Alternators, Batteries, Capacitors

Making sense of it all through all the chatter

Car electrical systems come with a simple single battery, alternator charged electrical system that is more than sufficient for the stock car's needs. It is even sufficient for a car with a relatively large custom audio system.
There is a lot of talk among the audio forums about what people "need to do" to their car's electrical systems to support moderately large audio systems. Unfortunately, often much of this talk is unfounded.

At some point, you'll find yourself pondering what you really do need in your vehicle to support your system and listening habits, and it's often difficult to see the forest through the trees, with all the rumors and myths.

One big cause of confusion for many is the application and use of capacitors.

There are many rumors out there that "caps don't do anything", or that "it's been proven that they aren't effective". These rumors began largely because of some tests conducted by Richard Clark (Autosound 2000), tests that were ironically enough performed to dispel some myths that capacitors help amplifiers make more power (which they certainly don't).

The scope of Richard's testing was along a much larger span and grain of time than the fine-grain, short-duration time period that capacitors contribute when activated. As such, Richard's plots had a minimum resolution of full seconds, spanning a period of several minutes.
As capacitors typically discharge down to their lowest charge level in an installation in under a second, it is easy to understand the misinterpretations, and hence the rumors.

Anyone who has experienced poor sounding MP3's (ones that were encoded at a bitrate low enough to cause audible degradations) should understand that our ears and brains don't process sound with minimum time resolution of a full second.
Understand also, that a the intent of installing a capacitor in the car is not to provide any semblance of a continuous supply of current - that is the alternator and/or battery's job.
So what is the purpose of the capacitor?

Read along as we explore the purpose of all these components in your electrical system.

Understanding what each does

You can't make an educated decision on what should be upgraded, if you don't understand what each component fundamentally does, much less what the limitations of each are.


An alternator can supply higher current levels when the car is running, but provides nothing when the car is off.
In addition, it puts a higher drag on your engine, which reduces your horsepower at the wheels and accelleration, it is expensive, and it might not even be avaliable for your car, depending on what car you have obviously.
They are also tricky to shop for, because you don't want to end up with a "200 amp" alternator that in reality produces the same or lower output current at idle.
But obviously, the benefits are that a good one can produce more current any time the car is running, if you do fall into the category who needs this.
Realistically though, that's not the majority- there are a whole number of interesting factors to consider though, including not just the wattage of your amps, but your listening habits, the "duty cycle" of the bass that you are playing, recording levels, volume levels, and your batteries recharge/discharge rates and the "duty cycle" there, also...

A Battery is always connected, but not always used. When the car is off, obviously it is. Most of the time, the alternator can supply enough current at it's 14.4v level that everything gets it's juice from the alternator.
When more current is demanded though, for that instant, the extra current is drawn from the battery... when that happens, your electrical system temporarily drops to the 12v level of the battery... and since lights aren't as bright at 12v, you see them dim. This isn't a symptom of a problem, the electrical system is doing what it's supposed to. As long as the battery has enough time to recharge in between (it only needs to recharge the small amount of current that was drawn) - as long as the battery voltage is able to rise back to 12v each time, it's OK.
Adding a second battery, close coupled to the amp, will help things in several ways... the most important being that your main power wire is resistive, and the second battery will essentially eliminate that, speeding the response of the battery to current demands from the amp.

A capacitor is added because batteries are slow to give up charge, relatively speaking, and are slow to recharge, where capacitors are nearly instantanious.
Think of it like eliminating a speed bump in the road.
Your amp demands current, to really slam a huge instantanious explosion or bass drum... in that instant, it exceeds the current capability of your alternator, so it must draw the extra current from the battery... the battery is like Droopy Dog.. it's saying "Ooooohhh... kaaaayyyy... heeeeeerrreeeesss yooouuurrr currreeeennnntttt", and finally the amp gets the current. But there is a rise time involved. Granted it is tiny, but here's where the lightning fast capacitor can help out. It gets rid of that speed bump. While waiting for Droopy Dog, your capacitor discharged for the amp.
Capacitors are also good, in scenarios where you have fast kick-drum type bass. They don't help much at all on those long drone-tone extended bass tones like bass CD's, like sine waves, but that's fine... they don't hurt performance in those scenarios, either.

Picture what happens when your system suddenly demands 200a of current for some huge, high-volume transient burst:
You have a point in time where the alternator reaches it's limits.
Before this point, there's no need for a capacitor.
And you have a point in time after that where the battery was called upon due to the dropping voltage level, where the battery actually has risen to the demand, and is fully providing the current that's being demanded.
After this point, there is no need for capacitor.
There's a gap in between those two points in time, where the voltage is initially falling, and where the current flow is rising, where the supply of current isn't meeting the demand for current.
And that gap in time only is milliseconds.

Reference the illustration below, to compare how a capacitor can augment the electrical system, during this transition time, when the current capacity of the alternator has been exceeded, but before the battery has fully risen to the task at hand.
This shows the effect on voltage of a heavy bass note beginning at the point in time of T1, and ending at T4:
Illustration showing voltage momentarily dipping when no capacitor installed

Headlight dimming - what does it mean?

Possibly your concern is headlight dimming.
It certainly doesn't seem good. Especially if you don't understand the dynamics at play.
It certainly does beg the question: Is dimming really a bad thing?

Bear in mind what is going on:
Normally, with your car running, the alternator provides over 14v of power to the electrical system. The amplifier is happy with this, and your lights are all at their brightest.
When something in your electrical system exceeds the alternator's current capacity (ie. your amp), for the moment that it needs to draw more current than the amp can supply, it pulls the remainder of the current from the car's battery..
And for that moment, your car's voltage level drops to the voltage level of the battery... 12v.
Now, of course, your lights aren't as bright at 12v.. hence, dimming.
This isn't a bad thing, a harmful thing.. that's just what happens.

It can be harmful if you see your headlights dimming darker, and darker, and darker each time... meaning your battery discharge exceeds your battery's charge time.
That's definitely a sign that your listening habits are warranting an alternator upgrade..

But most of us aren't anywhere near that, and won't have an alternator upgrade in our near future.
Most of us might even find a capacitor to be a benefit, particularly with "real" music, on realistic power levels...
And if it doesn't help, it might simply indicate that there is a weak link in the chain elsewhere... most likely a system design or installation that has created an inherent electrical inefficiency, such as expectations of truly high-output from tiny enclosures.
This is certainly not a sign that "a capacitor is useless".

Prioritize your upgrades

Now that you understand what role each component potentially performs, it's not too difficult to see what limitations will be reached before the others.
While I'd typically recommend upgrading in that order, the expense of each upgrade needs to also be factored in, in terms of prioritizing what upgrades to tackle when.

Here's what I usually recommend:

#1 would be the wiring from the battery to the amps.
I try to advocate even overkill on your main power wiring, as affordable wiring supplies are out there.
Use the largest gauge wiring that you can. Using "appropriately sized" wire is good, but "going overkill" with even larger gauge wire is simply even better.
It's fundamental to your install, it's your most "gotta have it" type of thing, and overkill now will not only support tomorrow's upgrades, but arguably provide a more ideal supply for your amps now.
It's easy to find power wiring surprisingly inexpensively online.

#2 would definitely be the "magic three" as they are called, mostly because they are so inexpensive to do, and can often yield a noticable improvement.
These are the "magic three" underhood wires: A tip, as you are replacing or augmenting (There's no need to flat out replace, after all) these three wires, do everything you can to ensure clean, bare-metal on bare-metal solid and secure connections.
Remember that "paint is an insulator". Take a wire wheel and chuck it up in a drill to clean any paint off your ground points.
These are cheap and easy upgrades.

#3 would be to add a capacitor or three.
You might have different needs for "why" and "where" though...
Capacitors installed near the amp can help your amp with that brief transient time when you do exceed your alternator's current capacity, and the extra current is drawn from a relatively slow-responding battery... the relatively lightning fast cap can fill in here.
Capacitors installed near the battery up front can help filter noise more effectively, and even still help with the current flow situation.
Capacitors will also help minimize light dimmage, even if none will eliminate it.
These are relatively small, low cost devices, with low installation effort, and without negative side effects, placing it high on my list.
Side note: You may want to avoid those large, square style capacitors that seem to offer huge amounts
of capacitance. These are not electrolytic capacitors, but rather carbon-type capacitors, which have more
ESR (internal resistance) than the more common round-style electrolytic counterparts.
This ESR is harmful - it slows the flow of current from the cap, nullifying the benefit of much
of that capacitance. Not to mention, it makes them more expensive, and less beneficial to your electrical
system. I recommend sticking with the simple and inexpensive round electrolytic caps.

#4 would be to add a second (or third) standard battery.
I avoid deep cycle types... deep cycle batteries are the slowest charging - and discharging (ie. transient response) type of battery.
Ideally you might install that near the amps, minimizing the wiring between battery and amp, for those moments where you exceed your alternator's current capacity.
Also, due to the relatively inferior transient response of a deep-cycle battery, I would recommend using a standard battery at least in the back by your amplifier (if adding a second battery back there), if not underhood also.
Even if your average current draw exceeds your alternator's capacity for some period of time, this may provide sufficient supply to endure this abuse.
Batteries are large, expensive, and heavy, and may require significant planning to properly install (most sanctioning bodies require that a secondary battery be installed in a sealed compartment vented to the outside of the vehicle).

That being said, You don't need to buy anything special to add a second battery, electrically speaking.
No "isolator" required.
It's as easy as wiring the + pole right to your distribution block, and grounding the - to the same place as your amplifiers are grounded.
The only downside to a non-isolated approach is that realistically if you ran your system with the car off, you could drain both batteries, leaving you needing a jump-start. But... you'd have two batteries, making the system run longer before that could happen, and you'd have two batteries to start your car from.

And chances are, with that much power on tap - unless you have some real fundamental inefficiencies in your installation (which would imply you could get more output going on with less power) - chances are you won't be able to stand having it cranked up to that level of output for more than short periods of time at any rate...
Because as soon as you do turn it down, you also inherently are turning down your current demands, of course.

Only if you suffered actual symptoms, after making all of the above changes, would I go for...

#5 is the alternator upgrade.
Bear in mind, it's not a "happy happy" upgrade.
There are compromises and tradeoffs... it's expensive, you'll lose useable horsepower, and even gas mileage.
You might find that the current capability at idle RPM isn't much higher than your stock alternator was... it might make it's higher current number... but at a higher RPM, like 3000.
That's something you should know, while shopping for them, if you ever do get there, someday. You should look for the plot of current capacity vs RPM, because current capability is a function of RPM.