My 78 F250 starts well enough in moderate to warm climates but I've found it's useless below -20. All I get is a weak dull throb from the big 460, no prayer of starting. A new battery with high CCA would probably help that somewhat but then I thought to myself if one battery is good, two is better. The truck has a second battery cradle sitting unused so I'm wondering if I can stick a new battery in that and wire it up in addition to the existing one for twice the cold cranking amps presumably. Would it cause electrical problems and if not how should I run the cables?

 

I did that on my OTF = "Other Than Ford". all you do is get cables to connect + to + and - to - and you should buy your batteries at the same time and kind. I use mine in my camper and I bought a marine switch to use any combination I wanted to. Do a god job and use correct wire and make sure your batteries are bolted down good
Jim

 

This is very important. You obviously have a problem somewhere, since 460's start all day long all over the world in -20 degree weather. If the problem happens to be your battery, and you buy a new battery and parallel it with the old battery, then the old battery will drag down the new one.

 

1+

Much better to find a better single battery than two as mentioned above.

A second battery isolated for a camper that is deep cycle is nothing like a primary battery for starting.

Most of the time it is not a battery issue if the battery is new or up to par anyway.

Read this post I put up as it answers the most common problems I hear about. It is in this forum.

Quick tests for charging systems-basic

https://www.ford-trucks.com/forums/6...ems-basic.html

 

i would first check starter draw amperage. it sounds like your starter is going bad, or you have a bad cable or ground.

if all checks out good, then you can run dual batteries, just like the diesels do. secondary battery negative to ground, and the secondary positive to the primary battery positive. use as large a cable as you can get. i use 4.0 welding wire.
but a 2.0 welding wire will work fine for a gas job.

 

definitely check the grounds.
i live in montreal and had the same prob.
had the battery tested and it checked out.

went over all the wiring and found one ground was slightly looose, but it was enuff combined with the cold.

good luck