Dual Batteries - shoot holes in my idea please! [Archive]

ericbendler

02-06-2007, 12:30 PM

As someone who definitly sees his headlights dim when the bass hits, I was wanting to get some opinions on a dual battery setup in the 525i.

We are all lucky enough to have two mounting points. Either under the hood or under the back seat right? Whether you are using one or the other.

My factory spot is under the hood, but my rear seat mount is not being used.

Instead of the 3 farad cap that I have under there currently, I was thinking about using the lucky setup that I have and installing a second battery under the rear seat as well.

I have read (exhaustively) other articles on dual battery setups, and wanted to get the final word from this board.

I have two power leads (one black and one red) that I will attach to the new secondary battery at the rear seat, keeping their 12volt power supply in tact.

I will vent it as it should be with the factory tubing and the tubing supplied by the battery maker.

I have identical (age and type) batteries as I am not going to be isolating them. I am aware of the possibility of having two batteries and stil needing a jump without the isolator.

My question is this........

Do you forsee any problem specific to BMWs? I have had the two rear power wires connected to my cap for years, so why would connecting them to my rear battery be any different.

I searched the board for "dual battery" and so on with little success. If anyone knows of a good thread, I would appreciate it.

Other than that, just wanting to hear people shoot holes in my idea to increase my amps while keeping it at 12 volts.

(I run two soundstream amps that really make my lights dim).

Hopefully there are no threads about dual batteries in the e34, so this will be the first.

RallyD

02-06-2007, 01:26 PM

before you go into dual batteries:
Being that you have soundstream amps and a capacitor, it appears you may have a clue about quality and such. I always run 2ga welding cable off the alternator and make sure my earth ground system can handle the power. 4ga wires to the amp and 4ga grounds to chassis from the amps.
If you have an upgraded alternator cable already you might want to go with 2 batteries after all.
http://www.pirate4x4.com/tech/billavista/dual_battery/schematic.jpg
http://img.shopping.com/cctool/PrdImg/images/pr/177X150/00/01/b9/c6/64/28952164.JPG

i used 2 batteries in one car that I had. I ran the Odyssey PC 680. It is compact and quite powerful.
http://www.odysseybatteries.com/applications/auto.htm

Tiger

02-06-2007, 02:09 PM

pingu

02-06-2007, 02:47 PM

What kind of cap do you have? The cap is supposed to prevent this from happening. How did you hook up the cap?

Battery will never give you fast enough of power to handle the bass. What amperage is your alternator? What is alternator power output? Have it been tested?
I'd be very surprised if a battery couldn't give enough current - we're only talking a few tens of Hertz for bass, not tens of MHz. The internal resistance of a car battery is of the order of a few milliOhms and while it will rise with frequency (due to inductance, amongst other effects), I think this will be trivial.

On the other hand, I'd be wary of connecting two batteries in parallel. I wonder if the one with the lowest open-load voltage will hogg all the charging current - in other words one of the batteries might end up fully (over?) charged while the other is almost devoid of charge.

ericbendler

02-06-2007, 03:02 PM

What kind of cap do you have? The cap is supposed to prevent this from happening. How did you hook up the cap?

Battery will never give you fast enough of power to handle the bass. What amperage is your alternator? What is alternator power output? Have it been tested?

I don't know about the internals, but the cap is 3.0 Farads. I have what seems to be an OEM 8 guage wire running from under the hood to the rear seat. That is what I was going to use for the positive supply for the rear seat battery.

Then I could just run the power leads to the amps from there (with the correct fusing).

My alternator is a french 130 amp.

I had the cap hooked up under the seat as though it were a battery.

ericbendler

02-06-2007, 03:06 PM

Don't diode based isolators drop the voltage on your lines? I don't want that to happen. If I can get past the charging issues - (one battery getting all the charge, while the other one discharges) without putting my impedance on my 12v lines, I can move forward.

