Cap - Honda Prelude Forum

92-h23a1 · 01-17-2005, 01:18 AM

Hey guys, right now I'm running a 12" Solo Baric Kicker. No Cap. What Caps would you guys recommend? I think its causing my battery to corrode.

ExodusFX · 01-17-2005, 01:49 AM

I don't see how running that would cause your battery to corrode. If it's been a few years since you put a fresh battery in, by all means replace it if it's bad. You can have it tested for free at most auto parts stores. But if your lights are dimming on big bass hits, you could use a capacitor to try and remedy it. Stiffening caps are really just a band-aid for when your alternator can't supply enough power for a given instantaneous draw, and you can't afford a high output alternator. I'd clean off your battery terminals and upgrade the big-3 if you haven't already. If that doesn't help, then you can decide if you want to try a cap or save up for a HO alternator. As far as what brand cap to buy, it doesn't really matter. They're pretty much all the same, unless you want fancy connectors and/or a digital voltage display on it. (if you want to show off the cap/install)

apocalypse · 01-17-2005, 11:47 AM

how many amps is your amp?
do you have stock battery terminals?

TimeRacer · 01-17-2005, 12:53 PM

Actually the main car battery just isn't designed to discharge in lumps, plain and simple. You could very well be overdrawing your alternator as even though we have a 100 amp alternator it still isn't quite enough to power the entire car and some sound systems. With a cap just get one that's reputable and cheap. You don't have to go with name brand or anything, I got my 1 farhad cap for I believe $30 off a seller on a car audio board over 6 years ago and it's never given me a problem. Though if you want the "best" it's the 15 farhad Alumapro one.

crashandburn · 01-17-2005, 01:52 PM

If you want the "best" capacitor, get the lowest ESR/ESL you can find. Equivalant Series Impedance (ESR/ESL) determines how much power (time-dependent) the cap can deliver. The farad rating tells how much energy (not time-dependent) the cap can deliver. So a 1 farad cap with very low ESI can deliver more power than a higher farad cap with a high ESI -- even though the higher farad cap stores more energy, it can't pump it out as fast because of the higher ESI so it supplies less power.

A better solution would be to forget the capacitor and just move your battery to the trunk so it's close to the amp.

ExodusFX · 01-17-2005, 09:10 PM

Good luck finding that spec on most caps though.

crashandburn · 01-18-2005, 12:05 AM

^^ because it sucks on most caps and the manufacturer knows it.

TimeRacer · 01-18-2005, 03:05 AM

You're really not going to find that spec, just because it requires testing on each invidual unit and caps aren't all that expensive in the first place. As every set of connections will have a different spec and you'd have to test each one of of them (charge it up, set it up, discharge it and then measure the resistance of connectors). But in reality for audio systems this spec is almost worthless because the variance will not be incredibly large between any given quality cap of same capacitance. All the everyone really cares about is if the cap holds enough power to discharge when the bass not hits and not put a high drag on the electrical system (allowing the alternator to run at max capacity). It matters when everything is required to be on the #'s, but in audio it really doesn't matter. Which is why I recommend you go after the cheapest one that's reputable. Specs between caps will not vary by much.

crashandburn · 01-18-2005, 08:59 AM

Actually, you can find that spec on some capacitors. I have a 1.5 farad cap I bought many years ago and it's printed right on the cap -- "E.S.R. < 1.5 milliohms". The manufacturer knows what they're building without having to test every single unit: they know their statistical distribution, & they know their production Cpk. If they don't give you a spec, it's almost always because they know it won't make them look good so they don't even bother to measure it.

Most very high farad caps (> 5-10 farad) use an activated carbon-type construction that gives them insane interior surface area compared to traditional "can" caps with separated foil spirals. This translates to higher energy density and thus higher farad ratings but the tradeoff is that it usually has higher resistance. It may not look like much (0.015 ohm vs 0.0015 ohm), but when your system needs 100A of transient current, that 0.015 ESR will drop 1.5 volts as opposed to the other that drops only 0.15V. They're continously finding innovative ways to reduce that ESR but it is still generally higher on very high farad caps and that's why they will NEVER list a spec for it until they get it down to a competitive level.

Quote:

Originally Posted by TimeRacer

...All the everyone really cares about is if the cap holds enough power to discharge ...

This is where so many people get tripped up about caps -- they don't store power, they store energy. Power is a measure of how quickly energy can be applied but energy itself has no time dimension. Since you buy a cap for quick transient response, high power is far more important than high energy and the maximum power a cap can deliver is determined by the farad rating AND the ESR/ESL. If capacitors were squirt guns, the farad rating would only tell you how much total water the squirt gun could hold but the ESR/ESL would tell you about the actual spray nozzle -- it would tell you how quickly you can soak somebody (assuming you have enough water or "farads" to soak them).

Quote:

Originally Posted by TimeRacer

...Specs between caps will not vary by much...

Specs between similarly constructed caps will probably not vary by much. You have to know what you're buying. If the cap you buy has an ESR > 0.010 ohms, simple circuit analysis can demonstrate that it won't "stiffen" the voltage very well no matter how many farads it's rated or how reputable the manufacturer.

m610 · 01-30-2005, 05:54 PM

Just curious, but how do you charge such a high capacity cap initially, and after starting the car, and after draining it somewhat while in service? Caps charge as quickly as they discharge, so unless you take precautions it seems you'd still be stressing the battery and alternator even worse than before when you start the car. Caps will discharge slowly on their own, like when the car is parked overnight. Do you use a resistor to limit the charging rate, or does the battery keep it charged? I have some old computers that have, in total only a tenth of what you guys are describing and they dim the lights in the house momentarily when I turn them on. These power supplies use current-limiting resistors to keep the caps from charging so quickly that they blow out the rectifiers.

Minor comment, I thought caps stored electric charge, spare electrons in a can, so to speak.

HeavyDLuder91 · 01-30-2005, 09:15 PM

get the alumnapro 15 farad cap. Thats what I got and I only get the best!!!!

92-h23a1 · 01-30-2005, 09:45 PM

How much do they run and where would I go look for those?