[Crankster]

Manufacturers went to 12 volts because the era of high output high compression engines was dawning, along with a host of power hungry electrical accessories. There was simply no way a 6 volt generator electrical system was viable heading forward.

The key to maintaining any automobile electrical system whether 12 volt or 6 volt is to ensure clean, tight grounds and connections. Low voltage DC is susceptible to voltage drop due to corrosion, even invisible. By the time it is visible the connection has usually failed altogether. This is why people end up brushing battery terminals off in a dark parking lot on below zero mornings.

But it doesn't take much resistance to slow cranking speed. A related problem is the ability for the charging system to bring the battery back up to 100% to ensure fast tickover - a negative (heh) feedback loop. Just 0.06 of an ohm - that is a few hundredths of an ohm, barely measurable, will cripple any alternator, by about 30%!!