Phatty5BMW

02-06-2007, 03:08 PM

8ga from under the hood is not nearly big enough. Run atleast 4 guage to your back seat and they sell adapters to hook that to 6-8 ga. which is fine to hook up to your amp. You loose power the farther you get from your supply. I would run the 2/4 guage to your capacitor then run the other guages to your amp.

Pat

Tiger

02-06-2007, 04:19 PM

8 ga is too small for big amp... I don't know what your total wattage is but I would like to know. 8ga is good for up to like 300 true watt I think... I am not sure. If you have more draw than this, then you are starving the amp.

As for the cap, the only way I hook them up is positive cable only. From battery positive to negative on capacitor and from positive capacitor to amp. With this way, capacitor is always in work.

Tiger

02-06-2007, 04:24 PM

AngryPopTart

02-06-2007, 04:33 PM

On the other hand, I'd be wary of connecting two batteries in parallel. I wonder if the one with the lowest open-load voltage will hogg all the charging current - in other words one of the batteries might end up fully (over?) charged while the other is almost devoid of charge.

I don't think that happens. The batteries may very well output different voltages, but I don't think overcharging one is a concern here.

If you're going to run two batteries, you may as well take this opportunity to just throw in a deep cycle battery with a cut-off switch (to disconnect it from the charging system) and use this to power the stereo as there will be a significant difference in quality when it is seperated from the factory system. Batteries are a good thing to run audio equipment off of, but the charging system as well as the ignition system introduce alot of garbage into your audio. You may want to play with different routes, like running only the sub amps off the second battery.

In the case that you decide to run any of the audio equipment off of the secondary battery alone, make sure you completely isolate the grounds on those pieces of equipment. Don't ground them to the body, and don't use screws which will effectively ground the chassis of said equipment to the body, use nylon washers or whatever you have to do to keep them away from the ground of the main battery and run seperate wires to the negative post of the secondary battery. Otherwise you will have ground loop problems, oscillations, etc.. Don't forget about the audio cables that run from the main deck to your amps either. The ground will find it's way in this way as well so you'll have to alter your RCA cables and lift (cut or otherwise disconnect) ground from both channels.

pingu

02-06-2007, 06:11 PM

Bass draw massive amount of power in ms timeframe. Battery just cannot give that out that massive amount in that timeframe... that is the job of capacitor.

Second battery is used mainly for storage in the engine off condition. Majority of people with serious system usually upgrade the alternator for higher output capacity.

130amp or 145amp as in US is plenty of power.
Eh? a ms timeframe is about 1kHz - that's not bass, that's higher than middle C (440Hz). To take an example, 400W RMS would need about 40 amps of current from the car battery: that's quite a lot but a car battery should be quite happy pumping out hundreds of amps. Basically I'm sceptical about the need for capacitors as opposed to a battery (and a battery is simply an electrochemical capacitor!) but each to their own!

BillionPa

02-06-2007, 10:32 PM

this is what you need to do.

remove battery from engine compartment

install D31M Optima marine battery in trunk using thickest cabling you can get

if your alternator is not 140 amps, upgrade.

double check all engine grounds and the front power distribution junction for nut torque and corrosion.

.. just read about your french alternator, should be ok at 130, but if its old replace the voltage regulator (not just the brushes) and the diodes.

attack eagle

02-06-2007, 10:58 PM

I'd be very surprised if a battery couldn't give enough current - we're only talking a few tens of Hertz for bass, not tens of MHz. The internal resistance of a car battery is of the order of a few milliOhms and while it will rise with frequency (due to inductance, amongst other effects), I think this will be trivial.

On the other hand, I'd be wary of connecting two batteries in parallel. I wonder if the one with the lowest open-load voltage will hogg all the charging current - in other words one of the batteries might end up fully (over?) charged while the other is almost devoid of charge.

OK I have actually run dual identical batteries in parallel without an isolator (1974 MGB, dual 650 cca Interstate megatron 26) And yes they only lasted about a year before needing replacement... and they were only 1 foot apart, with ground on one side and positive lead off the other, not multiple feet apart with losses between them from resistance.
The second set I installed a battery charger in the car itself and plugged the car in nightly, which extended the life to about 18 months...

