# Re: Cap or Second Battery?

> 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
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