Re: Cap or Second Battery?


"Tony F" <tbone407*nospam*@charter.net> wrote in message
news:mz3Fe.17323$bG4.12327@xxxxxxxxxxx
> 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

"serveral thousand Farads" ???

that's a big amp!!



>> 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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