Don't do it.

IF you are really bothered by the bass dimming the lights (at idle only? off idle???) you have a couple of choices...

1) relocate your cap to within 12 wiring inches of the sub amp. Anything longer is a waste of time.

2) bump your idle speed when the stereo is on. Alternators don't put out rated power at lower RPMs.

3) install a second alternator along with a second battery, and use this only to run the amps and head unit

Blitzkrieg Bob

02-06-2007, 11:30 PM

John tesh

Enya

Yani

Slim Whitman

Vaneglis

And that douche bag master of the pan flute

AngryPopTart

02-07-2007, 02:00 AM

Basically I'm sceptical about the need for capacitors as opposed to a battery (and a battery is simply an electrochemical capacitor!) but each to their own!

Batteries cannot charge/discharge at the rate a capacitor can. There is no comparison in slew rates. Trust me, every amplifier, whether it's running on 220, 110 or batteries, has to have caps to compensate for big bass notes... of course they also act as filters.

In OpAmp applications, the datasheets often state that the power caps should be as close as is possible to the power pin of the IC or OpAmp. Similarly, your stiffening cap should be as close as possible to the power lug of your amp since their whole purpose is to compensate for line-loss as well. Another thing in OpAmps is that, when possible, using several smaller, but adequate sized caps can be better than using a single large cap because everytime you add a cap, it cuts the output impedance of the caps. This increases the effectiveness of the charge/discharge rate. All that's left then is the physical charge/discharge speed of the caps, but even with normal capacitors it's hard to find manufacturers who will list this sort of spec so just forget about it.

You may want to try several .5F caps instead of the single large 3F cap.

Paul in NZ

02-07-2007, 03:15 AM

its not rocket science,after all some cars(mine did) came with a extra battery in the boot,it is a little 25 amp hour,its conected via a cable od about 4mm incl insulation to the main battery via a relay looking device....i was surfing channels the other day and came across a 4wd program.This truck(land cruiser) had at east three batteries,each charged sequentially only after the previous battery had reached a certain voltage.They had just installed a fridge in this thing and it obviously had a pile of other electrical equipment,incl a electric winch.my 2c

BigKriss

02-07-2007, 07:12 AM

Why do you recommend the D31M Optima marine battery?

this is what you need to do.

remove battery from engine compartment

install D31M Optima marine battery in trunk using thickest cabling you can get

if your alternator is not 140 amps, upgrade.

double check all engine grounds and the front power distribution junction for nut torque and corrosion.

.. just read about your french alternator, should be ok at 130, but if its old replace the voltage regulator (not just the brushes) and the diodes.

pundit

02-07-2007, 07:21 AM

John tesh
Change your taste in music

Enya

Yani

Slim Whitman

Vaneglis

And that douche bag master of the pan flute
You forgot to mention Latvian Nose Harp! :p

Tiger

02-07-2007, 09:12 AM

Agreed that the cap must be nearby the amp... like 1 feet... not 5 feet. I just don't think he hooked up the cap right either.

Ferret

02-07-2007, 09:33 AM

*Sigh* Forum borked my first attempt at this reply, so I cannae be f*cked going back into so much detail again, so in summary:

1) Batteries can supply 400-450 amps yes, but only at 1-2Volts stable
2) If you direct link two batteries, you may well end up blowing your starter motor as it'll be supplied with 7 volts stable at crank stall rather than it's usual 2.
3) If you connect 2 batts together one at 10% charge and another at 90%, you'll have several hundred amps flow with a bang.
4) If you flatten your secondary battery and shut down the engine with the charge regulator still connected between the two, by the time the two settle out they'll be at about 15-20% charge - this is because charging wet cells is only about 40% efficient. You might get lucky and start your car off that...
5) A 1kW rig around 100 amps draw only needs a resistance in the circuit of just 0.12ohms before the amp will cause the cars main voltage bus to completely brown out at amp peak power.

