I'm working on a dual battery setup for my 75 e150 using a deep cycle battery as my main power source. Basically, I am connecting the fuse box wire that would normally go to the powered side of the starter selenoid to the deep cycle battery. A 200 amp relay will connect the positive sides of the batteries using an oil pressure switch as a ground. All the deep cycle battery has to do is have enough power to activate the selenoid and connect the fresh battery to the starter. Once the engine fires up, the oil pressure switch completes the ground to the 200 amp relay between the batteries and both are connected together as long as the engine is running and alternator is charging.
My question is- Will the action of activating a starter selenoid from a second battery create an internal connection with the first? To put it simply, does the energizing of the small posts create a direct connection to the big ones? It probably would not be a good idea to have a connection between a full battery and a discharged battery through a selenoid, but if both circuits in the selenoid are entirely separated I should be fine with my wiring setup.

 

I can't really picture what you're trying to accomplish without a diagram, but to answer your question regarding the conventional solenoid: applying a voltage to the small 'S' post bridges the two large studs together. If the solenoid has a second small post labeled 'I', the 'I' post is shorted to both studs as well. When the voltage is removed from the 'S' post, the two large studs and the 'I' post are electrically isolated.

A couple things to keep in mind when it comes to this stuff:

1) As you suspect, you do not want to load a battery with a discharged battery. In fact, generally speaking, you should not put too different voltages together in parallel. Even if the batteries are healthy, one will charge the other to a degree. Battery isolators are often used for these type applications.

2) Typical starter solenoids (like the ones mounted on the fender) are not rated for continuous duty. They're meant to handle the current for the short time the engine is cranked, once per trip.

There may be other concerns, but since I can't really picture what you're trying to do, that's all I've got. Why not just use a typical battery isolator for this seemingly common application?

 

Essentially what I'm wanting to know is, does a starter selenoid keep two isolated circuits even when activated? Or does the the activation momentarily connect all posts? I've never torn one apart to see what exactly is going on in there.
The reason I am going this route is because I have read that typical isolators do not fully charge either battery.

 

I don't understand your question (missing word maybe?). At any rate, the behavior of the solenoid was described in my previous post, and repeated here:

Voltage on 'S' post: two large studs (and 'I' terminal, if present) all shorted together.

No voltage on 'S' post: no connection between the two large studs, or between either large stud and the 'I' terminal (if present).

To add, there is never any electrical connection between the 'S' post and either large stud, or the 'I' terminal (if present). The solenoid acts just like a relay, with an internal winding between the 'S' post and solenoid mounting flange (ground) representing the relay coil, and the two large studs representing the relay contacts. The 'I' terminal, if present, piggy-backs the relay contacts.

It's still not clear why a conventional battery isolator would not work for what you seem to be trying to do, but that's probably me just not understanding the application.

 

When the oil pressure switch activates the solenoid, the two large posts would be connected.

fmc400 is correct. A regular starter solenoid is not rated for continuous use. Your idea is a interesting one, but you need a solenoid that is rated for continuous use, you may have more luck finding one using the word "relay" rather than "solenoid".

With the isolator, there is a voltage drop of about .7v through the isolator. That's really the only thing that happens with them, they short the battery that much charge voltage. Not a really big deal that I have heard about.

 

What do you think about the fancy relay in this thread at the bottom?

https://www.ford-trucks.com/forums/1...discharge.html

 

The starter selenoid is only going to start the engine.
I'm disconnecting the wire from the positive side of the selenoid that supplies power to entire electrical system and connecting that too a deep cycle battery. Therefore, the deep cycle battery is responsible for activating the starter selenoid when the key is turned, connecting the starter battery to the starter. I also have a 200a relay that will connect both batteries together once the engine starts, using the oil pressure switch as a ground for my relay. When the engine is shut off, the ground for the relay is lost and both batteries are isolated. In the event that the deep cycle battery is too low on power to activate the selenoid, I will have an emergency button under the hood that will use the starter battery to activate the selenoid.

 

If I do have issues, it may be that my emergency button connecting the hot side of the selenoid to the ignition post to activate it will create a momentary connection between the two batteries during starting. I'm not sure if that would fry anything. Then again, with all the power going to the starter, there may not be too much of a draw from the deep cycle through the ignition wiring before the engine starts and the relay connects.

