Dual Batteries - shoot holes in my idea please!

[attack eagle]

Ferret: amps are designed to run on anywhere from 11v to 14.4.
Specs at 12v on a tightly regulated amp = same power at 14.4 (using more current)
but on a loosely regulated amp, as most are nowadays, the extra voltage translates into higher output, making that missing volt or brief voltage drop much more audible on a long bassnote or a fast bass line..
Someone used a water analogy earlier, it was lame but I like using something we should all be familiar with, shop air. Voltage doesn't exist in a vacuum.

Voltage is a difference in potential, current is flow.
Think of it as voltage being PSI in an air line, while current is the CFM.
your battery is a large airtank, and the compressor is your alternator...

One of those 12v portable inflators can supply 60 psi to fill a bike tire, but just try using it to fill a 44" tire or run an air tool!

Resistance across the battery is irrelevant to anything USING the alternator supplied current, only to charging it.
As long as there is enough PSI AND enough Current to do the work, things are fine, but if you use more air than the compressor can put out, the tank will drain, psi will fall and your spray pattern will change or your tool will work slower... If you use that tool or gun intermittently you will be able to use it longer and may not even see an issue as time when not at max draw will give the tank time to charge.

But no matter what you do, a second tank, larger airlines, nothing will make any difference under constant duty cycles unless you supply MORE air than your current Compressor can provide. Under light duty (infrequent use of air hogs like plasma torches, spray guns etc) adding a second tank might give you enough time to get finished with a moderate job before pressure falls off too much, and adding larger airlines to the outlet might bring the tool up to speed a few milliseconds faster like a cap does.


Like gammite, i'm not a believer in caps since the normal 1 farad caps don't have enough capacity to do much... They are simply intended to provide a few milliseconds of higher voltage until the charging system can catchup and up the output. Unless you have several farads of capacitance you are going to draw the caps down to system voltage where they become an additional current hog, and yet another device to charge up.
Worst of all (to me)they never fully charge, and are a constant drain on the battery even when not in use, and must be within 12" or better yet 6" of the sub amp to provide any appreciable benefit.
If you really beleive in caps, you should be using 5 farads + or a bat cap, particularly if you have a current hog amp.

I'm also not a big beleiver in adding a second battery in parallel, particuarly without an isolator as the two will constantly discharge each other seeking an unachievable balance (due to resistence in the wiring). A second battery is for key off playing time.

IF you really want two batteries, you can take a hint from older Brit cars and run two 6v batteries physically next to each other in series to make one larger 12v battery. With the 900+ CCA batteries often used in BMWS I see no reason to go thru the trouble.


Otherwise I suggest getting a tightly regulated amp since output will not drop off nearly as much assuming you supply ample current capacity in the form of short high current capable low resistance (large gauge and HIGH STRAND COUNT) wiring.

C'mon guys, we have 130-140 amp alternators, with rear mounted batteries (mostly) while most everyone else starts with a 75 amp or a 90 amp alternator and it works out fine for them.

the problem here isn';t with batteries, it's with voltage sag due to excessive consumption and inadequate output.

IF you are that hardcore you need ot invest in a second charnging system and battery dedicated to the stereo... much like how a paint and body shop (at least a good one) will have a high output compressor and large tank for both the paint booth and a second for the shop itself.

[Ferret]

Lol, voltage doesnt exist in a vacuum?

EDIT :- I might remind everyone here about 'valves' which worked on the principle of electron/voltage acceleration in a vaccuum

Since when? And I'd like to see all the satellites whose electrical systems are open to space suddenly drop out of orbit when that happens

The ability to arc doesnt exist in a vaccuum, as there's nothing to break down.
Anyways, I'm being needlessly pedantic.

You've forgotten something on your analogy as well : air tanks dont have an internal resistance. Unless you're going to point out that your air tank has a stupidly constricting valve on it, that just gets worse as your flow rises?

You're also assuming (at least your text is saying that, forgive me if I've misunderstood it) that everything is running from the alternator : this is a fallacy unless you like your devices running on bridge rectifed AC.
Your alternator cannot supply more than 130-140 amps peak - and bear in mind that it's a rectified AC output, so you've gotta convert this to RMS power. What happens when your 2kW amp suddenly demands a ~200Amp peak?

"I'm also not a big beleiver in adding a second battery in parallel, particuarly without an isolator as the two will constantly discharge each other seeking an unachievable balance (due to resistence in the wiring)."