I'd stress that cable isnt particularly a huge problem but your cable termination is the killer - check all your connections are spotless and your battery clamps etc are shiney bright metal.

BillionPa

02-07-2007, 01:10 PM

why the D31M? because its BIG and works well in deep cycle, like if he turns his engine off but listens to the stereo for another minute... or 5, and it also has accessory leads right on top without the attatchment of additional harware. and it has enough CCA to start our cars in the dead of winter, doesnt need a vent tube, and is the toughest battery alive.

but mainly because it has very low internal resistance compared to anything else (even some capacitors!) which allows higher levels of current.

AngryPopTart

02-07-2007, 03:36 PM

I'd stress that cable isnt particularly a huge problem but your cable termination is the killer - check all your connections are spotless and your battery clamps etc are shiney bright metal.

Our body shop is on a trucking yard, and one of the guys came in complaining one day that he needed to replace the batteries in his rig cause the thing was having all kinds of problems starting, etc. We aren't the mechanics, but we went out and looked at his batteries out of curiousity and one of the mechanics had simply tried replacing a single cable that connected two of the batteries together... there were 4-6 batteries, and aside from that new piece of cable, all the posts and connectors were totally corroded. We took them all off, cleaned every post and connector and it worked flawlessly. :p

As far as cable size is concerned, there was an article up a while back on a 3rd gen camaro site where the owner, after making the connections as good as possible, still wasn't happy with the brightness of his headlights so he rewired that portion of the system with heavier gauge wiring and was able to achieve around 15% more brightness. That's not huge, but it shows there is room for improvement.

BigKriss

02-07-2007, 06:58 PM

thanks

why the D31M? because its BIG and works well in deep cycle, like if he turns his engine off but listens to the stereo for another minute... or 5, and it also has accessory leads right on top without the attatchment of additional harware. and it has enough CCA to start our cars in the dead of winter, doesnt need a vent tube, and is the toughest battery alive.

but mainly because it has very low internal resistance compared to anything else (even some capacitors!) which allows higher levels of current.

Ferret

02-08-2007, 05:44 AM

but mainly because it has very low internal resistance compared to anything else (even some capacitors!) which allows higher levels of current.

Again I'll just reiterate, watch out for your motor stall voltage - it wont be a huge problem if the motors relatively new, but it might finish off an older starter.

Claude

02-08-2007, 09:21 AM

...
As for the cap, the only way I hook them up is positive cable only. From battery positive to negative on capacitor and from positive capacitor to amp. With this way, capacitor is always in work.

Are you sure of that ? Capacitor should'nt it be install in parallel with the battery instead of in serie ?
http://www.caraudiohelp.com/car_audio_capacitor_installation/car_audio_capacitor_installation.htm

Some other ref. on the subject:
http://electronixwarehouse.com/car/accessories/capacitors.htm
http://www.zena.net/htdocs/alternators/AutoSound.shtml

Ferret

02-08-2007, 09:42 AM

Are you sure of that ? Capacitor should'nt it be install in parallel with the battery instead of in serie ?
http://www.caraudiohelp.com/car_audio_capacitor_installation/car_audio_capacitor_installation.htm

Some other ref. on the subject:
http://electronixwarehouse.com/car/accessories/capacitors.htm
http://www.zena.net/htdocs/alternators/AutoSound.shtml

Er capacitors should definitely be hooked up in parallel, without you're just creating a whopper of a high pass filter. In series basically it stops any DC current.

ericbendler

02-08-2007, 03:39 PM

Well I have a new starter that's for sure.

I just want to do this dual battery experiment because if it is stable it will be the finishing touches of a great setup.

I just finished uploading some photos to webshots.

http://community.webshots.com/album/556700959Ynxlvn

if anyone is interested.

AngryPopTart

02-08-2007, 04:11 PM

In series basically it stops any AC current.

Fixed.

DaveVoorhis

02-08-2007, 07:18 PM

Fixed.
Eh? In series with a load, a capacitor passes AC current and blocks DC. In parallel with a load, it filters out AC.

