Originally Posted by Amnesia II
The thing is, there is already a neg cable returning from the battery bank to the engine block. The alternator is securely bolted to the engine block so therefore is directly connected to the neg side of battery bank.
So what is the point of another neg battery cable returning as Mainsails photo shows?
The point is that the "case ground" run though the engine block is a HORRIBLE way to get a hundred + amps back to the battery bank without suffering some serious voltage drop issues either now or in the near future.
Running an alternator through numerous different metals, that offer lower conductivity than copper, through pounds of rust, grease, corrosion
causes problems with HO alts. It actually causes just as many problems with factory alternators that don't have a way to "sense" and try to compensate for voltage drop....
It also causes issues with starter motors and I often run a neg wire directly to the ear of the starter not the opposite side of the engine block 18 different connection points away. The differences in the way the motor
starts are often quite noticeable and dramatic...
I am the guy often called in to fix alts that suffer from voltage drop and the resultant chronic under charging that accompanies this voltage drop. More often than not the majority of the voltage drop is in the NEG side of the circuit but it is often a decent split especially with factory wiring
I fix this by using the alts "isolated ground" terminal or I clean a good spot on the alt case when it is case grounded. I also use conductive terminal grease to maintain this connection. I lead a new wire, the same size as the POS, directly back to the bank or the load side of a shunt if so equipped.. There is a good reason HO alternators usually require a NEG cable the same size as the POS cable. Creating the most direct path back to the battery bank, on both sides of the circuit, results in the best alternator performance.
I often deal with & fix voltage drop issues by adding a new NEG cable the same size as the POS direct from the alt to the bank.. It is not uncommon to see a 1Ga POS wire and a 10GA NEG run through the factory wiring
harness, to a rusty engine block, grease, corrosion
, paint and multiple engine components before it gets to the cable that takes it back to the battery bank.
After this new cable has been installed I regularly see 90%+ of the current
flowing though the new cable vs. the engine block path... The before and after is easily measured with a clamp meter. The path of least resistance is the best path when dealing with something as important as charging voltages and not allowing them to be eaten up though poor conductive paths..