too many amps

[BiggSherm1988]

So I am contemplating getting a E-fan but I also want another light bar and I still have 2 amps to install for my speakers. The amps are not too big its just like a 500 and a 750. I just don't want to over load my alternator (I have a 130 amp alternator). How could I go about calculating the draw from adding all these new bits. If it ends up being too much I will probably scrap the E-Fan idea as I haven't purchased it yet.

 

[franklin2]

On the light bar, it will usually give the wattage of the light. Take this wattage number and divide it by 12, that will give you the amps of the light.

I would think the fan would give you the amp number, but it works the same as the light if they give you a wattage number. The amplifiers will be a transient draw, each time a bass beat hits the draw will go up.

 

[ericbphoto]

One thing to note; not all loads will ne running all the time. The fan will be on and off. As mentioned, the stereo amps will change. They don't use a full 500 or 750 amps all the time. So, 500watt amp = 41.5 amps. But that is peak power used at full volume during a really loud section of music or hard drum beat. Bass uses more power, by the way. So, you can de-rate that. Your amp might have a "RMS" rating which is a weighted average power used. If not, I would estimate about 70 percent. That gets you just below 30 amps.

Your battery will supply the amps for your electrical loads. The alternator is just there to charge the battery. If you are going to use the stereo at loud settings often, I would add a second battery.

 

[dirtymike]

ohms law prevails

 

[Uncle Gump]

I think you'll be fine...

 

[55trucker]

So I am contemplating getting a E-fan but I also want another light bar and I still have 2 amps to install for my speakers. The amps are not too big its just like a 500 and a 750. I just don't want to over load my alternator (I have a 130 amp alternator). How could I go about calculating the draw from adding all these new bits. If it ends up being too much I will probably scrap the E-Fan idea as I haven't purchased it yet.

Make use of an LED light bar to keep the load down when in use

 

[franklin2]

Your battery will supply the amps for your electrical loads. The alternator is just there to charge the battery. If you are going to use the stereo at loud settings often, I would add a second battery.

The alternator is there to charge the battery AND run any electrical loads. If you are using the battery for electrical loads, the alternator has maxed itself out.

 

[franklin2]

ohms law prevails

P=I x V works better in this case.

 

[RonD]

And just to complicate it more, lol

Alternator's rating is with engine at 1,800RPMs not at idle, 750RPMs
At idle an alternator is only capable of 60% of its rating
130amp alternator would have about 80amps at idle
Which is fine

Most vehicles with all the lights on, engine running, heater fan on High might draw maybe 40amps, so 80 to 90amp alternators were what most car makers put in as stock
That gave enough "head room" for constant power window up and down, and cycling power door locks all you want, lol

130amp alternator would give you 40 amps to use at idle
Cooling fan may draw up to 30amp when they START UP, but that will drop down to under 20amps once they are running

I can't see light bars drawing more than 20amps unless you are running Air Craft Landing lights, lol
LEDs are brighter and draw less amp so that would be the go to in my opinion

Sound systems are a different story
Sound watts are different than electrical watts, so any watt/amp calculation should then be multiplied by 0.7 to get the actual electrical amps needed, and as said thats at FULL VOLUME

And sound watts can be misleading
2 watts is not twice as loud as 1 watt
10watts is twice as loud as 1 watt
So its a factor of 10
Most vehicles have a 40 to 60watt system, 10 to 15watts on 4 channels, which is VERY LOUD
To get twice as loud you need 400 to 600watts, i.e. times 10

Low frequencies need the most "power"/wattage, a 200watt powered sub-woofer would rattle the windows in a vehicle with a stock sound system
As said the sub-woofer draws the most AMPs in the system and especially when it hits a low "note"
You can get capacitors that install on the 12volt wire to the powered sub-woofer
Capacitors are like mini batteries, when the sub-woofer hits a low note the capacitors provide that brief extra voltage needed so alternator doesn't need to


As said the Battery is not used at all when engine is running, battery's sole purpose is to start the engine, period
Battery voltage is at most 13.0volts, usually 12.5volts
Working Alternator's lowest voltage output is 13.5volts, average 14.0volts
So voltage flows TO the battery when engine is running, keeping it charged up, never from the battery, if so Battery light will come on and there is a problem in the charging system

 

[BiggSherm1988]

Thanks all. This actually gave me a pretty good idea on how to move forward. I only have two shallow 8s for under the seats so I don't think it will give me any issues.

 

[ericbphoto]

My point above, concerning the battery, is that it will fill in for momentary periods when the stereo amps hits a high peak load. I may not have stated my case clearly earlier. When that peak drops off, the alternator recharges the battery and all is fine. As RonD mentioned, capacitors are installed to do the same thing with very large stereo systems.

 

[franklin2]

My point above, concerning the battery, is that it will fill in for momentary periods when the stereo amps hits a high peak load. I may not have stated my case clearly earlier. When that peak drops off, the alternator recharges the battery and all is fine. As RonD mentioned, capacitors are installed to do the same thing with very large stereo systems.

I am sure that would happen if the stereo is being played loud at idle when the alternator's output is low.

 

[RonD]

You know how the Battery light comes on when key is on and engine is off(no alternator output)
Same thing would happen if engine was running and any Battery voltage was used

Battery light comes on when voltage is flowing from the battery to the alternator, i.e. key on engine off
Battery light goes off when voltage is flowing from alternator to battery

 

[franklin2]

You know how the Battery light comes on when key is on and engine is off(no alternator output)
Same thing would happen if engine was running and any Battery voltage was used

Battery light comes on when voltage is flowing from the battery to the alternator, i.e. key on engine off
Battery light goes off when voltage is flowing from alternator to battery

I will have to disagree on this one. As long as the alternator is putting out juice, even if the battery has to help it out, it flips something in the regulator and turns the light off. If the alternator is putting out nothing because of a failure or the belt came off, then the light comes on.

If the engine is getting ready to stall, it's turning so slow the alternator is putting out virtual nothing, I have seen the light flicker. But as long as the alternator is trying to keep up and has output, I have never seen the light come on.

 

[RonD]

The light green wire on alternator is one terminal for Battery light, the other terminal comes from ignition switch or cab fuse box in later years

The Battery light, like any 12v light, lights up only when current is flowing thru it, which only happens when one terminal has a different voltage than the other terminal
If one terminal is 12v and other other 0v(negative) then light is bright
If one terminal is 12v and the other 11v it will be less bright but still on, because current is flowing thru it because there is a difference in voltage between the two terminals

When key is on one terminal has battery volts, the other terminal, at alternator, has 0v so current flows and it lights up

When alternator is running the whole electrical system is now at 13.5volts, battery is at 12.5volts, so voltage is flowing to the battery because its a lower voltage

Battery light has 13.5 volt on both terminals now, so no current is flowing and it off

IF..........you were to try and pull too many AMPs from an alternator its voltage level would drop
If it dropped below 12.5volts(battery volts) then Battery light would have one terminal at 12.5v(fuse box, battery volts) and one terminal below 12.5v(alternator) and current would flow thru the bulb, may not be much, i.e. a flicker, but it would indicate a problem that needs to be addressed