How Do OEM Dual Batteries Work?
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Soundguy
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the 2 bat setup in my 99 f450 uses 1 long cable to hook both bat hots together.. suspect my f350 is the same.. 
soundguy
soundguy
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OK, I did some checking on the truck. The battery on the passenger side is mounted by the firewall. It has heavy gauge positive cables coming from it. One cable runs down to the starter and the other one runs to some big terminal type device (circuit breaker, isolator??) on the firewall and then splits into two wires going over to the driver's side. One of these two goes directly to the fuse box, the other runs thru a relay to the other battery on the left side.
So I'm thinking the firewall battery does all the starting because (1) it has the heavy cables and (2) is the one closest to the starter.
The battery on the driver's side only has a light gauge positive cable going thru some kind of fusible link and then over to the relay which connects to the other side. I have a feeling in the 3.5 years I've had the truck this battery has only been along for the ride. When I do something stupid like leave the dome lights on overnight it only runs down the starting battery.
This truck has the factory HD trailering package and also the snow plow prep package. I have a feeling if I had a plow installed or was using the accessory power to the trailer, then the 2nd battery would come into use, and if you ran it down thru the trailer plug with the engine off the starting battery wouldn't be affected.
Maybe it would be a good idea to switch the two batteries for now. I don't think either one is "bad" yet, but I did run the one down a few times by leaving something on in the truck overnight and I'm sure that doesn't do the battery any good. But what do I know, maybe I'll ask at the dealer when I get a chance.
So I'm thinking the firewall battery does all the starting because (1) it has the heavy cables and (2) is the one closest to the starter.
The battery on the driver's side only has a light gauge positive cable going thru some kind of fusible link and then over to the relay which connects to the other side. I have a feeling in the 3.5 years I've had the truck this battery has only been along for the ride. When I do something stupid like leave the dome lights on overnight it only runs down the starting battery.
This truck has the factory HD trailering package and also the snow plow prep package. I have a feeling if I had a plow installed or was using the accessory power to the trailer, then the 2nd battery would come into use, and if you ran it down thru the trailer plug with the engine off the starting battery wouldn't be affected.
Maybe it would be a good idea to switch the two batteries for now. I don't think either one is "bad" yet, but I did run the one down a few times by leaving something on in the truck overnight and I'm sure that doesn't do the battery any good. But what do I know, maybe I'll ask at the dealer when I get a chance.