Thread: High Amp Alt vs. Bigger Cap

Caps and batteries are a band aid fix for a bigger alternator. Does it help? It can but eventually they both have to be recharged by the alternator while supporting gobs of electronics at the same time. However batteries and especially capacitors help smooth out the electrical signal from the alternator.

Also, every amplifier has capacitors in the power supply section.

I have a 2.5F cap that didn't do a damn thing. I did the alternator rewire and my headlight dimming has nearly disappeared.

Why you don't need a capacitor - Realm of Excursion
Easy read to learn alot

yeah that's alot of bla bla bla i can go ahead and point out several critical mistakes that post....basically, when you simply do not have enough power because you have taken more power than your alternator can produce, no battery or cap will save you. You have to stop the power consumption and get the battery charged back up to maintain voltage.

But when your electrical system has plenty of power available for your amplifier but cant deliver it 'quickly' enough.....a cap does one simple thing...stores the energy for brief moments when the amp draws more power than what would regularly be available in your electrical system. That's it, there's no magic to a cap. A fast discharging battery. A cap can maintain its output voltage down to 20% charge. It simply buffers out light dimming/flickering with hard bass notes. It does nothing more.

The problem is when a battery is charged it's not a load on the alternator. When you listen to music it's not one constant tone so not a constant draw on the alt and a good battery will charge as fast as a cap.

With a setup like his a second battery is cheaper and going to be more effective smoothing out large spikes in current draw. A daily driver doesn't need a 200 amp alt. to run an amp that will only draw a 100 amps max.

My question to you Nick is way is it that every professional car audio installer is taught to increase power storage before installing a High output alternator,and why is it that most H.O. alt builders won't warranty any alternator they build without a battery bank installed? Last question, Do you have a large aftermarket stereo in your car?

Mine will do 146.2DB on the TL all day long without a high output alternator and no voltage drops. The only thing I've added is a group 31 Deka Intimidater in the truck.

Battery= 12 volts .....alternator =13.8 volts. By default, anything electrical runs at the highest voltage...which is the 13.8 (or more) volts the alternator runs at. Now when you over draw the alternator (which pretty much happens every time you draw huge current from an amplifier) the current falls back on the battery, which has plenty of power to deliver without a doubt, but that power from the battery is 12 volts and not 13.8v from the alternator, so when you over draw the alternator, you get severe light dimming when the current is taken from your battery. A cap is easily charged to 13.8 volts all the way up to 18 volts easily (unlike a battery)...in the brief moment your system goes BOOOOOM the cap will quickly provide the current required for that energy spike, preventing the lights from dimming, Obviously if you have an adequate alternator the cap should be charged up almost instantaneously ready for the next hit, but if you over tax your charging system, its like a beaver dam in the nile river...it will eventually fail. Electrical systems on their own are somewhat 'sluggish', so the big BOOM will dim the lights before the current can be delivered. Think of when you turn your lights on, or the cooling fan kicks on...the current falls down but then the engine picks up to compensate for the lost electrical charge. With amplifiers, a cap should buffer the light flickering out...


I have 2 Image dynamics 12's with two Memphis monoblock amps, total fusing = 200 amps...I run a slightly done up alternator (100 amps @ idle, 140 peak) and a 2.0F cap....the cap defiantly helps the light dimming, period.

oh, and i was in a mobile electronics course for over 4 months before i was put in a shop with a coach mechanic. Never was I taught to increase power storage before upgrading the alternator in a regular everyday system. For the guys who were doing car stereo competitions, then yes the more batteries the better. Sure they are running at 12 volts but there was a hell of a lot of current available to them for a short period of time for super loud explosive bass. For someone who wants to listen to their stereo in the real world, with the engine running, it comes back to my argument about the difference in voltages between the alternator and the battery...keep the current coming from the alternator and don't fall back on the battery, because if you fall back on the battery you will be down to 12 volts from 13.8 volts, and when you drop voltage you lights will dim.