 

This emergency button idea is not going to work. I'm assuming you're planning on continuously monitoring two independent voltmeters and relying on yourself to kill the switch at the appropriate time? Not only is that impractical, it also isn't reliable. A discharged battery can report the same voltage as a healthy battery depending on the load placed on both batteries.

Furthermore, as I mentioned earlier, two healthy batteries can still charge or discharge into one another. In general, you should not connect batteries in parallel. A battery isolator, which is the proper tool for your application, prevents either battery from charging the other using a diode array. This is responsible for the 0.7-volt drop that Franklin describes. In practice, this is of no consequence, just as he says.

 

Here is my diagram

 

Maybe instead of the e button there, I should make an alternate ground for the relay as a button under the dash to self jump start if one battery is low.

 

I get what you're doing and after here, you want to power the bulk of the vans smaller power draws off the deep cycle and be able to run the deep cycle down and still start the van from the starting battery, is this correct?

To answer your question on the solenoid, no, it will not back feed they are two separate circuits. Look at it this way, if they weren't two separate circuits and the large post could power the small post then the solenoid would get stuck on when ever activated. The small post goes directly to ground, the large posts only connect each other.

Actually a rather clever way to go about it, I do see one problem though that I don't see that you have considered, also a why.

First the why, if my assumption on my first paragraph is correct about your goal then it can be accomplished much simpler with better function. They way you are going about it has issues in that if the deep cycle gets too low then the ignition power will be too low to reliably start until the oil pressure connection is made(which has issues). A better way to go about it would be to simply switch the power source for the things you want to power, or the main circuit you intend to already if the simpler option is preferred. Do this with a rather standard 5 pin relay. Here is a very common standard diagram for the common 5 pin.



If you connect 85 to ground, 86 to an ignition on source, 30 to the circuit you wish to power(fuse box) 87A, to the deep battery, and 87 to the starting battery. Then when the ignition is off the coil is not powered, the connection is made between 30 and 87A, powering the fuse box from the deep battery, when the ignition is on, the connection between 30 and 87 is made connection the fuse box to the starting battery.

This will also completely isolate the deep battery from the act of starting the van preventing a discharged deep battery from drawing juice from the starting battery. You can then connect your deep battery to the alternator via an isolator(not starting battery, see next paragraph) via the separate relay as planned, though I would not use oil pressure. It would be better to isolate/delay the charging of the deep battery until starting is finished and the starting battery has it's surface charge back. Using something like a temp switch, or manual switch is a better way.

Ok back to the problem I see, as you have it in your diagram you are connecting the two batteries directly to each other. The connecting of the higher voltage and being charged starting battery directly to the lower voltage deep battery is very likely to overcharge the deep battery and damage it. This is why you need some form of isolator, to prevent a direct connection between a high voltage source and a low battery. A modern diode version will do the job just fine, or you could rig one up using a couple high amp diodes or a couple alternator rectifiers.

 

That is a good point above. What do you really need to power with this deep cycle battery? If you separated those circuits, it may eliminate some of your problems.

 

Subscribed.

 

Those are all very good ideas bruteford. You have kind of racked my brain trying to think of how I would wire all that. I already have all the components needed to make the setup in my diagram work, plus a five pin relay.

As far as my overall goal, I plan on installing a head unit-less stereo using a Minidsp sound processor and a 480w rms 4 channel amp, and a power convertor. Maybe even a small mini fridge for the occasional music festival/camping trip with the girlfriend. I would like to be able to run the stereo for a few hours, then run the engine for about ten or fifteen minutes to recharge without the worry of the not having enough power to start the engine.

The power converter I am looking at has a built in digital voltage meter and sends off an audible warning, then kicks off when voltage reaches below 10v. The amplifier kicks off at 9v. Assuming I am away, an already cold minifridge should be able to run for about six hours with a meager 35 amp hour battery. If the inverter kicks off, it should still be decently cold for a few more hours and I would think that without the load of a starter, 9v should enough power to activate a starter solenoid. If that is not enough, the emergency button comes into play.

This is all what if scenarios and may only happen once or twice a year. This is just a save-my-*** setup if I need it. Otherwise, 99% of the time, I would not need a second battery.

I really don't want to have to cut into my stock wiring too much, but if you could draw me a diagram, I would be able to get a better idea of what guage wire I would need and where to better make this happen, and possibly help the gentleman subscribed above. I will also take pictures when finished and report my findings.