Tiger

02-08-2007, 07:51 PM

Mmm... I guess it is the new way to install caps now... I have been running this way for years... like 15 years... from positive battery to negative on capacitor... and then from positive capacitor to amp positive.... amp negative is direct to bare ground.

It is not about AC/DC nor high filter... etc... it is just the old way of hooking it up. In a sense... all power must go through the capacitor... not only draw power from capacitor when needed...

Claude

02-08-2007, 10:47 PM

... It is not about AC/DC nor high filter... etc... it is just the old way of hooking it up. In a sense... all power must go through the capacitor... not only draw power from capacitor when needed...

To help figuring out how this works just make an analogy between the electrical power system of the car and an hydraulic system:

- the battery is like the main water reservoir,
- the alternator the main source of water,
- the cable between the alternator and the battery the water pipe;
- the capacitor installed close to the amplifier is a secondary water reservoir;
- the amplifier is the load that drains current like a tap drains water;

With that model in mind the electrical power in watts that may be delivered to the amplifier (or the amount of water that could be push through the water tap by unit of time) is restricted among other things by the cabling (or the water pipe) because it introduces some electrical resistance (water flow constrain) in the system.

When the amplifier requests suddenly a great amount of power (or water) the fully charge capacitor (secondary water reservoir) installed near it will discharge in or feed the load so you avoid the effect of energy drop in the cabling (water pipe) between the battery (main water reservoir) and the load.

Because power peaks (bass punchs) are needed only part of the time, not all the time, if the system is well design the capacitor will have time to be fully refilled between those peak power requests.

Also in a well design system the battery (main water reservoir) is big enough to refill the capacitor (secondary water reservoir) between 2 requested power peaks and the generator (the water source) is itself big enough to completely refill the battery (main water reservoir) in a resonnable amount of time.

For sure it help to have the bigger water pipe (bigger cable) possible to link the different components of the system and that the secondary water reservoir (capacitor) be real close to the load (power amplifier).

That's it for my analogy.

Tiger

02-08-2007, 11:34 PM

That's an outstanding anology. I totally agree with you on this.

Ferret

02-09-2007, 06:25 AM

Fixed.

I think not! Lol get off my internets!

A capacitor in series is a high pass filter - IE it will let AC through and drop DC.
A capacitor in parallel is a low pass filter - it'll stop AC and smooth it out - so long as some sort of rectification is present.

At least that's what my electrical engineering degree lecturers were telling me...

Hence where Tiger's capacitor's in series I'd love to hear the results, it must screw around with the sound something awful. The amp can only charge internal caps... AFTER a massive bass beat... I'm sure that's gonna be screwed up.

EDIT : and I know the physics behind capacitor action hasnt changed in the last 15 years. Putting a cap in series is only going to cause limitation in available current. Yes your lights etc wont dim any more, but that's because you've caused a physical disconnect between battery and amp - there's actually no phsical connection between the positive and negative cap terminals - just air or electrolyte usually.

ericbendler

02-09-2007, 09:39 AM

Fixed.
I see the qoute of "fixed", and wonder what is fixed? There is a lot of disagreement on the outcomes of two batteries in parallel with (or without) an isolator.

WTH - I my as well just buy two identical optima yellow tops with an isolator and do it right.

Isolator drops my line voltage .2 - .4 down, but allows me to raise the amperage of the whole system for things like (stereos, tv visors, 12v adaptors).

hmmm- $ (red tops x 2 = $150) + (PAC-200 - $60) = $210

That puts it 2nd on my list behind a new compressor. NICE!!!

AngryPopTart

02-09-2007, 10:22 AM

Eh? In series with a load, a capacitor passes AC current and blocks DC. In parallel with a load, it filters out AC.

Aren't these polarized caps?

DaveVoorhis

02-09-2007, 12:26 PM

Aren't these polarized caps?
With polarized caps, subjecting them to AC will damage them. In that case, replace "AC" with varying DC. The cap will filter out DC variations.