Originally Posted by reinke

i will be the first one here to post it. DO NOT USE A CAPACITOR. they are useless. but a small bettery instead. either a powermaster, optima, or a kinetik.

times eleventybillion

Originally Posted by offroadfury6

there not useless

they are useless, unless you are competing for a sound quality world championship, they are useless.

Originally Posted by _nick_

yeah that's alot of bla bla bla i can go ahead and point out several critical mistakes that post....basically, when you simply do not have enough power because you have taken more power than your alternator can produce, no battery or cap will save you. You have to stop the power consumption and get the battery charged back up to maintain voltage.

But when your electrical system has plenty of power available for your amplifier but cant deliver it 'quickly' enough.....a cap does one simple thing...stores the energy for brief moments when the amp draws more power than what would regularly be available in your electrical system. That's it, there's no magic to a cap. A fast discharging battery. A cap can maintain its output voltage down to 20% charge. It simply buffers out light dimming/flickering with hard bass notes. It does nothing more.

but once a cap is discharged it has to recharge, therefore putting a bigger strain on your alt in a time your system needs the current.

If you think your stereo is loud enough to need a high output alt. or a cap., get a sponsor and let them pay for it. If your stereo is not that loud get a sealed dry-cell, or gell-cell in the trunk, and call it a day!

Originally Posted by Toofastgs

Wait....
if your battery is seeing 12v but your alt is putting out 13.8 there is something wrong. From all i have been told and picked up on. If there is more then a 10th of a volt drop between the 2 either the one of the other is bad or bad cabling.

My alt puts out 13.8-13.9 after start up. Battery sees the same voltage that the alt puts out

batteries are only designed and effective at delivering 12 volts. When the alternator charges it, the battery does get a 'surface voltage' of 13-14 volts but that quickly falls back to 12 volts when a load is put on it. It has to do with the voltage of each cell in the battery. Ive studied it and tested endlessly and extra batteries do nothing but hurt you in the long run.
like i mentioned, they don't hold anything over 12 volts for any length of time, they take a long time to recharge
and to discharge and they weigh a tonne. A cap has a relatively low capacity, and its design can be discharged instantaneously and recharged almost as fast with little to no effect on the charging system considering your within the charging systems limits. It will simply buffer the light dimming issues in a charging system that is not overdrawn,but has light flickering issues. If you have a trunk full of amps and a stock alternator...add as many batteries or caps as you want it will not help... all that happens is your alternator lasts a few months instead of 5 years. Bigger alternator is always the way to go if your short on power, if your power is within limits but lights still dim, then try a cap.

your batterie should always have 13.7-13.8 volts whether it is being charged or not. If you have a large load on your batterie for a long time and its not getting recharged, it will slowly lose voltage, not quickly jump down to 12 volts. If your batterie quickly drops to 12 volts when there is a load on it your batterie is junk!

hmm.... I already know this but for reference wikipedia states:

Automotive starter batteries (usually of lead-acid type) provide a nominal 12-volt potential difference by connecting six galvanic cells in series. Each cell provides 2.1 volts for a total of 12.6 volt at full charge.

Car battery - Wikipedia, the free encyclopedia


A battery that holds more than 12.6 volts is just showing surface voltage, not a true output for any length of time. Its just the way she goes, batteries will not hold any more than 12.6 volts reliably.

An AGM battery is different than a wet cell if your putting wet cells in the trunk your asking for trouble. What you fail to mention is that your caps up to 2 farad only have the potential to hold as much as a 9 volt battery. There is no difference between a .5 farad and a 2 farad cap until the voltage is increased past 24 volts. A capacitor is a marketing gimmick always has been. Their used to make newbs feel like they did something to help there alternator when if fact they only hurt the performance of the electrical system. A surface charge on a wet cell still has more capacity than 10 2 farad caps. By adding a small yet efficient AGM battery you help stabilize voltage so the alternator can do its job if you add a large less efficient battery you will need a high output alternator.

i sent you a pm