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batteries, 1200 watts, lights dimming


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DYohn 
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Joined: April 22, 2003
Location: Arizona, United States
Posted: January 06, 2012 at 2:34 PM / IP Logged  

shadowm891 wrote:
Yeah i did lastnight at work with a headlights on and my heater on at idle. With the system on all the way up it drops down to 13.2v each time the bass hits and goes back to 14.2v. But after a while it will drop to 12.6v for a split sec and go back to 14.2v. But with the system turn down it stays at 14.2v and never moves even if im driving.

That could be many things, but first I'd check the ground return resistance and make sure it's less than one ohm.  Make sure the Big 3 upgrade is intact and all connections are tight and there is no corrosion at your battery.  Very likely your alt can't keep up with the load either because it is too small or it is turning too slowly.  Try upping your idle speed and if that works, you may want to change the alt pulley or upgrade to one with more output at idle.

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shadowm891 
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Member spacespace
Joined: March 11, 2009
Location: Oklahoma, United States
Posted: January 06, 2012 at 3:17 PM / IP Logged  
Alright ill check that out but i have a question. I was surfing youtube and i came across a 101 lesson thing and i did the math and it said i need a 100 amp per hour battery for 1200 watts. And the red top i have now is 50 amps per hour do you think this could be the problem. Btw the red top up front.
oldspark 
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Location: Australia
Posted: January 06, 2012 at 5:26 PM / IP Logged  
DYohn wrote:
....check the ground return resistance and make sure it's less than one ohm.
That's the second time I have seen such a high figure...
Where does it come from? I know AC ground systems usually specify a max 1 Ohm resistance(sic - it should be impedance!)
1 ohm at 100A means a 100V drop. That is not possible in a 12V system.
Are you sure you don't mean 1 milli-Ohm else 10mOhm (for a 1V drop)?
shadowm891 - you do realize that a 180A alternator is NOT 180A at idle - maybe not even 100A?
Also some alternators are slower to respond than others.
Except for the Ohmage, I can only repeat what DYohn wrote.
If the dip occurs across the alternator, then it is the alternator.
Anywhere else is a distribution (wiring) issue.
And that battery size won't effect dips - they still have similar internal impedances and should supply adequate current when needed. The AH rating is a total capacity measure and does not (on its own) reflect it current or short-circuit current capability.
shadowm891 
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Member spacespace
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Location: Oklahoma, United States
Posted: January 06, 2012 at 9:33 PM / IP Logged  
Yeah i understand im not getting 180 at idle i'm only getting 105 but to my car useing some of that 105 as well just not sure how much. All i know is my stock was 75amps and a car use 40% of that but im not sure if that is at idle or with car moving now.what gets me is i have a underdrive pully on it to so its moving but ill check my grounds and all.
DYohn 
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Joined: April 22, 2003
Location: Arizona, United States
Posted: January 07, 2012 at 10:30 AM / IP Logged  
Less than one ohm is just a good rule of thumb for car audio.  Ideally you want it to be zero (or at least less than the meter's ability to indicate) of course.
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oldspark 
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Posted: January 08, 2012 at 2:42 AM / IP Logged  
DYohn wrote:
Less than one ohm is just a good rule of thumb for car audio. 
So a 10V drop for a 10A audio load is acceptable?
I don't think so....
One Ohm for speaker wiring maybe, but NOT the power path.
I think there is some confusion over what that ROT refers to.
DYohn 
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Location: Arizona, United States
Posted: January 08, 2012 at 9:31 AM / IP Logged  

Please, of course you're right.  That is unacceptable.  Like I said, ground return resistance should be as low as possible,.  Zero is the goal.  0.1 ohms is typical.  But I always use "less than one ohm" as the rule of thumb when trying to help people troubleshoot via the Internet because 1) it makes them measure, which most people never do 2) who knows what kind of meter they may have? 3) who knows if they know how to use the meter properly to remove lead resistance from the indication? 4) it keeps the conversation going.  And remember this is DC 12V power, not AC.  Impedance is not part of the equation.

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oldspark 
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Posted: January 08, 2012 at 9:46 PM / IP Logged  
It sounds like an "old Ohmmeter" rule - make sure the "meter" reads under 1 Ohm...". Alas these days with digital (DMMs etc) we should be more precise.
And the voltage drop will be V = I x R. I'm saying that to the novices so they can check what info they are given (ie, is 100A thru 0.1 Ohm reasonable? IE: V = 100A x 0.1R = 10V which is obviously not a good GND voltage drop in a 12V system!
As many know, I rarely quote or use resistances. I ask what a reasonable max voltage drop is, then calculate the R from V/I where the "customer" decides the V and should know the I (or power from which I can be guesstimated) - eg, 1V @ 100A => 1V/100A = 0.01 Ohms max resistance.
Besides which the voltage across something (like a GND path) is easier to measure than resistance - and more "practical" assuming that is at max else known Amps
IMO if a figure like "x Ohms" is stated, it should have its assumption (ie, max 1V drop or max 1A load etc).   
And yes - a minimal voltage drop (ie, minimum resistance).
Sorry if impedance confused things, but that was wrt to house earthing/grounding Standards (the point being that a ground stake 300m away is useless for lightning protection etc), and batteries which have internal impedances rather than an "internal resistance" - though that does not impact the sort of DC stuff we do here. (Dare I mention Bleeder's and OldFart's that suggest AC on a vehicle's +12VDC due to motors and SMPS dc-dc converters (USB chargers, big amps, PC & laptop supplies etc?)
Thanks DYohn. (I like your tease for conversation. $%$#@!!) (LOL)
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