DaveVoorhis

02-09-2007, 12:32 PM

Mmm... I guess it is the new way to install caps now... I have been running this way for years... like 15 years... from positive battery to negative on capacitor... and then from positive capacitor to amp positive.... amp negative is direct to bare ground.
Does the capacitor have a ground strap? Are you sure about how it's wired? If so, then it's wired in parallel, but with two positive side connectors. If it's genuinely wired in series, then it's either not a capacitor or it's shorted out. See http://www.caraudiohelp.com/car_audio_capacitor_installation/car_audio_capacitor_installation.htm

pingu

02-09-2007, 02:13 PM

With polarized caps, subjecting them to AC will damage them. In that case, replace "AC" with varying DC. The cap will filter out DC variations.
Almost right: subjecting polarised caps to reverse polarity will damage them.

For example, suppose you have a polarised cap connected to a voltage that varies from 12V to 14V. The cap will see an average of +13V (which should be fine, as long as the cap is connected with the correct polarity) with 1V (i.e. 2 volts from trough to peak) of AC superimposed on the 13V of DC.

If the AC were to increase to 14V superimposed on the same 13V DC, then on the troughs the cap would see -1V (i.e. 13V DC minus 14V), which it wouldn't like.

DaveVoorhis

02-09-2007, 02:52 PM

Your point about reverse polarity damaging a polarized cap is true, but AC is, by definition a current varying in magnitude and direction, i.e., polarity, at regular intervals. A 2 volt P2P AC current superimposed on 13 volts DC is, strictly speaking, varying DC. 14 volts P2P AC superimposed on 13 volts is AC. See http://en.wikipedia.org/wiki/Direct_current and http://en.wikipedia.org/wiki/Alternating_current

Felixdacat

02-09-2007, 03:21 PM

How many watts is he running? I would upgrade the capp before I ran two batteries. I'm running about 2500watts for my subs with one amp, and have a 5mf capp. I do however see my lights dim slightly when I have the fogs, and heat on. More amp draw from the alternator.

pingu

02-09-2007, 07:11 PM

I forgot to add in my post that I knew what you meant. It's just a matter of how strict a definition one uses for DC. I was in a strict mood!

AngryPopTart

02-09-2007, 07:48 PM

Are you a goon? :) The posts are getting too cluttered for me to follow, but I thought polarized caps helped eliminate any AC characteristics. IIRC this is why using a bipolar cap in series on an audio channel is ideal, though most of the time you find polarized caps cause they're cheaper. When rectification is present, isn't the rectifier what eliminates the AC? The cap just smooths out the waveform, right?

A capacitor in series is a high pass filter - IE it will let AC through and drop DC.
A capacitor in parallel is a low pass filter - it'll stop AC and smooth it out - so long as some sort of rectification is present.

DaveVoorhis

02-09-2007, 09:18 PM

Whether a capacitor is polarized or not makes no difference on its behaviour as a capacitor. It's simply cheaper to make a polarized large-capacity cap than a non-polarized cap of equivalent capacity.

Ferret is essentially correct. A capacitor in a series circuit passes the AC component, i.e., the voltage-varying-over-time aspect of a current source, and blocks steady DC. A capacitor in parallel filters out the AC component, i.e., the voltage-varying-over-time aspect of a current source, and leaves the (relatively) steady DC component.

In passive crossover circuit design, which you alude to via "audio channel", a capacitor in series with a tweeter will filter out the lower frequencies (more DC-like, er, sort of) and pass the higher (more, uh, AC-like) frequencies. A capacitor in parallel with a woofer will filter out the higher frequencies (er, more AC-like, if the analogy helps) and allow the lower (more DC-like) frequencies to pass to the speaker. See http://www.bcae1.com/xoorder.htm

A rectifier converts AC to varying DC, or what is sometimes known as DC with an AC ripple, i.e., steady DC plus an AC compone- ...

