Posted by apb (in NY) on Thursday, March 02, 2006 at 10:54PM :
In Reply to: Re: how to hook up 2 batteries in series? posted by Willy-N on Thursday, March 02, 2006 at 9:27PM :
Willy-N is correct. The full picture requires two pieces of apparatus: the isolator, and a way to "un-isolate" the batteries when the full amperage is needed.
Assuming a battery isolator is installed:
Put a low-amperage (20 A. capacity)pushbutton on the dashboard. Feed it with + 12 V. via a fuse from either battery.
Run the other lead from the switch to the coil terminal of a Ford starter relay, mounted in the engine compartment. Run a heavy battery cable from each battery's + terminal to one of the two large terminals on the relay.
Ground the - terminal of both batteries.
When you push the button, it ties the two batteries together momentarily to allow all the amperage to start the truck. When it is not pushed, the batteries are again independent, and they will charge independently, via the isolator.
For you information, the guts of a battery isolator is just a couple of giant diodes and a heat sink. The diodes act as check valves for electrons so that the concerns of unequal charging that The Red-headed Stranger discussed are not a factor. The other component of the isolator, the fins, sink the heat away from the diodes.
Every diode has an inherent voltage drop of 0.7 V. That means, in practical terms, that whatever voltage your alternator is putting out (13.8 to 14.2 Volts) will be reduced by 0.7 volts to each battery. This will not adversely affect your ability to charge a battery, although in Ford products, it is less of a problem, since their alternators historically have put out close to 15 volts.
APB
Follow Ups: