cap w/digital voltage display

i got this scosche 500k cap with a sweet red led readout an blue led backlight, im sure most know what im talkng about im not really wantin to use the capacitor part of this thing just the volt display, what do u guys think if i were to use a relay off my remote turn-on an run a pair of wires from my amp + an - to my cap ? to use my display which id like to mount around my drivers seat. i kinda wish there was a way to seperate the 2 from each other. is there a way to disable the capacitance? would i be gettin a accurate voltage reading?1000 watts of lp, now need sum hp an mids

Just buy a 3-digit voltmeter from eBay, you can get blue LEDs or backlit blue LCDs for around $10.    Otherwise just hack the meter off the cap, it should only have 2 wires (ground & +12V). The cap is probably useless unless you know why you need it...

sell the cap and buy a digital volt meter. that's my 2 cents@0 Hertz Xpertz/Team SPL

well the top of the cap does unbolt an seperate, its really easy to do i just need sumthin to mount it to now, probly end up boltin rite too the dash of my car, a bolt on meter who woulda thought an as far why i need it old spark, it filters the voltage spikes but adds load to ur alternator so its not really needed cause alts have a built in regulator1000 watts of lp, now need sum hp an mids

sthrnbmpn wrote: ...but adds load to ur alternator so its not really needed cause alts have a built in regulator

Ah yes, the same old crap! Who told you that? A cap does not add a load to the alternator other than when charging (much the same as a battery). But it LESSENS the alleged "strain" on the alternator by providing a store of electricity (much like a battery, only a heck of a lot smaller). All alternators have voltage regulators. The cap is irrelevant. And batteries filter spikes - that is the main reason you shouldn't disconnect the battery with the engine (alternator) running. (It used to be because batteries provided a voltage reference for the regulator (alternator), but nowadays they provide their own voltage reference.)

well it doesnt add load unless its charging, an if ur amp discharges the cap alot while playin music thats a lot of discharge/recharge im not tryin to open an old can of worms but im just goin by what majority rules about caps, an it seemed to make sense when i think about it, a 500k farad is a smaller cap in the car audio realm, so maybe it would help clean up the the voltage goin to the amp1000 watts of lp, now need sum hp an mids

Your not opening a can of worms. But you may be stressing those that simply don't know and don't or can't think too hard. Think about what you said - the capacitor supplies energy when the alternator can't. Then the alternator replaces that energy. So, has that saved the alternator or stressed & damaged it? Without the cap, the alternator would have to supply that same energy/power. So the cap saves the alternator from having to supply the burst of power, and the alternator thanks the cap in return by recharging it. If that whole bullsh "extra load" argument were true, we'd be using the smallest battery we could get just to "save the load". Do those same people put bigger batteries on their system? I bet if it's an audio forum the answer is yes. And yet the battery is worse from that POV than a cap. In part because it is so much bigger (electrically) than a cap, but more so because of the charge & discharge inefficiency - you put about 30% more electricity in to the battery than you get out. (I'm not sure of the figure(s), but I think that is a conservative estimate.) Compare that to a cap which has essentially 100% efficiency - eg, over 99%. So why aren't those dorks using caps instead of batteries if "alternator load" is such an issue? Oh yeah, I forget, caps lower peak SPL - so they use low ESR AGM batteries instead. (Of course if they placed their AGMs in the engine bay - or better still - used flooded batteries, they'd get even higher peak SPLs if the low-ESR caps are the reason for the lower SPLs.) Don't worry, though there are many audio-forum contributor's that excel in stupidity, there are a few that understand the basics. LOL - if you don't know what I am talking about, see the poo chucking between floridaspl and caraudioclassifieds regarding "How many batteries do I need?" (as linked). Not that the OP is the brightest of kids, viz: "Why would it work harder to recharge 10 batts versus 1 batt? Its constantly charging at a certain rate based on RPM (this case being 160A max). I'm just confused by electrical systems I suppose. I'll ask my EE/Physics degree'd father what he thinks tomorrow" (see extra batteries like extra caps apparently does NOT load the alternator!), but he calls the knowledgeable people dorks, nurds & ridiculous despite his admission that he knows squat. But floridaSPL is another site that reckons to add more batteries rather than upgrade the alternator (first). I can only assume that despite their assertions that voltage is important for an amplifier that they don't use the amps they often refer to (which increase output with increasing voltage) and that they actually have high-quality amps whose output does NOT vary with input voltage - ie, whether you have 12V or 14.4V will not effect the max power output. Alas I ramble, but for people like that, ANYTHING is a can of worms. That's because they aspire to that level of intelligence. They may be smarter than a worm, but a can of them would indeed be a challenge. I guess my suggestion or warning is to be careful where you get your info from. And I'd suggest when it comes to "power", you should ignore the majority of the stuff from similar audioforums (as per floridaSPL - unless it is from one of their good people).

