Recently bought two Die Hard Platinum batteries. After two days the truck would not start - in fact no electrical whatsover (e.g. radio, lights, etc.). Checked the batteries and found the driver side (secondary) fully charged with the passenger side (primary) completely dead. Brought the battery back to Sears and they confirmed it was bad and replaced it under warranty (it didn't even register on their tester as it had zero juice). Put the new battery in and truck started up perfectly.

In any case, I was surprised I had no electrical whatsoever running on one battery. Shouldn't I expect that the second battery should at least run the radio, lights, windows, etc.? Also, does the truck really need both batteries to start? It is as if the second battery is doing nothing.

Any ideas why having one dead battery (passenger side) causes no electrical whatsoever?

 

They are run in parallel. Very weird that one died on its own. Must have been a bad apple. The truck needs both to turn the truck over to start. The magic number is 10.5volts. Any lower than that and it will not start...
As far as the accessories...no idea....Im sure someone else will know more than me!

 

When the batteries are wired in parallell the voltage tries to equalize per say. The stronger battery tries to compensate for the weaker battery nad the voltage reduces. If you had disconnected the dead battery from the system then your 12 volt accessories probably would have worked.

 

I had this same issue with the motorcraft batteries.. Although it took 18 months to fry the one.. The other one is fine tho..

 

There is no primary and secondary. The batteries both work equally and need to both be good. In parallel the voltage stays at 12 but the cranking amps available is doubled.

 

exactly as they said above. the two bats in parallel act as one big bat, so if in your one big bat there is a dead cell then the battery wont work properly. there is no harder task for the truck electrically than a dead battery. our trucks and most diesels are designed with two batts, big ones, because of the potential for prolonged draw when starting. the glow plugs, the vac pump, the fuel pump etc... then the starter.
you should have both batteries, you need both if you live in a colder environment. the colder it is out side the less juice your battery has.

your dead battery was causing the truck to think that it had no voltage, so it wouldn't work anything.

 

depending on if you feel like reading some, this link is a real good one for us normal guys in explaining batteries.

The 12volt Side of Life (Part 1)