I agree with what you guys are saying about the floor pan as a ground return. However in alot of these ground planes that are a combination of crappy welds, glued together unibody panels and recycled pop cans, resistance can play into the equation. Yes I do not always run a line back to the battery if the ground resistance is low enough, but I will always check and then inform the customer so they can make the decision. It is not a case of way more area at all, it is a case of resistance in that area. I've asked for another well respected person in the industry to post on this as well, maybe he'll be able to explain something that I am missing out on. At one point in time I knew no different and I've been in this industry quite a while now.

To answer your question though I have seen many cases where it was necessary to ground to the battery and have seen even more cases where failure to do this has led to a failure of an amplifier.

 

you have to remember that voltage drop will be proportional to resistance and current drawn.. so if you've got a fairly high resistance and nothing hooked up to that cable, you will barely notice a voltage drop.. but start drawing 100 amps, and you'll notice different. You should measure resistance, not voltage drop.

 

^And the formula is V=IR assuming no current limitations R is constant. As voltage changes so will current (current MUST follow voltage in DC circuits) if we were measuring insulation resistance or leakage voltage the R value could change (due to the high voltage) but this is a ‘constant’ voltage system.
As you said, and it is a good point, what if there is a fixed resistance at no load but the chassis is actually a current limiting device (akin to a power line that is too small, this could also be similar to a fuse) then the deviation from linear behavior would go unnoticed by my original plan of testing unloaded. You are correct, but I really can not recommend doing this test to anyone who is not an experienced electrician, I mean it is just not healthy to be dicking around a system drawing 100 amps. After reading a recent series of electrical follies (in this forum) it is obvious to me that people do not treat low voltage systems with enough respect.

 

Once again, this statement can be picked apart electrically but the basic concept is correct. I do not recomend anyone attempt to measure resistance in an energized circuit (even if it is the chassis of a car)completing a circuit between + and - with your expensive DMM in a resistance measuring setting will prove unfortunate for your DMM, voltage measurements will give the same info though resistance changes will be an implied result..

 

I guess we all just have to know how to do things right the first time.

Here's what I have...pretty basic: One 0 gauge ground running from battery to body (bare metal). My amp/device grounds are all compounded to one spot on in the back through a distro block, I used the trunk floor as the ground plane. The previous owner of the car must have had a system in it because there was a scratched off spot and the hole was patched with a piece of putty. That ground spot ended up to be right on one of the chassis structures, so I had assured myself of a good ground (I think). I bolted the grounding post down, set things up, and as of now I have about 1kW RMS running through the system. I haven't noticed any headlight dim or voltage drop at moderately high volumes. [img]graemlins/thumb.gif[/img]

So, aside from the technical and scientific scheme of things, having the right grounding has its desirable results. [img]smile.gif[/img]

 

^^^^i sort of did the same thing, except i ran my ground wire thru a grommet in the cab floor, and attached it directly to the frame of the truck using a stainless steel bolt and nut, the thru some undercoating over the whole thing for some added protection. and all components of the system will be grounded to the one dist. block also, includung the deck.

 

I wasnt saying you should to and measure resistance on a circuit drawing 100 amps.. i was just offering my input to your method..

 

And it was needed, thank you

 

a difference of .1 ohms can make a difference, but thats not signifacant enuff to warrent the grounding directly to the bat

i have worked with a few circuts where if the resistance jumped over .6ohms i knew there was a falty connection, not nessasarly lose or broken but not 100%

anyhow, i must go driving whiel there is still some snow on the ground, hit some puddels and just enjoy it