making a battery

[SunEagle]

It is my responsibility to learn what I need to know before I start working on any DIY project, and its my own risk. And its my job to make sure I have the proper equipment to do whatever it is safely.

Fine. You have accepted the risks involved with your project. All I ask is for you to mention those risks so that others do not just jump in without first knowing there could be some dangers.

[J.P.M.]

If I had a friend that knew anything, I wouldn't have came here to bother you guys. Trust me. But if I need to leave or have too I will.

A person is known by the company they keep.

[jflorey2]

It is my responsibility to learn what I need to know before I start working on any DIY project, and its my own risk. And its my job to make sure I have the proper equipment to do whatever it is safely.

Well, good for you; that's the right approach at least.

So let's start with the basics. Why do you want a lithium battery for your car radio?

Is it because you can't supply the energy (i.e. power over time) that the stereo needs? If so, you need a bigger alternator first.

Is it because you can't supply the peak power (i.e. your voltage is sagging during audio peaks) that the stereo needs? If so, you will be better off with a big capacitor, or a very small, low impedance battery.

Or is it something else?

[ahdiofreak]

Well, good for you; that's the right approach at least.

So let's start with the basics. Why do you want a lithium battery for your car radio?

Is it because you can't supply the energy (i.e. power over time) that the stereo needs? If so, you need a bigger alternator first.

Is it because you can't supply the peak power (i.e. your voltage is sagging during audio peaks) that the stereo needs? If so, you will be better off with a big capacitor, or a very small, low impedance battery.

Or is it something else?

Already have an upgraded alternator. Because of my engine size I can only get a 200amp alternator and running a 2nd alternator isn't an option for my car. I do plan on adding a bank of maxwell super caps when I can afford too.

My amplifier is a 7000 watt amplifier. Adding a very small low impedance battery is not enough. Agm batteries are big, and heavy. Lithium batteries put out better power at a constant rate, and weighs less and takes up less space. So yes there are other options like make space for heavy agm batteries, or spend $1000+ on a lithium battery made for car audio, but I dont want to do those things.

[Mike90250]

Just bite the bullet and install a 240V inverter and a mega watt amplifer

And just how does a 7,000w amp work on 12V ? That's over 500A DC the cables have to carry. Even going into 2 ohm speakers, it's near impossible.

[bcroe]

Those audio amps tend to be totally over rated for output power. The number is not average power,
its some peak that can only occur for an instant. If a square wave to maintain that power were fed
in, the amp would fall on its face.

To deal with the problem, real, realistic numbers need to be established. Even a starter doesn't use
7000 watts, just what voltage and current must be maintained, for what kind and frequency duty cycle?
Most audio has an average power of a tiny fraction of the possible peak. When these are understood,
solutions might be possible.

To actually get into those numbers, sounds like a job for at least 24VDC. I suppose the amp might
convert to such voltages internally, relying on the idea of a far lower average current at 12V. If so
part of the issue could be there. A good storage scope would be a beginning of instrumentation.
Bruce Roe

[BoloMKXXVIII]

Or strap a generator to the bumper...

[jflorey2]

Already have an upgraded alternator. Because of my engine size I can only get a 200amp alternator and running a 2nd alternator isn't an option for my car. I do plan on adding a bank of maxwell super caps when I can afford too. My amplifier is a 7000 watt amplifier.

OK great that's a start. 7000 watts is 600 amps. That's doable, but it means multiple 4/0 conductors to a very large capacitor bank. At those currents, forget batteries; all of it has to come from a low impedance source. Impedance is the only thing that matters (wiring and source.)

Next you are going to have to figure out average power. Your system does not take 7000 watts continuously. Get a scope with a current probe and measure the AVERAGE current you see. If it's less than 200 amps (and it most likely is) then you need only capacitance, not battery storage.

Adding a very small low impedance battery is not enough.

Again, for your case, impedance is the ONLY thing that matters. It determines how much the 12V system will sag when the amplifier hits its peak draws. You are much, much better off with a 1 amp-hour ultracap with a series impedance of .01 ohms than a 1000 amp hour battery with a series impedance of 1 ohm. Do the math; the .01 ohm cap will allow the 12 volt bus (really 13.8 volts) sag to 7.8 volts; the 1 ohm impedance will allow the 12 volt bus to sag to almost zero.

Agm batteries are big, and heavy. Lithium batteries put out better power at a constant rate, and weighs less and takes up less space. So yes there are other options like make space for heavy agm batteries, or spend $1000+ on a lithium battery made for car audio, but I dont want to do those things.

