Car Battery Current

[willtucker]

hey

ive just started looking into building a new amplifier for my new car. My main concern is the output of the battery. Im not sure what the current is that im dealing with. Also if the current is high (which i assume it is) can anyone suggest a current limiting circuit that may work in this situation.

Thanks

 

[stevez]

A car battery can deliver hundreds of amperes if the wiring and load present a sufficiently low impedance/resistance. What it will deliver in your situation is somewhat dependant on the load and the wiring - at some point the battery characteristics play a part but probably not in your case.

In order to determine the current requirements you might work backward from the power you expect to deliver - and then add something for the inefficiency of the system. If we start with 100 watts per channel, 2 channels thats a total of 200 watts - with 12 volts we'd have to supply 16 amps - all of this presumes DC - it won't be but this helps illustrate orders of magnitude. You might double this to account for a 50% efficient system.

If you plan on significantly more power then your amplifier will have to run on something much higher than 12 volts unless you have uncommonly low speaker impedances available to you.

Fuses, fusible links and circuit breakers are used to limit current. The fuse has to be sized to protect the equipment AND the wiring. Example - if the equipment could handle 50 amps but your wire can only safely handle 30 amps then you have to limit the current to 30 amps.

 

[audioguru]

If you use a current limiting circuit so that an amplifier doesn't draw too much power from the battery, then the amplifier won't supply its full power. You might as well use a lower-output-power amplifier without a current limiting circuit.

The "boom-boom" kids use car amplifiers that deliver hundreds of watts per channel. They use a voltage step-up power supply and draw peak momentary currents of around 80A from the 12V battery. To avoid voltage drop in the cable between the battery and the amplifier, they use a huge capacitor at the amp to supply the peak momentary current. :lol:

 

[Nigel Goodwin]

As already suggested, big amplifiers take BIG currents from the battery, and you need extremely thick wiring to reduce the losses as much as possible.

However, you should be aware that specs on in-car amplifiers (and speakers) are wildly exaggerated - a 1000W amplifier is usually only 200W per channel (for two channels), or 100W per channel for four channels (and may be less than that?).

Building your own in-car amplifier isn't a viable proposition, you're not going to build one anywhere near as small and neat, and it's going to cost you MANY TIMES the price of a commercial one.

As for your suggestion of current limiting - FORGET IT! - you can't afford any voltage drop from a 12V battery. The only thing you need is a suitable fuse, designed for this purpose, and even that loses you power you can ill afford.

 

[audioguru]

Hi Nigel,
People can go to any car radio shop that has amps on display. With see-through covers you can see the hundreds of expensive parts inside.

My son bought a scratched store-demo car amp on E-bay for a very low price. It is name-brand with very good continuous RMS-rated low-distortion power spec's. He wired it with very thick cables, four paralleled 25A fuses and it drives his subwoofer with a tremendous amount of power.

I had to go and boost his 120A/hr battery after he ran the amp while playing basketball for only 1 hour. Until the alternator's belt was tightened, it squealed with every beat. The alternator gets very hot on the road with the subwoofer playing loudly but still works. :lol:

 

[Oznog]

The "boom-boom" kids use car amplifiers that deliver hundreds of watts per channel. They use a voltage step-up power supply and draw peak momentary currents of around 80A from the 12V battery. To avoid voltage drop in the cable between the battery and the amplifier, they use a huge capacitor at the amp to supply the peak momentary current. :lol:

Yeah you need to size a fuse/breaker for high surge currents. Turn-on's pretty high too. I've always thought circuit breakers were relative high for resistance compared to fuses (this is correct, right?) so you might want to stick with a fuse. You shouldn't blow one in practice, but just keep a spare around anyways.

Do those caps serve any real purpose? I mean, a 1 farad cap means a 1 amp load makes it drop at 1 volt per sec. Say you want to avoid creating a 1v drop that you might see under a 100 amp surge due to a heavy bass tone. It's kinda hard to say how much current the cap supplies versus the battery, but the cap can only supply 100 amps for 10mS before the max voltage difference is exceeded.

