# cap makes voltage problems worse?



## DJSPANKY (Dec 15, 2009)

Hello all,

I recently visited a local car audio shop (not a chain- a local "mom and pop shop" where the installer seemed to know what he was talking about and wasn't a 17 year old kid just interested in bass) and had several discussions with the installer. One discussion was about adding a high amp alternator to one of my systems. The system is in a Dodge Ram with (2) RF 18" XLC subs powered by a RF Power 1000a2. There is also a RF800a4 running components all the way around, plus the truck has a snow plow. Needless to say the factory alternator isn't cutting it.

I have an aftermarket 200amp alternator to add to the system. When I mentioned this the installer asked if I had 1 or 2 batteries. I only have 1. He recommended that I get (2) batteries (1 additional) when I add the alternator, claiming that upgraded alternators can be damaged in a system with only 1 battery. That was the first I'd heard of that, but I pressed on with the conversation. I mentioned that I caould add an Alumapro 15 farad cap instead of an additional battery (mostly b/c I already have it and it's on the shelf not doing anytihng). The installer then said that adding a CAP would actually make the problem worse. He justification was that the CAP is always drawing current from the charging system as opposed to the amps only drawing mass amount of current during bass heavy portions of the music. He actual statement was that CAPS are good if you just want the illusion of fixing a problem (dome lights don't dim) instead of actually fixing a problem.

I have never heard this before. 

It didn't seem as if he was just trying to sell me something. I'm not even sure if they sell batteries or alternators. At a minimum they doen'y stock them in the shop. I guess he could have been motivated by the desire to sell me 2 batteries- but I'm not sure. I'd like to give him the benifit of the doubt and find out more information before I pass judgement.

So has anyone ever heard of:
#1) a single battery system buring up a part of a high current alternator simply b/c it only had 1 battery
#2) a CAP making charging system problems worse.

Thanks for the help,
DJSPANKY


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## dragonrage (Feb 14, 2007)

A cap can make a small problem better but if you have a large problem it can make it worse because they only hold a small amount of energy and when the amp drains it, you then have both the amp and the cap demanding energy from the alternator which makes your system even more strained.

I recommend an HO alt with a single good battery unless you are looking to play your system with the car off in which case I recommend multiple batteries. Having just a single battery will not burn out a HO alt, even if you had a 400A alternator.


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## adrenalinejunkie (Oct 17, 2010)

Never heard of #1, but have heard of #2.
http://www.diymobileaudio.com/forum/car-audio-truth-myths-industry-dogma/96044-capacitor-myth.html


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## airbornflght (Jan 18, 2012)

A good alternator and batter is far better than a capacitor. Capacitors by their very nature can only handle very small transients, but most of the time they won't do much at all.

FYI a 1 fard capacitor holds 1 amp/second energy @ 1 volt, or conversely, 1/12 amp/second of energy at 12 volts. Now, lets say out musical transient (a bass punch) is 1/24 of a second long. 

Our capacitor would be able to provide an additional 2 amps of stored energy through the transient. 

They aren't completely useless, but I see no need for them.


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## DJSPANKY (Dec 15, 2009)

Thanks for all the info guys. This really helps


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## MarkZ (Dec 5, 2005)

Yikes. You should be more discriminating about the info you accept. :/

1) A cap will not make a problem "worse", unless it's defective. It cannot draw more current than it delivers. This is because it's (practically) lossless. Furthermore, it tends to draw current only when the charging system can afford it (see point #2...).

2) A cap only charges (i.e. draws current) when the charging system's voltage is higher than the capacitor's voltage. This is expressed mathematically on wikipedia's site (the 5th equation): Capacitor - Wikipedia, the free encyclopedia This means that if the charging system is being taxed during a bass hit, for example, the capacitor will have discharged to the same voltage that the charging system sits at UNTIL the charging system starts to rebound. In other words, it's _impossible_ for the capacitor to draw current from a current-starved charging system. In fact, the capacitor is delivering current to the amplifier at the same time the charging system does, reducing the peak amount of current that the charging system has to deliver.

In effect, a capacitor serves the same purpose in a vehicle's electrical system as it does in a crossover: it resists changes in voltage, and therefore acts as a lowpass filter (when placed in parallel to the load).

Will a capacitor always reduce the symptoms of dimming lights? No. Sometimes it just can't deliver enough energy to be a positive factor. But it's hard to imagine a scenario where it would be a negative factor, unless the alternator has an odd rebound time course due to some crappy electronic control system.