Or spend $140 on a BMOD0058; that's .02 ohms, which is a great deal for that impedance. Two in parallel gets you to .01 ohms. Or spend $500 on a BMOD0500; that's .002 ohms.

[jflorey2]

And just how does a 7,000w amp work on 12V ? That's over 500A DC the cables have to carry. Even going into 2 ohm speakers, it's near impossible.

Most big (well designed) amps boost the bus voltage to 100 volts or so before sending it to the output stage. That's a common strategy for class-D amps, for example.

[bcroe]

Most big (well designed) amps boost the bus voltage to 100 volts or so before sending it to the output
stage. That's a common strategy for class-D amps, for example.

That is the way I would do it. The supply for the internal bus most likely cannot supply 7000W for
any significant time. But it will do better if battery stays up.

A 1000AH battery does not have a ESR of 1 ohm, 1 milli ohm would be in the ballpark. The problem
with caps is, the only way to get much of the energy out is to severely discharge them to a much
lower (possibly unacceptable here) voltage. Batteries are different in that a fairly useful voltage can
be maintained for much of their discharge. I would suggest that the correctly chosen battery, though
small, might perform better than comparable size caps.

Auto alternators, they have regulators and rotating magnets with inductance. No tests
run here, but they may be incapable of following higher frequency audio loads. Bruce Roe

[jflorey2]

A 1000AH battery does not have a ESR of 1 ohm, 1 milli ohm would be in the ballpark. The problem
with caps is, the only way to get much of the energy out is to severely discharge them to a much
lower (possibly unacceptable here) voltage.

Right. But this guy doesn't want energy - he wants power. That 7000 watt number? The only way to claim that is to claim that it can drive a transient of that power through a speaker. The amplifier, speaker, heck even the wiring won't sustain that for anything longer than 100 milliseconds or so.

So let's take that 100 millisecond number. A 120 farad cap (two BMOD0058's) will droop by .5 volts if you draw 600 amps for 100 milliseconds. Again, the ESR dominates.

[bcroe]

So let's take that 100 millisecond number. A 120 farad cap (two BMOD0058's) will droop by .5 volts if you draw 600 amps for 100 milliseconds. Again, the ESR dominates.

That also seems to be in the ballpark at $300 and 11 milli ohms. But a 600A surge causes 6.6V drop
in addition to the discharge drop, not a good at that current level. Bruce Roe

[jflorey2]

That also seems to be in the ballpark at $300 and 11 milli ohms. But a 600A surge causes 6.6V drop
in addition to the discharge drop, not a good at that current level. Bruce Roe

Right - the drop from IR losses completely dominates the equation; voltage sag due to lack of energy storage really isn't a factor.

[ahdiofreak]

Just bite the bullet and install a 240V inverter and a mega watt amplifer

And just how does a 7,000w amp work on 12V ? That's over 500A DC the cables have to carry. Even going into 2 ohm speakers, it's near impossible.

Exactly. If you have the correct equipment, its easily possible. I could explain but I'd rather not get off topic. Especially since I'm already barking in the wrong territory.

[ahdiofreak]

Those audio amps tend to be totally over rated for output power. The number is not average power,
its some peak that can only occur for an instant. If a square wave to maintain that power were fed
in, the amp would fall on its face.

To deal with the problem, real, realistic numbers need to be established. Even a starter doesn't use
7000 watts, just what voltage and current must be maintained, for what kind and frequency duty cycle?
Most audio has an average power of a tiny fraction of the possible peak. When these are understood,
solutions might be possible.

To actually get into those numbers, sounds like a job for at least 24VDC. I suppose the amp might
convert to such voltages internally, relying on the idea of a far lower average current at 12V. If so
part of the issue could be there. A good storage scope would be a beginning of instrumentation.
Bruce Roe

My amplifier specifically is made to put out 7000rms (Root-Mean-Square) with the speakers wired down to 1ohm at 14.4v. (there are many factors that can cause the amplifier to not put out exactly that rating other then power supply) It is not a peak power rating, and its not a cheap amplifier that is sold through mainstream companies (they list unrealistic peak power ratings). 24vdc is not optional because the average car does not run higher then 14v with the alternator charging. 12v need to be maintained the best way possible. It does not need to be perfect at all. Music plays at multiple frequencies at multiple times and current being pulled changes. If my voltage is dropping to a low unsafe number (like below 11v), I turn the volume down. There are way too many factors for me to worry about to make sure I'm pulling exactly 600amps current and 7000watts are going out to the speakers..