There is some truth in there. By your own arguments: 1 you put more energy into a battery (or capacitor) than you can get out because of efficiency loss 2 the capacitor supplies energy when the alternator can't 3 the alternator replaces that energy Therefore, adding a capacitor increases the energy required from the alternator because of some loss of efficiency when storing and discharging through the capacitor. For the absolute best explanation of how a capacitor functions in a car audio application, this thread is fantastic. Highly recommended, read the whole thing! http://web.archive.org/web/20080117064421/http://forum.realmofexcursion.com/showthread.php?t=17970

oldspark wrote: Compare that to a cap which has essentially 100% efficiency - eg, over 99%. So why aren't those dorks using caps instead of batteries if "alternator load" is such an issue?

Caps suffer a voltage drop proportional to the current it supplies by Ohm's law V=IR. With currents (I) running upwards of 50 or 100 amps or more, a tiny internal resistance (R) of only 0.02 ohms would lower the voltage (V) at the capacitor's terminals by 2 volts or more. The alternator would have to drop it's voltage due to an extreme load for the cap to begin discharging, and if the cap is helping supply a significant current its voltage will also drop as shown above, which leaves the humble battery to help supply current starting at about 12 volts. The amount of energy that can be supplied by a capacitor at high currents, is a tiny fraction of the amount of energy that can stored in a capacitor, which is in turn a tiny fraction of the energy that can be stored in a battery. Read that link to see all the details. -JMalcom: "This is the captain. We have a...little problem with our engine sequence, so we may experience some slight turbulence and then...explode."

Jayne: "We're gonna explode? I don't wanna explode.

sthrnbmpn wrote: i got this scosche 500k cap with a sweet red led readout an blue led backlight, im sure most know what im talkng about im not really wantin to use the capacitor part of this thing just the volt display, what do u guys think if i were to use a relay off my remote turn-on an run a pair of wires from my amp + an - to my cap ? to use my display which id like to mount around my drivers seat. i kinda wish there was a way to seperate the 2 from each other. is there a way to disable the capacitance? would i be gettin a accurate voltage reading?

Lots of good suggestions above. The voltage reading would be reasonably accurate, should be just fine for your needs. -JMalcom: "This is the captain. We have a...little problem with our engine sequence, so we may experience some slight turbulence and then...explode."

Jayne: "We're gonna explode? I don't wanna explode.

teenkertoy wrote: Therefore, adding a capacitor increases the energy required from the alternator because of some loss of efficiency when storing and discharging through the capacitor.

What inefficiency? Are you saying that a cap resistance is similar to vehicle wiring? In comparison, a cap is close enough to 100% efficiency. And if a cap has a resistance of 0.02 ohms, then I would certainly ditch it and use an AGM battery (as far as ESR & resistance is concerned). You could draw a circuit or schematic for a vehicle's charging system. Show the stator winding resistance as well as the distribution resistance, the battery and its resistance (assume it is to spec & fully charged), and that of the load. If you come up with a something profound, I'll change my stance. Remember that the cap is a parallel device - it is not a (say, 0.02 ohm) series resistance. (Its terminal voltage will always equal its circuit voltage - there is no voltage drop, but feel free to model and ideal capacitor etc.) Or do some rough power dissipation calcs for the cap, the protection (fuses) and wiring.    However, you should realise the above "efficiency" is irrelevant. A battery "saves" the alternator and it has way more overhead than a cap. (You might find around ~100-fold more "inefficiency" with a battery.) And, is it more efficient to not have a cap (or battery) and let the alternator provide the load(s)? (And do you like dipping headlights and stalling engines?)    I put it to you - do you recommend a bigger battery for its increased capacity (as well as lower resistance) to offload the alternator, or a small battery (to offload the alternator)? I eagerly await your response.