Terminal voltage while cranking will tell the tale, but only if the connections are clean and tight and the cables are big enough. 6 volts leaves absolutely no room for error where voltage drop across connections are concerned. Dropping more than 1/10 volt per connection is too much.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!