Explain to me how trucks start up then after standing overnight, when they're rigged with anything upto 4 batteries in parallel? Sure if one is at a lower charge level, current will flow, but they'll balance and stabilise - admittedly at low efficiency.

Capacitors will only leak around 2-3mA when at 98% charge - that's practically nothing and shouldnt be considered a 'constant drain'

[attack eagle]

Ferret, you are being too literal.

Voltage doesn't exist in a vacuum, meaning you can't have voltage without some AMPERAGE, as you seemed to be discussing in your posts. Without discussing amperage as well, voltage is only 1/2 the equation when discussing work performed (W)as I was indicating with my comparison of a small 12v air pump compared to a large multi Hp shop compressor.
VxA=W You have to have BOTH to make any discussion of work relevant.

I actually like my analogy, PSI is pressure above a reference like voltage, and CFM is a rate of flow like amperes. Excessive consumption causes a drop in PSI and available CFM with a loss of performance, just like in an electrical system. Especially if (like me) you keep working away when the compressor kicks on.


A tank does have resistance if you want to be technical, the inlet and outlet orifice diameter is a form of resistance.


Everything runs off the alternator... the alternator's output (DC voltage) is what runs everything in the car. Don't believe me? Start the car and remove the positive cable. The car will keep running, and everything will work just fine. Ever jumpstart a car? You know why you stick it in first or second instead of 4th where it is easiest to turn the engine over?


Big trucks run batteries in series to get 24v, They don't run on 12v like cars. I have actually run parallel batteries, and I know they fail quickly and need constant topping off of the charge. I could measure a V loss after several days of the batteries being connected with the car's negative cable lead off. that means no drain because they were electrically isolated from everythign but each other.


When your 2kw amp demands 200 amps, the voltage sags to near battery voltage and the battery provides current until the transient is over. And it probably won't get the full 200 amps either and output levels will drop to what is available until the note ends. After which point the alternator begins to recharge the battery if there is sufficient voltage and current left over to do so and it recovers. Adding another battery gives you more current capacity after the sag to ~12v, but even with 900+ CCA, but does nothing to prevent the initial voltage drop required before the battery can supply voltage. Rocks don't fall into space, and electricity flows from higher potential to lower potential... (well actually the negative is the positive and the positive is the negative in terms of electron flow...) That is why a second battery is known to be for key off time. You have to be down to 12v (actually 12.4 ish is normal battery charge i think) before the battery will begin to discharge, at which point you have already lost output with an unregulated amp.
The apparent brightness of bulbs is very voltage dependent which is why you have people complaining of lights flickering to the bass. Also alternators generally do not have as much output at idle as at faster RPM, which is why you see complaints of flickering when stopped at a light.

In the above hypothetical 2kw case, you should be concerned with hearing protection, and should have the balls to go high output alternator (and replace them annually) and have separate audio system battery etc.
Otherwise all the capacitance in the world wont do you any good when you have 260 amps of current required and only 130 amps available...once you sag, you get the rest from the battery.
your average 1,2 or 5 farad Caps can help the initial attack of the note, but they do not have enough current to sustain it, and they will immediately fall to system voltage as they discharge. As I said they just give the alternator enough time to ramp up output, but then become a load themselves temporarily.

okeley dokely?

You wanna use a second battery, go for it. You'll get lots of key off time. It will do nothing for dimming etc.
You want a cap? Use a big one, It'll hide the worst of the problem, and that may be all you need to get by.
Want to have enough power for a mongo amp? get an alternator big enough to supply it... or get more efficient speakers so you can use a smaller amp to get the same output.

[ericbendler]

I have printed this out to read through your comments back and forth. I appreciate the back-and-forth.

I think the French Alternator can hold a PAC-200 and two good batteries.

Two good batteries? Could be the Odyssey or Optima. I am up for anything really. Reliability would be my number one with Current management number two.

[pingu]

As I've mentioned, I'm sceptical about caps (I suspect their main benefit comes from being mounted close to the sub amp, i.e. with short [i.e. low resistance] wires).

Anyway, I've just done back of envelope maths to work out what the effective capacitance of a car battery is (a battery is effectively an electro-chemical capacacitor). Bear in mind that a battery is non-linear so this is only to give an order-of-magnitude indication:

Assuming a 50 Amp hour battery, and assuming the voltage is down by 2V when the 50Ah has nearly all been used up, the capacitance of a battery is 90,000F. That's not a typo - 90 kilo farads!

Kinda blows a 5F cap out of the water!