Aw, never mind. Nobody cares, right?

pingu

02-10-2007, 05:16 AM

What colour is an electron? ;)

DaveVoorhis

02-10-2007, 05:44 AM

What colour is an electron? ;)
An electron has 2.4x10-12 metre wavelength at rest. :D

AngryPopTart

02-10-2007, 10:14 AM

In passive crossover circuit design, which you alude to via "audio channel", a capacitor in series with a tweeter will filter out the lower frequencies (more DC-like, er, sort of) and pass the higher (more, uh, AC-like) frequencies. A capacitor in parallel with a woofer will filter out the higher frequencies (er, more AC-like, if the analogy helps) and allow the lower (more DC-like) frequencies to pass to the speaker. See http://www.bcae1.com/xoorder.htm

A rectifier converts AC to varying DC, or what is sometimes known as DC with an AC ripple, i.e., steady DC plus an AC compone- ...

Aw, never mind. Nobody cares, right?

I care! I'm into DIY audio so I'd like to know, hehe!

Actually, I was talking about audio channels... err, inputs. Most amplifiers have capacitors in series on the inputs to help bump up the low-frequency cut-off and or to remove the DC offset coming from the source. This is important, especially on headphone amplifiers since you can mess up your cans something awful if the DC offset is big enough.

gmannino

02-10-2007, 10:50 PM

Hey guys, let me drop some advice. I've been a car audio installer and electrical tech for over 7 years. I installed thousands of high output subwoofer systems, and hundreds of capacitors. Throughout my experience, I've come to the conclusion to never use a capacitor in my car. Caps have a 1 volt drop across the circuit, lowering your total "at amp" voltage.

Also, they are no where near the benefit of having an extra battery. Batteries provide much more current capacity over caps. caps, when drained, will become a heavy burdon on the alternator.

Basically if you are looking to improve your electrical output to aftermarket stereo equipment, this is what I've found to be true from experience.

first step: upgrade your alternator to yield the best results.
second step: add another battery, include heavy gauge wire.
third step: caps? I DONT suggest them at all. They do not provide benefit to your stereo system, except for cleaning up audio interference through your power wires, which should be avoided by correctly running your wires.

gmannino

02-10-2007, 10:50 PM

Ferret

02-12-2007, 05:08 AM

Lol, my comment about a cap stopping AC was technically correct!

You plug AC straight into a cap and you're going to get either zero volts output at high frequency (and possibly blow up your source)... or if it's mains frequency a phase shift.

I dont mean to question gmannimo's experience...

BUT

when the engine's running, you've got a voltage of 13.5 to 14.5v present anyways, for an amp that's designed around 12v?

Besides, because these batts have a far higher resistance internally than a cap, they surely should voltage drop a lot more on a huge ass bass beat?

I totally agree on the beef the alternator bit up though - a 135amp alternator is only about 1300 watts RMS. Take off half of that for vehicle systems in the dark/rain and you're gonna be buggered with an amp of 1kW or more... you'll be running the battery down despite the engine running.

I is gonna go away and look up the facts on this, then post a statistical analysis.

Ferret

02-12-2007, 05:49 AM

Okay part 2:

Wet cell Lead Acid batteries have non-fixed internal resistances. (Standard Car battery) With no load on them, they're in the 20's milliohm ratings, by the time you're pulling crank amperage out of them, they've dropped to around 10milliOhms. This can be dropped significantly by using much much larger batteries - but you'd need a much better alternator to cope with the lowered internal resistance. Your alternator would be running at 100% load for a much higher proportion of the time, risking regulator burn out.

However, large capacitors of the 3-4F range have an internal resistance of sub 2 milliohms.

So in the case of standard car batteries, you get approx 10volts drop per 1000w consumed.

This means that for the same drop of 10V, a capacitor can supply over 5kW of power....

However as I pointed out above, battery choice appears to be critical in these systems, I've seen a couple of lead acids on the RS website that have 0.5milliOhm internal resistances... but they're not car application ones. If you can retrofit they'd be brilliant - again match your alternator.

attack eagle

02-12-2007, 05:52 AM

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

02-12-2007, 08:21 AM

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

02-12-2007, 09:33 AM

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.