>
> Here are some functional differences between a cap and a battery:
>
> 1) The operating voltage of a cap is defined by the charging voltage,
> whereas a battery's is roughly constant (at full capacity, at least). The
> alternator operates at around 13.8v-14.5v (usually less at engine idle),
> depending on the car and condition of the alternator. The battery
> operates
> at around 12.5v-13v. So, when the engine is on, the alternator is going
> to
> be supplying the bulk of the current since its voltage is higher than the
> battery's. A capacitor's voltage follows the alternator's voltage, so for
> the sake of this discussion we'll consider it to be equal to the alt's
> voltage. When a quick current draw from the amp occurs, the alternator
> voltage drops a bit and so will the capacitor's. In order for the
> capacitor's voltage to drop, it has to discharge current through the
> electrical devices that are drawing current - and that's how it benefits
> us.
> [I've reversed cause and effect here for clarity] Meanwhile, the
> battery's
> voltage is still lower than the alternator's voltage, so it's not even
> participating in this process (kinda sorta).
>
> 2) Another difference is that the battery has a higher internal resistance
> than the capacitor. This simply means that the capacitor has the ability
> to
> discharge a *greater* amount of current during very fast transients when
> compared with the battery. It also means it will be recharged quicker
> than
> the battery when the transient is over.
>
> 3) Some claim that the battery "reacts" more slowly than the capacitor.
> While this may be true from the internal impedance standpoint (and can
> therefore be included in point #2), I don't think there's much of a
> consequence from a functional standpoint. We all know that the battery
> has
> the capacity to deliver extremely large amounts of current reasonably
> fast -
> that's how it's able to start the car. A capacitor can't start the car
> simply because it doesn't store enough energy to do so [unless you had an
> ungodly amount of capacitance]. I'm not convinced that the battery is so
> slow that it would be "too slow" to deliver current for transients. After
> all, the amplifier already has several thousand Farads of capacitance
> inside
> to help with the immediate current demands. So maybe a cap can deliver
> high
> instantaneous current, but it doesn't do squat to help during a sustained
> current draw. I think the same might be said of alternators as well, to a
> certain extent. How long can the alternator deliver a lot of current
> before
> it starts to suffer from droop due to the feedback process inherent in
> alternator operation? If anyone has the answer to this question, I'd love
> to hear it.
>
> As you can probably tell, batteries and caps serve different purposes in
> the
> system. A battery is really there to start the car, filter some ripple,
> and
> that's about it. It also delivers current when the current draw is so
> high
> that the alternator's ~14v output is lowered down to what the battery's
> voltage is. So it's not completely pedestrian. It's important to have a
> good battery because it's also charging up when the car is on, so it's
> stealing away some of the current from the alternator. Old crappy
> batteries
> are going to take longer to charge (or continually charge) and draw more
> current while doing so. The moral of the story: if your battery is old,
> replace it. Get a halfway decent one too. I've had great luck with Die
> Hards, but admittedly I've never done a true comparison of different
> brands'
> capabilities.
>
> Capacitors, on the other hand, provide *very* brief bursts of current to
> supplement the current delivered by the alternator. This is useful for
> short-duration transients. To illustrate with an example: if the current
> demands of the amplifier are causing the alternator's voltage to drop from
> 14v to 13v every time the bass hits, the capacitor might smooth this
> fluctuation a bit - say, making it go from 13.9v to 13.5v instead. [I've
> oversimplified things a bit, and completely fudged the numbers, but you
> get
> the idea] Enough capacitance installed correctly should help with the
> headlight dimming, unless you're so far gone that your alternator is still
> recovering a second or more after the peak.
>
> It might also help the amplifier's power supply charge a bit faster.
> Although, by that time the reservoir capacitors are already "depleted" (so
> to speak - of course they don't fully discharge), so it's not going to
> make
> much of a difference to the amplifier's output. Any claims that the
> capacitor is going to help with bass response or anything like that is
> bogus. You're not going to hear a difference between a supply voltage
> difference of half a volt - even moreso since it's on "the other side" of
> the power supply (ie. the transformer's primary). Yeah, I know, we hear
> of
> all these people who insist that their bass is better now with their
> mega-Farad cap installed. Not sure how that's possible. Besides, most of
> them are operating under the assumption that the supply voltage has an
> effect on the *gain* of the amplifier, but it's really only affecting the
> voltage rails (which helps determine at what volume level clipping will
> occur). So many of them claim that the bass is *louder* after installing
> the cap. I don't see how that's possible. The perceived volume should be
> the same in both cases. Even if they're correct in assuming that the
> capacitor is significantly helping to maintain the rail voltage [which
> it's
> not], the symptom would be greater distortion without the cap - not lower
> volume! What I suspect is REALLY going on in their case is masking - a
> perceptual phenomenon whereby some sounds sound diminished when
> accompanied
> by louder sounds. And this has nothing to do with supply voltage. So,
> naturally, we might get the impression that our midbass driver isn't
> pumping
> out as much bass when the subwoofer hits.
>
> Anyway, this newsgroup's FAQ, located at http://www.mobileaudio.com, has
> some more information in the "electrical" section regarding capacitors -
> such as how to go about determining whether a capacitor, battery, or
> alternator upgrade is your best course of action.
>
>


.

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