Dual battery jump starting question...

[Martinjmpr]

I've seen this come up in a couple of other threads but I wanted to ask here because I'm not certain how this works.
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I put a dual battery system into my Suburban, isolating the house battery with a 200a solenoid that is powered by being connected to the fuse block on a circuit that is only powered when the ignition is on.
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If I understand correctly, when the ignition is on, power flows to the solenoid (200a Stinger SGP32) and closes the switch, connecting the two batteries together, yes?
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So, here is my question: Let's assume that something has happened to my starter battery to cause it to drop below the point where it will start the truck. Not 100% dead, mind you, just dropped to the point where it is incapable of starting the truck.
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If I turn on the ignition and power then flows through the ignition circuit that then energizes the solenoid, doesn't that close the switch and connect the batteries together? So, wouldn't it be possible to "jump start" the starter battery then? If the "house" battery is fully charged, wouldn't turning on the ignition switch send power to the depleted starter battery? Which would then allow me to start the truck, yes? Or am I missing something?
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I seem to recall learning in the past that in order to send maximum power to the starter, sometimes other circuits are momentarily cut off, so does that mean the ignition circuit that I've tapped into for the solenoid power would lose its power (and open up, disconnecting the house battery from the starter battery) as soon as I turn the key to crank the starter?
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But if that is the case, shouldn't I be able to "charge up" the starter battery, for example, by turning the ignition on (closing the switch and connecting the two batteries together) and then leaving it in that position for a few minutes (so the house battery will charge up by the now-connected starter battery) and then turn the switch to crank the engine?
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I understand this would not work in a situation where the starter battery is 100% dead and doesn't have enough power to energize the circuit with the solenoid, but I'm talking about the (IMO much more common) situation where the starter battery has some power, just not enough to actually crank the starter.
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The reason I'm asking is because I'm wondering if I need to carry a small set of jumper cables to connect the batteries in the unlikely event of a dead starter battery (unlikely because my fridge - biggest current draw on my setup - is connected to the house battery, not the starter battery.) It just seems strange to have to carry cables to connect the two batteries when they're already connected to each other.
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Thanks in advance for any pointers here.

 

[dwh]

I've seen this come up in a couple of other threads but I wanted to ask here because I'm not certain how this works.
.
I put a dual battery system into my Suburban, isolating the house battery with a 200a solenoid that is powered by being connected to the fuse block on a circuit that is only powered when the ignition is on.
.
If I understand correctly, when the ignition is on, power flows to the solenoid (200a Stinger SGP32) and closes the switch, connecting the two batteries together, yes?

Yup.

So, here is my question: Let's assume that something has happened to my starter battery to cause it to drop below the point where it will start the truck. Not 100% dead, mind you, just dropped to the point where it is incapable of starting the truck.
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If I turn on the ignition and power then flows through the ignition circuit that then energizes the solenoid, doesn't that close the switch and connect the batteries together?

Yes.

IF the battery still has a high enough voltage to actually cause the electromagnet to pull hard enough to overcome the disengagement spring and pull the main contacts into position.

Otherwise, nada.

Don't ask me exactly how much voltage is high enough, it's probably different for each type and depends on the strength of the opposing spring.

So, wouldn't it be possible to "jump start" the starter battery then?

Absolutely. But only if the starting battery has enough juice to engage the solenoid. Otherwise, you'd have get power to the solenoid from the house battery.

If the "house" battery is fully charged, wouldn't turning on the ignition switch send power to the depleted starter battery?

Only if the solenoid engages.

I seem to recall learning in the past that in order to send maximum power to the starter, sometimes other circuits are momentarily cut off, so does that mean the ignition circuit that I've tapped into for the solenoid power would lose its power (and open up, disconnecting the house battery from the starter battery) as soon as I turn the key to crank the starter?

No. If the ignition were cut off during cranking, the truck would never start. That's one circuit that would never be rigged to cut off during cranking.

But if that is the case, shouldn't I be able to "charge up" the starter battery, for example, by turning the ignition on (closing the switch and connecting the two batteries together) and then leaving it in that position for a few minutes (so the house battery will charge up by the now-connected starter battery) and then turn the switch to crank the engine?

Absolutely. But only IF the solenoid engages.

I understand this would not work in a situation where the starter battery is 100% dead and doesn't have enough power to energize the circuit with the solenoid, but I'm talking about the (IMO much more common) situation where the starter battery has some power, just not enough to actually crank the starter.

Just depends on how much is "enough" for that particular solenoid.

The reason I'm asking is because I'm wondering if I need to carry a small set of jumper cables to connect the batteries in the unlikely event of a dead starter battery (unlikely because my fridge - biggest current draw on my setup - is connected to the house battery, not the starter battery.) It just seems strange to have to carry cables to connect the two batteries when they're already connected to each other.
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Thanks in advance for any pointers here.

You could also rig a switch. Power from the house battery to switch to solenoid to temporarily engage the solenoid.

Or you could just use a little jumper wire to jump between the large aux battery terminal on the solenoid to the small positive terminal on the solenoid. Once you've caused the solenoid to engage, power will flow from the aux side to the chassis electrical system and provide power to keep the solenoid engaged (as long as the key is turned to ignition that is).




Some Blue Sea units have a lever built-in to the top so the switch contacts can be manually engaged in in a pinch (or manually locked out so they can't engage):

 

[Martinjmpr]

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You could also rig a switch. Power from the house battery to switch to solenoid to temporarily engage the solenoid.
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Or you could just use a little jumper wire to jump between the large aux battery terminal on the solenoid to the small positive terminal on the solenoid. Once you've caused the solenoid to engage, power will flow from the aux side to the chassis electrical system and provide power to keep the solenoid engaged (as long as the key is turned to ignition that is).

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Ah, I hadn't thought of that but it makes sense! IOW, instead of having to hook up heavy jumper cables to the battery terminals, I could run a small 12g wire from the positive terminal on the house battery to the same terminal that the isolator that connects to the ignition switch and that would do it, too!
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Thanks again! I figured I'd get the right answer here. :26_7_2:
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EDITED TO ADD: Or, I guess I could make my own "jumper cable" by taking a single piece of 12g wire, putting alligator clips on either end. One end clips to the positive on the solenoid, other clips onto the positive terminal on the house battery. As soon as the truck starts, the "jumper" would be removed and put away. I think I'm going to make one this weekend.

 

[dwh]

EDITED TO ADD: Or, I guess I could make my own "jumper cable" by taking a single piece of 12g wire, putting alligator clips on either end. One end clips to the positive on the solenoid, other clips onto the positive terminal on the house battery. As soon as the truck starts, the "jumper" would be removed and put away. I think I'm going to make one this weekend.

Um, I did say that.

But the positive terminal on the house battery is connected to the solenoid, so your jumper could actually be just an unfolded paperclip because it only needs to be about 2" long.

 

[rayra]

You'll want the alligator clip jumper / test lead, if you are having to do this alone. Unless you want to stand there and make contact long enough to dump enough juice into your starter battery.

You can get one for just a couple dollars at any electronics store (if your local Radio Shack is still open, mine's having their 'everything must go' sale right now).

 

[Martinjmpr]

You'll want the alligator clip jumper / test lead, if you are having to do this alone. Unless you want to stand there and make contact long enough to dump enough juice into your starter battery.
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You can get one for just a couple dollars at any electronics store (if your local Radio Shack is still open, mine's having their 'everything must go' sale right now).

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Gosh I was just going to get a couple of alligator clips and some shrink wrap and make one. I'll keep it in a zip loc bag in the glove compartment of the truck.
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Realistically, of course, I would hope that I'd never need such a thing. After all, the whole POINT of going to the House battery/starter battery setup is to protect the starter battery so it will start when I need it to (and I should point out that the starter battery is a new and expensive one that I put in right after buying the truck in 2015.)
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So this was posted more out of curiosity than anything else. If I'm having to jump start the truck then I miscalculated something BADLY.

 

[dwh]

Unless you want to stand there and make contact long enough to dump enough juice into your starter battery.

Doesn't work that way.

The instant that power is applied to jump the solenoid, the solenoid is energized, power flows from the house battery side through the solenoid to the engine battery side and supplies power to the ignition circuit which then keeps the solenoid engaged.

There is no standing around holding anything because the jumper is only needed for a second.

 

[crazysccrmd]

I just self jumped last week using my ML-ACR. A simple push of the button to engage the solenoid and connect the house batteryand I was able to start the truck right up. The starting battery was dead, wouldn't even click the starter over.

 

[LulaNord]

Hi...as per my knowledge Clipping on a jumper wire not needed. A momentary actuation is all. Aka, dwh prior comment of using an unbent paperclip...
If the car 'ign switched on, actuating the solenoid via any sort of temporary jumper (or even manually pressing its plunger closed) the solenoid will energise & latch. You wont need to stand there to "dump juice into starter battery".

 

[NMC_EXP]

The more I read about the ACR units for dual battery set-ups, the more I wonder why the 4-position battery selector switches fell out of favor.

Other than having to remember to do what is required with the manual battery selector switch, what is the downside?

 

[NMC_EXP]

Locating the switch in readily accessible/handy location.
Big copper conductors are expensive & bulky. Its better to minimise distances it needs to run.
Those switches are quite large too. Do you really want one attached next to your 8-track player ?

Was that intended to be humorous or sarcasm of the offensive kind?

 

[dwh]

Was that intended to be humorous or sarcasm of the offensive kind?

Humorous. Verk is not a native English speaker. Another bloody immigrant. Just what the U.S. needs.

(Disclaimer: Intended to be humorous.)

Those battery switches are primarily designed for boats. Personally, being absent minded, I prefer the automated thing.

 

[DiploStrat]

Its Automatic

The more I read about the ACR units for dual battery set-ups, the more I wonder why the 4-position battery selector switches fell out of favor.

Other than having to remember to do what is required with the manual battery selector switch, what is the downside?

Two points:

-- Automatic switches are, well, automatic. Manual switches are easy to forget or require big tell tales.

-- A relay lets you place the manual control one place and the switch another, saving wire runs, etc. Putting a manual switch on the dash would require massive wire runs. Under the hood, not a problem.

 

[NMC_EXP]

If I were to add a 2nd battery and selector switch, the switch would be in the engine bay.

 

[NMC_EXP]

Humorous. Verk is not a native English speaker. Another bloody immigrant. Just what the U.S. needs.

(Disclaimer: Intended to be humorous.)

Those battery switches are primarily designed for boats. Personally, being absent minded, I prefer the automated thing.

Apologies to Verk.

I'm absent minded myself. But since I drive old junk and have the hood open a lot anyway, and the selector switch is big and red, maybe that option would work.

Plus, I'm just a simple country boy struggling to do my best in a complex technological environment. Therefore, simple solutions have some appeal.