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## minbari (Mar 3, 2011)

^^^ x2

1) an alternator, HO or not, will only put out as much current as your demands. adding a second battery will add an additional load on the alternator. if you need more engine off time, then get another battery. if not, then 1 battery is fine.

2) the cap info is totally retarded and only shows how little this guy actually knows about electronics. weather you have 1F or 30F, the cap is gonna follow the input voltage based on current draws from the amplifiers. a cap will NEVER put out all of its charge, if it did then it would be at zero volts! if your systems reaches zero volts when playing, you have bigger problems than a cap! 
once a cap charges to its voltage it is essentialy an open to the system. there is a small amount of leakage current with any cap, but with a low esr cap like the kind used for audio, it is nearly non-exsistant. you could charge that cap off your car battery and let it sit in your basement for a week and it would still be charged. 
the only thing this guy said that is right, is that caps give the illusion of fixing a problem. that is true. they are bandaid. HO alternator are the real fix. since you already did that, you dont need one.
(as a side note, most of the time caps are completely useless for audio systems, dont bother with it)


everything that guy said is completely baseless and I wouldnt beleive anything he said.


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## DJSPANKY (Dec 15, 2009)

MarkZ said:


> Yikes. You should be more discriminating about the info you accept. :/
> 
> 1) A cap will not make a problem "worse", unless it's defective. It cannot draw more current than it delivers. This is because it's (practically) lossless. Furthermore, it tends to draw current only when the charging system can afford it (see point #2...).
> 
> ...


MarkZ,
thank you for the information- it helps. Or should I be more discriminating about YOUR information- ha ha.

Notice that I said that everyone's information helps. I didn't say that I bought it all hook line and sinker. Obviously I don't do that or I wouldn't have started the thread in the first place.

I appreicate your comments, and will take them into consideration while I further research the topic.

Thank you for taking the time to reply.


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## AKheathen (Sep 10, 2011)

what you have to look at with the strain on the alternator with both batteries and caps. caps, really don't hold much compared to batteries. where much strain of the alternator comes from is duty cycle. point blank, your alternator should be capable of feeding your current needs, or you need to upgrade it. say, you have a high current hit, well, there is that amount of current pulled from the battery and alternator, a moment later, then there is the time the alternator takes to bring the voltage back up to working voltage. the heat created in the diodes and the rate of dissipation sets the alternators duty cycle. adding more capacitance via a cap. or battery, increases the duty cycle put on the alternator to recover to the same working voltage. so, while the cap can overload the alternator in this way, a second battery can be much worse, since the battery is going to require more power to bring the voltage back up, and a battery also takes more power than it puts out. both, however, dampen and maintain a more mean voltage, where an alternator would not be quick enough to respond to demand, and carries an ac ripple in the voltage. capacitors are quicker at responding, which can help with the ripple "whine" problem, if you have it. i do remember that exact alumapro 15 capcell, and it actually did a heck of a job masking the need for an alternator, but, as mentioned, all it did was mask it, and also had a problem with slowly draining the battery...not sure which effect eventually killed the stock alternator in my buddy's pathfinder, but it did nonetheless. i wouldn't mind having one to mount on the wall for nostalgia, though, if you'd be interested in getting rid of that one. as for the second battery on a h.o. that is within it's working range- you don't need it, but it does have it's advantage, especially on a plow truck, with decent sized current spikes from the pump motor.


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## MarkZ (Dec 5, 2005)

AKheathen said:


> say, you have a high current hit, well, there is that amount of current pulled from the battery and alternator, a moment later, then there is the time the alternator takes to bring the voltage back up to working voltage. the heat created in the diodes and the rate of dissipation sets the alternators duty cycle. adding more capacitance via a cap. or battery, increases the duty cycle put on the alternator to recover to the same working voltage.


Not true at all. The cap is lossless. Any current it draws while the alternator is recovering, it has already delivered during the initial current draw. It's a zero-sum system.

A battery, as you correctly point out, is lossy. So the same does not necessarily hold true. [Although you're also correct that its energy capabilities are much greater, and so for that reason, it can often produce more noticeable results]


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## AKheathen (Sep 10, 2011)

If you pull from a cap, it still needs to accept the same amount it discharged. I have a 2farad mounted on my bench with my power supply setup to clean it up, and get to watch current and voltages on the meters. Power in=power out


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## cmayo117 (Aug 14, 2009)

airbornflght said:


> A good alternator and batter is far better than a capacitor. Capacitors by their very nature can only handle very small transients, but most of the time they won't do much at all.
> 
> FYI a 1 fard capacitor holds 1 amp/second energy @ 1 volt, or conversely, 1/12 amp/second of energy at 12 volts. Now, lets say out musical transient (a bass punch) is 1/24 of a second long.
> 
> ...


