alternator queston, opinions needed

[walkychalky 10+ year member]

u calculate how many amps u will nedd by dividing ur wattage by 12??

 

[davidwm88]

Which is Volts the formula is amps=watts/volts

 

[JonJT 10+ year member]

While the formula is correct, your also assuming that your amps are !00% efficient, which is not the case. Class D amps are very efficient, in the 90% range IIRC, but most other types of amps are not, and I'm willing to bet that most everyone has a non class D amp in their car. Also, efficiency decreases as temp goes up. Your going to draw more amps than your going to calculate using that forumla.

 

[n2audio 5,000+ posts]

For the record...

In a car like that (I'm assuming not a ton of stock electronic accessories) 1400w of music power shouldn't be a problem with any alt over 100A or so.

Anything over 150A would be complete overkill.

BTW - it's a shame an auto repair shop employee doesn't know how an alternator works.

 

[iceteebone 5,000+ posts]

While the formula is correct, your also assuming that your amps are !00% efficient, which is not the case. Class D amps are very efficient, in the 90% range IIRC, but most other types of amps are not, and I'm willing to bet that most everyone has a non class D amp in their car. Also, efficiency decreases as temp goes up. Your going to draw more amps than your going to calculate using that forumla.

most class D amps are not 90% effecient. most are in the 60-75% range with a few in the 80's.

 

[walkychalky 10+ year member]

well i have one orion 2500D (running my subs) and one audiobahn A4801T (running my mids and highs)....i'll be running the orion at 2500 RMS @ 1 ohm and the audiobahn at 600 RMS @ 4ohm....the orion is a bit underrated though and the audiobahn might be a bit overrrated....but with just the rated RMS i will be running about 3100 RMS.....divided by 12 thats 258.3....so would i need to get an alt tahst about 275 amps?

 

[iceteebone 5,000+ posts]

well i have one orion 2500D (running my subs) and one audiobahn A4801T (running my mids and highs)....i'll be running the orion at 2500 RMS @ 1 ohm and the audiobahn at 600 RMS @ 4ohm....the orion is a bit underrated though and the audiobahn might be a bit overrrated....but with just the rated RMS i will be running about 3100 RMS.....divided by 12 thats 258.3....so would i need to get an alt tahst about 275 amps?

a lot depends on install. i'll be installing an orion 2500D and PG ti500.4 and will be using a 250 amp alt. make sure you have good grounds, big 3, all 1/0 guage wire. it wouldn't hurt to get the 275 and have some headroom but you'd be fine with a 250 amp alt.

 

[n2audio 5,000+ posts]

Well - figure around 3000w.

Factor efficiency and call it 4000.

4000/14 = 285A

Assuming you're going to listen to music and not low freq. test tones you can divide that by about 3:

285/3= 95A

That would be approximate RMS current draw at full volume, but since you can't blast it 100% of the time you can cut that by another 20-50%.

IMO 150A would get it done. 200A would give you all KINDS of head room.

 

[davidwm88]

When you have a lot of watts like 4000+ is when you get into 2nd and 3rd batteries. I mean on Pimp my ride with the Jackhammer they had 4 batters and 6 caps. with a 200 amp alternator

with one batterie 6000/12=500 amps

with 4 6000/48=125 amps

bid difference

 

[walkychalky 10+ year member]

Well - figure around 3000w.
Factor efficiency and call it 4000.

4000/14 = 285A

Assuming you're going to listen to music and not low freq. test tones you can divide that by about 3:

285/3= 95A

That would be approximate RMS current draw at full volume, but since you can't blast it 100% of the time you can cut that by another 20-50%.

IMO 150A would get it done. 200A would give you all KINDS of head room.

wait.....so if i go ahead and get a 200A alt....how much headroom do u think i would have to grow on?.....in case i wanna expand in the future...RMS wattage wise?...jus so i know for future reference

And why did u divide by 14 instead of 12?

 

[ToyTundra 10+ year member]

And why did u divide by 14 instead of 12?

with the motor running you put out 14V or so

 

[n2audio 5,000+ posts]

When you have a lot of watts like 4000+ is when you get into 2nd and 3rd batteries. I mean on Pimp my ride with the Jackhammer they had 4 batters and 6 caps. with a 200 amp alternator
with one batterie 6000/12=500 amps

with 4 6000/48=125 amps

bid difference

Sorry, completely and totally incorrect.

Trying to draw a logical conclusion based on what they do on Pimp My Ride is the WRONG way to get accurate information about 12v electronics.

The batteries are wired in parallel, which means the CURRENT available increases, not the voltage. 24v would toast everything in any car, let alone 48v. It would be a complete meltdown -- literally.

They put in batts and caps in the PMR system b/c that's what their sponsors would give them. The only thing required in that situation is a big alternator. Extra batteries are nice, they allow you to use the stereo w/o the car running for an extended period of time, but they're in no way required to make a high power system work correctly.

When the alternator is the right size for the demand of a car's electronics the battery is electrically invisible to everything except the alternator as long as the car is running.

I used 14v because when your engine's running the alternator should charge at around 14v. 12v is approximately what the battery should maintain when the engine's off.

 

[Bigrick31 10+ year member]

heres a formula to help everyone out

p=power

e=voltage

i=amps

p= E x I

e= P/I

i= P/E

so if you want to power a 2500 watt sterio system and your car runs at 12vdc you take 2500 and divide it by 12 and you get 208amps neede for that sterio.