Your engine will run off the alternator until you hit a red light unless you keep the rpms up. On a factory alternator, (this is why they s@#$) the idle output is real low, to the tune of about 10 -15 amps output. This is not enough to run an electronic vehicle for long. You can shut off the A/C, radio, etc to conserve, but sooner or later, the alternator will put out insufficient juice to run the vehicle.
Older vehicles were not an issue since they had carbs and the only thing electronic on most was the ignition. This of course doesn't include the 1979 - up models which is when computer controlled autos came into their own.
As for the diagramming and photos, I started out shooting the project, but it became so huge that it would have sucked up every ounce of space on my website. Not to mention it became so technical that posting it in such a way that anyone could do it was impossible (I always try to write my how-tos so anyone can do it if they follow instructions).
I actually had to go in and repower my relay centers under the hood in order to power the PCM off of the new setup. I had to rewire the alternator, the starter and all other main power feeds based on where I wanted them to be fed from, the main or auxillary battery. The battery cables are 1/0 and 2/0 cable and all battery terminal connectors are hand soldered on and the cable custom made by me to fit. I preplanned what accessories I would be running off the batteries and made darn sure that wires were in place to make it work, including my voltmeter that reads voltage from both batteries at the flip of a switch.
It took me two months of planning and work to make it fly. I wish I could say it was a cake walk but it wasn't. It was expensive and time consuming, but damn well worth the trouble now that I look back on it.