hooking up a second car battery

[ahole-ic]

It would depend on the matterials of the frame in which many frames/sub frames in a vehicle are not as "solid" steel" as you might think. Some have alloys and are weilded/bolted to gether without very conductive materials. I drive a 89 chevy S-10 and I gained between .2 to .3 volts grounding my amp/amps to the front battery vs a short run to the frame. In a uni body car I wouldnt even take the time to find a rear ground as there is nothing solid front to back in most of them. The very reason why the excuse of having the shortest ground as possible is even used is to lower the cost of matterials (thiner wire) and the potential (huge) difference in grounds front and back in unibody cars.

With all due respect, if you gained .2 to .3 volts by making a long run to the back, then you did not do the big 3 and your rear ground correctly. I'm not trying to argue, but with no current being drawn and going through welds and pot metal, you'd still read a dead short. It's under a heavy load where the resistance rears its ugly head. In my particular case, I'm on the same beam of my frame. It is very thick steel and I'm connected to it up front and in the back. It's like I'm directly connected to the alternator. I realize not everyone has that luxury, but if you take the time to investigate your grounding path and make good solid and short connections you will in almost every case have less resistance than several 1/0 runs of copper. If you're lazy and just want to throw another run of 1/0 in there, sure that might work but it's almost never necessary or as good as if you took your time and grounded to your frame correctly.

 

[audiolife 5,000+ posts]

With all due respect, if you gained .2 to .3 volts by making a long run to the back, then you did not do the big 3 and your rear ground correctly. I'm not trying to argue, but with no current being drawn and going through welds and pot metal, you'd still read a dead short. It's under a heavy load where the resistance rears its ugly head. In my particular case, I'm on the same beam of my frame. It is very thick steel and I'm connected to it up front and in the back. It's like I'm directly connected to the alternator. I realize not everyone has that luxury, but if you take the time to investigate your grounding path and make good solid and short connections you will in almost every case have less resistance than several 1/0 runs of copper. If you're lazy and just want to throw another run of 1/0 in there, sure that might work but it's almost never necessary.

My big 3 is very correct and my rear ground when I had it was less than 20" right to a well sanded existing bolt hole into the center of the frame rail on the same side as the ground up front. I also lost my cd player hiss by doing this.