That's kind of a weird setup. I get what they are going for, but it could create a ground loop for one (which is bad and can cause weird electrical issues), and second that is actually creating the exact problem you say they are trying to avoid with bypassing the current sensor GM uses on the negative cables to the battery. ALL negative wires to the battery should run through the current sensor so the the PCM can monitor charge and discharge performance/usage. There are a few ground wires that go from the block to the battery, frame, and body i think, the larger one goes from the bottom of the block up to the battery if I recall correctly, and is what provides the path from the alternator to the battery. The smaller ground wire from the negative battery terminal goes to the fender and several other grounds mount to the fender right there as well.
Adding another cable from the alternator directly to the battery for negative and bypassing the current sensor means the PCM no longer really knows how much current is going through the system. The current is going to take the path of least resistance and most of it will go through the larger, lower resistance cable. Some will still go through the smaller OEM cable, but not enough for the computer to really know what is going on. This affects both charge and discharge current.
It would be better to replace the OEM cable completely and run the larger cable through the sensor. Then run another cable from the block to the frame in place of the existing cable (which I am pretty sure is the OEM setup as there isn't anything going from the battery to the frame as far as I am aware). You always want to replace an existing ground cable instead of adding another in addition to the existing one, so that you avoid the possibility of introducing a ground loop.
Frankly it concerns me that they are making these recommendations as it doesn't sound to me like they really know what they are doing. I'm not an electrical engineer, but I know enough about electricity and electronics and battery chemistry to question their understanding of any of it. The suggestion to run a negative cable directly from the alternator bracket to the battery negative is a bad idea on these trucks, the voltage being well past 15v when ambient is well above freezing is a recipe for boiling the battery acid/water mixture away, and the other problem you might have with those higher voltages is while many, if not all of the sensors and electronic components should be tolerant of the higher voltages, it's possible that the levels might be over some of their thresholds and could cause them to either be damaged, destroyed, shorten their service life drastically, or just not work quite right, and looking at the cable and terminals they seem to be using for the cable kit looks like they're just using cheap welding wire cable and screw down terminals.
Welding cable is fine and all but don't expect the insulation to last very long, it's not really meant for the heat and chemicals you find in the typical engine bay. The problem with the terminals is they do not make a gas tight connection between the terminal and cable, so you'll see poor connectivity there compared to a properly crimped terminal, and at those amps the alternator can produce it can cause excessive heating and resistance at the terminal.
I really hope it all works out for you, and I am not questioning your judgment as this kind of stuff is beyond the understanding and knowledge of the vast majority of vehicle owners and is easy to misunderstand. I just don't want to see you drop all this money and start burning through batteries and/or having other weird issues crop up because of it. The guys at mechman just sound like nothing more than hacks that figured out how to screw with the internal regulators on OEM units and rewind the coils to produce more amps and mess with the voltages coming out of them and are preying on all the guys that want more power regardless of the cost. I'd be interested to see what the output looks like on an oscilloscope compared to an OEM unit. I wouldnt be surprised if the waveform was garbage.
Sent from my SM-N950U using Tapatalk