1992 Ford E350 7.5L

What did it do? How did it do it?

The trailer tow battery charging relay closes when the ignition is on/run allowing the trailer battery to be charged.

So how about the OEM aux batt relay for a dual batt. installation? I was going to take one off a Super Duty desiel in the junk yard, but I'm not sure it will do what I want. I want dual battery cranking amps on start up(big 460 runs is hard to start warm-kills the cranking amps) and an aux batt for key-on-engine-off usage; lights, tv ect. This is what I'm looking to order DC battery isolator and split charge relay for RV, car, and truck applications

Any thoughts?

 

The OEM aux battery relay was a small relay designed for charging the aux battery. It was not designed to aid in starting.

If I were you, I would get a regular battery isolator. Buy a deep cycle battery for the aux battery, and leave the starting battery in the truck now to do the starting. If you read the description in your link, they say their relay can be used for "emergency" starting "if" the regular battery is dead. I don't believe it's designed for regular starting use, and really if you use the aux battery for it's intended purpose, probably 50% of the time it's going to be too low to help out the regular starting battery anyway.

If you feel you really need to use the aux battery as an emergency jump start battery, just carry some battery jumper cables with you. If you do need a jump, just unhook the aux battery terminals, and use the jumper cables to jump to the starting battery to give you a emergency boost.

If you want dual battery starting on a regular basis, you are going to need 3 batteries. Two starting batteries and one aux battery.

 

Can I go with 2 battery system hooked in simple parallel if I just want extra cranking juice? What are the risk? + to + and - to - using a 2 gauge wire of equal lengths?

I do have an isolator that will take a battery off line at 11.5V and leave the other to be run down. BatteryBrain

Any thought or comments.

Opinion about starting with 2 fresh batteries. My Car Quest guy says it doesn't matter much and neither does equal cranking amps.

 

You can do that, it's done on the diesels all the time. All you are doing is making one big battery out of two smaller ones. You would get the same thing from one of those large batteries they put in the big rigs, but there is no room in the truck for one of those, so they use two smaller batteries hooked together like you describe.

I didn't look at the link, but if you are still trying to do double duty(starting and aux) with one battery, I don't recommend it. When hooking two batteries together(+ to +, - to -) they must work together to provide any benefit. When hooked together, each battery is only as strong as the weakest battery. Like I said, when hooking them up this way, they are considered one large battery, so they need to be drained together, and charged together, and they need to be the same age(bought new at the same time) to get the most out of them. If one battery is weaker, or has double duty and is used more than the other battery, it's going to drag the other battery down with it.

Like I said, two fresh batteries is the best route. If you have a older battery, and you hook it in with a newer battery, the newer battery will never reach it's full potiential, it will only be as good as the older battery it's connected to. You can do it, and it will probably work fine, but it's not preferred way to do it. This rule applies to the + to + and - to - hook-up. If you have a new and old battery, and are using a isolator, then it doesn't matter, they are only hooked to together for charging purposes.