Battery, Wiring, and Wattage talk.

Ill admit im basically a noob when it comes to the three topics in the thread title. I was looking into a slightly larger amp then what I have to get the most out of the subs I will choose. Im thinking of doubling my current amps wattage if not almost tripling it. On a stock alternator with 0ga wiring what would i be up against if I were to install a D3100 in the trunk? I guess what are my limits? I dont blast my music and if I do it would be around a minute tops. I understand my stock 120amp alternator will be my limiting factor. How long can it take before a 2-3k watt amp drains a group 31 battery?

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Hope I do my math right:

1. Let's say, roughly, you're at 14v running the 3kW amp at 1 ohm at 80% efficiency. 3000/14/0.8=267A draw.

2. XS 3100 has a capacity of 110ah. Peukert's Law is then used as follows: t=(110)/(267^1.3)=110/1427=0.077hours=4.6min=4min and 36sec.

As you can see, with any voltage drop below 14 and efficiency below 80%, you will see a good decrease in the time before the battery is drained.

Hope I do my math right:
1. Let's say, roughly, you're at 14v running the 3kW amp at 1 ohm at 80% efficiency. 3000/14/0.8=267A draw.

2. XS 3100 has a capacity of 110ah. Peukert's Law is then used as follows: t=(110)/(267^1.3)=110/1427=0.077hours=4.6min=4min and 36sec.

As you can see, with any voltage drop below 14 and efficiency below 80%, you will see a good decrease in the time before the battery is drained.

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Peukert's law is better at predicting very small draws and gets a little wonky at larger ones. I don't think you (or your amp) would like the result if you were to try to draw 267A for 4 full minutes.

Besides your variable is generous for an AGM- as is your efficiency estimation. As well as the estimated current draw. Those things probably come out on the wash though due to the nature of music and what not.

Stock 120A + d3100 on a 3k... you'll live in the 11's but you should be able to wang out pretty good depening on where you're wired and how responsible you are on the knob.

Peukert's law is better at predicting very small draws and gets a little wonky at larger ones. I don't think you (or your amp) would like the result if you were to try to draw 267A for 4 full minutes.
Besides your variable is generous for an AGM- as is your efficiency estimation. As well as the estimated current draw. Those things probably come out on the wash though due to the nature of music and what not.

Stock 120A + d3100 on a 3k... you'll live in the 11's but you should be able to wang out pretty good depening on where you're wired and how responsible you are on the knob.

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lol I did my best! 4min did sound like way too much. Thanks for the clear up.

lol I did my best! 4min did sound like way too much. Thanks for the clear up.

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'twas an awesome post. I doubt many have even heard of Peukert's law. Kudos.

Odyssey battery always lists larger draws in their spec sheets. I always appreciated that about them. It looks like they just use Peukert, but still good info for reference.

I think it also depends on the battery itself though as well. If you're throwing back with a blue top group 31 I don't think that would handle as well as a north star group 31 or xs 31. But both would lose charge fairly quickly trying to keep up with a 3k for extended periods of time. Don't quote me I'm not an expert but that's just my 2 cents

lol I did my best! 4min did sound like way too much. Thanks for the clear up.

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If you're talking about a stand alone power supply the math is fine, but the assumptions are way off.

For one, you're STARTING at 12.6v. That is a fully charged battery. Above that is the "float" charge that can move very little current.

Then - generating 267A through 2.2mOhm internal resistance drops .56v off the top. You're basically starting at 12v where the amp's efficiency is likely to drop and you're fighting a losing battle.

Besides -- it's the alt that's at the heart of the matter.

Running 2-3kw on a 120A alt will have it basically maxed out at 1/2 volume or higher.

A big battery does little nothing to reduce the load on the alt.

The battery will be fine until the alt burns up -- and with that much power i'd give it 1-6 mo. depending on your listening habits.

Running 1.5kw in my old car (130A alt) needed an alt replacement 3x in 4 yrs and I don't blast it that often.

So basically, att what point (rms watts) should someone be looking to get an HO alt? And what size (amps...200...220...bigger??)

One more thing quick. How much wire will I need to do the big 3 as well as run wire to a second battery in the trunk? Should 40 feet cover me?

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40 feet is a bit much.. 24ft covered just to do big 3 and run to back of cab on my ext cab truck.

If you can afford 40 then why not.. If you mess up you will have extra.

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3k is when I tell people to start looking at upgrading their alt.

Hmmmm interesting.

I think a sticky should be made that lists the wattage of a system and what electrical upgrades should be done when you get to that point. I believe a lot of people would find that extrememly helpful....maybe there already is a thread like that but I just don't see it:
@TaylorFade

it's entirely dependent on the car ^ there's way too many variables. honestly, the best thing you can do is try to educate people on electricity and hope they can build a mental model of their car vs the amount of power they're pulling and then choose the right amount of power appropriately.

you might be able to run 2000w only dropping to 13 with a full size truck and then using only 1000w drop to 11v on a honda civic. the equations fly_marlin posted are a good starting point, but people don't usually like dealing with unfamiliar equations.