Actually, a 1 farad capacitor would be able to provide 12 coulombs of charge, or 12 amp * seconds, at 12 volts. So, if you were to discharge that entire 12 coulombs in 1/24 seconds, that would be 288 amps of current. Of course, this would not happen unless your entire charging system dropped to 0 volts, but it gives you an idea of how much energy a 1 farad 12 volt capacitor can hold. Now, if the system voltage dropped just one volt in 1/24 seconds, that would still mean that the 1 farad cap would supply 24 amps for that 1/24 seconds. 



AKheathen said:


> If you pull from a cap, it still needs to accept the same amount it discharged. I have a 2farad mounted on my bench with my power supply setup to clean it up, and get to watch current and voltages on the meters. Power in=power out


Yes, but any energy discharged from the capacitor is energy that does not have to be discharged from the battery or alternator. So, any extra energy the alternator has to provide to recharge the capacitor is energy it would have had to supply elsewhere if there was no capacitor. Therefore, as long as the capacitor is not wasting significant energy, and energy in is effectively equal to energy out, the capacitor will not create any extra significant strain on the alternator.


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## ATOMICTECH62 (Jan 24, 2009)

That guy has no business working in a stereo shop.
Since all modern alt are regulated that kills his first theory.
There are already multiple caps in parallel with the positive and ground of the trucks electrical system,mostly in the amplifier,that adding to them will only help.(IF DONE CORRECTLY).
However,big caps will slowly drain the battery if its not being used for long periods of time.
I had a 1 farad Rockford in my blazer that has dead taqs.It would go dead in two weeks.Once I disconnected the cap I can go about 3-4 months and it will still start.
Where are all the intelligent people?
They must be on this forum because there not in the shops any more.


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## AKheathen (Sep 10, 2011)

cmayo117 said:


> Actually, a 1 farad capacitor would be able to provide 12 coulombs of charge, or 12 amp * seconds, at 12 volts. So, if you were to discharge that entire 12 coulombs in 1/24 seconds, that would be 288 amps of current. Of course, this would not happen unless your entire charging system dropped to 0 volts, but it gives you an idea of how much energy a 1 farad 12 volt capacitor can hold. Now, if the system voltage dropped just one volt in 1/24 seconds, that would still mean that the 1 farad cap would supply 24 amps for that 1/24 seconds.
> 
> 
> 
> Yes, but any energy discharged from the capacitor is energy that does not have to be discharged from the battery or alternator. So, any extra energy the alternator has to provide to recharge the capacitor is energy it would have had to supply elsewhere if there was no capacitor. Therefore, as long as the capacitor is not wasting significant energy, and energy in is effectively equal to energy out, the capacitor will not create any extra significant strain on the alternator.


unfourtunately, there is no picking and choosing where the power comes from. there will be an equal voltage drop across the entire system, save for the effect that the wiring has on it. so, the voltage will drop at the alternator, battery, and cap. how much power is pulled from each, is equal to how much power is needed for that drop in voltage. as for the alternator, it takes hardly anything to drop the voltage there, since there is no capacitance in it and it will adjust shortly after the drop, but will need to catch up the other parts as well. the capacitor is easy, being loss-less, but the battery would be the wildcard... the thing about capacitors, is that they can react in short bursts, allowing the alternator to catch up before a significant drop, which will hide the dimming, or, i should say, not make it obvious. basically, if you look at the power flow and activity of the whole single cycle, you could see what is going on. without the capacitance, the alternator simply sees the voltage drop and adjusts output, though it's output is more-so in real-time use, and does not need to recover previous used power, simply supply the present demand. throw a battery in there, and once it is fully charged, the alternator operates in the floating range, which is anything above high-charge voltage, typically 13.8-15v. in that rancge, the battery is operating in the system as a low capacitance devise, and holding verry little power in comparison to a 1farad cap. now, the cap will have active working voltage all the way up to it's internal dielectric resistance voltage, where it would begin to short the voltage, usually over 20v. not only that, it completely shorts any ac voltage, up to it's limit. to give an idea, the 2 "plates" in a cap have a tremendous more surface area than 1 battery cell, which is where you find capacitance. in case you are wondering why i referred to 1cell, not 6cells, is because the 6 cells of a battery are in series, and when working with capacitor and similar devises, paralleling adds capacitance, and series adds voltage capability, so there is no more capcitance in a battery than a single cell.

so, in short- battery= small capcitance/recovery in floating voltage, and high recovery in charge voltage, and capcitor=large recovery to large draw


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