Using your calculations backwards, a 550wrms system should put a 26.6a draw on the alternator.

550wrms * 2 =1100w
1100w / 13.8v = 79.7a
79.7a / 3 = 26.6a

Plus another amp or two of current draw for a headunit with no amplified output and a cd changer we're probably looking at 30a current draw. This only leaves 14a for the rest of the accessories based on the 40% figure. Does anyone know for reference how many amps the headlights or a/c draw (I'm sure this varies based on your settings for fan speed and temperature)?

If this is pushing the limits of the alternator, I understand my options are to get a higher output alternator (brand? model? prices?) or get mine respun (prices?). I'm just wondering if this is something I should plan for or not.

 

headlights are 35W each. I believe the 60% is for the cars electricals itself, and the 40% is headroom. You def. wont run into any problems with a 550wrms system.

 

Cool.

 

Dave -- where did you get the 40% from?

also, i think Dave's number assumes you've got 100% efficiency for your amps... so depending on your amplifiers, you may not even be able to supply 900 Wrms

 

nm...

slow on the draw.

 

How about 950w?

I'll have the Audison 2.250 (120wx2 @ 4 ohms) running mids/highs and the MMATS 700 (700w x 1 @ 1 ohms) on my sub for a total of 950w.

 

Using the same calculation you'd be looking at:

950wrms * 2 = 1900w
1900w / 13.8v = 137.7a
137.7a / 3 = 45.9a

With a 45.9a current draw you are a tad over the 44a draw that Dave estimated. Of course taking into what Phil said about the efficiency of the amps there will probably be a bigger draw on your alternator. You might be OK as long as you didn't have your a/c running full blast, with your headlights on, and using the cruise control.

Although I would expect that when you hit big bass notes you might see some power drain in some of the electrical systems.

Guys, if I am off please correct me.

 

that seems about right...

not to mention, i'm honestly not sure how often anyone pulls full rated RMS from any amp continuously for any major period of time. a good battery should help, imo and the alternator should recharge the battery when it has time to catch up...

 

Phil...two things.....

first of all, most of todays cars allow about 40% overhead for electronics....whether it be tire pumps, cig lighter accessories, radars, headunits, etc. 40% is the average. Some cars are like 60% headroom (ford pickups and some cars are 20% high end light weight sports cars)


Next....my calculations are for 100% amplifier efficiency. (the x2 on the wattage is to account for amplifier efficiency)

However, peak current isnt being drawn all the time, (you arent pulling 700x1 every second...just on bass hits)....so the numbers turn out to be rather accurate...give or take a few watts.

 

A 950Wrms amp will pull around 67A at full volume...