I now have 3 graphs hooked to my cranking battery. I won't be using more than 2 graphs at any one time. My question is this. What should a fully charged cranking battery read on a volt meter? Should it be right at 12 or like 12.5 to 13 like an auto? Also, how low can the volts get and still be able to safely crank the big motor? Thanks in advance for any help.