And that's assuming the ESR of the cap is incredibly low. 10mOhms would be the max to avoid a 1v drop under 100 amps inside the cap itself. ESR specs are hard to come by but I did find one for a 20F cap at 1.6mOhm- I guess that would do it IF you believed the mfg number (generally these are simply made up by marketing folks).

Dunno how applicable these figures are. A 1v drop in the source voltage is a pretty generous allowance, but 100 amps may be an unreasonably high surge load estimate.

It can smooth abrupt changes in the voltage, but can't *stop* it. This doesn't really look like enough energy storage to carry it through sustained tones. Now unless I'm mistaken, the DC/DC converter inside a well designed amp itself should be designed to accomodate rapid voltage changes without measurable effects on the output so screwing around with small voltage changes with huge caps might be pointless.

 

[Nigel Goodwin]

Hi Nigel,
People can go to any car radio shop that has amps on display. With see-through covers you can see the hundreds of expensive parts inside.

That was my point, it's not cost effective to build your own.

My son bought a scratched store-demo car amp on E-bay for a very low price. It is name-brand with very good continuous RMS-rated low-distortion power spec's.

It's VERY, VERY unusual to see RMS specs on in-car amps, and if given it's usually in small writing below the grossly inflated figures in the main spec. What make and model is it?, and what do they claim?.

 

[audioguru]

Hi Nigel,
My son bought the smallest amp:

 

[Nigel Goodwin]

Hi Nigel,
My son bought the smallest amp:

Interesting! - one of the very few to advertise RMS?.

I hope he has suitably rated speakers?, they are usually massively over rated as well!.

 

[audioguru]

Hi Nigel,
There are a few high quality car woofers that can handle 500W or more in short bursts. In my son's car I hear something overloading, I think it is my hearing because it sounds fine about 1km away! :lol:

 

[Pyroandrew]

Car audio systems have always been a joke, they claim 1000W of output power, IF it was driving this much power your average car battery would only last 20 minutes when the engine is off. You would just about be able to listen to a single track if you had the 3kW version.

Nearly very manufacturer I have ever seen quotes "music power", which is some weird mess up way of calculation power that is not even standardised, so every manufacturer makes up their own way. The system might be rated to 1000W, but it will certianly not deliver.

One of the spec I've seen before was the rated power being the power disipation that would not heat up the componants to failure. "Marketing folk" often make up figures because they think they know how to. Some even claim they are Class-A devices without even knowing what that means. Class-A is a stupid setup to have on a battery operated system anyway, Class-AB is not much better either, a high quality Class-B or Class-C device would be a better product.

The 1 Farad caps the "boom-boom kids" use do improve the performance a little but pound for pound they are worthless. Costing sometimes over £100, they are a joke considering how badly they are made.

The worst thing about car amps is that they are crap. Unless you spend a packet on the speakers as well as the amp, they distort before they get to within even a quater of their "REAL" rated power output. I supose it does not really matter when it is that loud, and the content is not really that good! :lol:

 

[audioguru]

Hi Andrew,
Do you think that people listen to high power car stereos at full output?
With music playing, haven't you seen a VU meter moving all the time?

With music playing, the average output of a car stereo is nowhere near full output, but probably 1/10th or less. It is good to have an amplifier and speakers that are capable of producing continuous sound pressure levels exceeding the amount required for "headroom", freedom from distortion and reliability.
My son's sub-woofer amp is rated at 1kW RMS continuously into a 1 ohm load with very low distortion. But his speaker is 4 ohms and draws only (?) 500W from it continuously. With people in the car, the amp probably idles along at 100W maximum and 10W average. When he plays basketball (away from the car), he probably cranks it up but its average output is still only about 50W or less because the sub-woofer is used for only the beat of the music which has a low duty-cycle.

Sure there are manufacturers who inflate power claims for their products but there are also manufacturers who make high quality systems with good true spec's. Good ones cost a lot but can also be purchased for a bargain. :lol: