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Old 06-10-2020, 02:08 PM   #1
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How to wire for Emergency start from Aux Battery?

Hi,
Does anyone possibly know the best way to design for Jump starting from the Aux battery if and when the Starter battery is dead?

I mean, I have a 1996 Ford E350 van and it is equipped with the auxiliary battery package. I know the Aux and starter batteries are isolated until the engine/alternator is running.
Some heavy trucks with aux batteries have an "Emergency Start" button on their dash that connects the Aux and Starter batteries if a boost is needed from the Aux battery, mine does not.

I know if a pair of jumper cables is handy that they could be used to connect the two batteries. But what if jumper cables aren't handy at the time..

So I'm wondering how one might alter the wiring to retro fit such an Emergency connection on the dash or under the hood...?

I know there is a relay under the starter battery that connects the aux battery to the system when the engine is running.... Perhaps a switch that can manually energize that relay would be one way to go...but introducing voltage into that relay coil may go back to something else and effect things...

I'm also wondering if the system would already connect the two batteries if the key is just moved into the run position without starting the engine, to allow the starter battery to charge from the aux till you turn further to start. But no that can't be because then if the Aux battery was dead, it would pull down the starter battery.... Hmmmm

Thank you for any input

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Old 06-10-2020, 04:04 PM   #2
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With the battery separator installed by Sportsmobile in my 2004 van, it just happens automatically unless the starter battery is very dead.

When the starter battery is very dead, there's no power to activate the relay so I have to manually run a wire. This happens to me often enough that I added a switch to temporarily make this connection.

I have the Sure Power "Model 1315" separator under my rear seat in my RB50. Looks like this. You can see the switch I added to temporarily connect the AUX battery (+) to the Separator's brown wire:



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Old 06-10-2020, 07:25 PM   #3
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Thank you Geofff for a reply and pics
So are you saying that the starter battery voltage energizes there connection between the starter battery and the auxiliary battery automatically when the ignition is switched to the run position?
But if the starter battery is dead, you manually energize the relay with the auxiliary battery power so you can use the auxiliary battery to charge the starter battery to start the vehicle?
If ithe connection is automatic in the run position, what keeps a dead aux battery from draining the starter battery voltage and then not being able to start?
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Old 06-10-2020, 08:15 PM   #4
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Seeing as you are talking about the stock Aux battery and not a sportsmobile supplied aux battery, Geoff's solution won't work, because you don't have the Surepower unit.

This circuit should be able to modify the existing circuit to work with the Stock aux battery solenoid. You would need to cut the existing positive solenoid coil connection. It would go to the NC (87A) input of SPDT relay,the pole (30) would go to the aux battery solenoid. The wire in a switch with power from the aux battery, to NO (87) and the 85 positive coil. Turn the switch the relay is energized and sends power to aux battery solenoid. When switch is off the existing circuit works as intended before mod.

Note: you should check the current ratings of existing aux battery solenoid, wiring size and see if it we meet your starting-current needs.

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Old 06-10-2020, 09:27 PM   #5
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Thank you Very much Scalf77!

I believe the specific relay is in a bank of relays next to the radiator.
I’ll have to investigate the amp rating of the solenoid as you say...I believe I’ve read that it’s like a 250 amps solenoid or so..which is what a starter motor might draw in very cold weather or ...

Well can you explain a bit how it works?

I mean:
Does the solenoid connect the starter battery to the Aux as soon as the key is turned to the run position without starting or is there a delay?
I mean if it does, then perhaps no alteration is needed...?
Although if it does, that would mean a dead aux battery could pull down the starter battery in the opposite situation..?

I’ve read that trucks with the emergency start switch, it’s a momentary switch and for a real weak starter battery, they must hold the button for a few minutes, allowing the starter battery to charge from the aux before they try to start the vehicle. Well it sounds like a nice feature to add, I just want to understand a bit more to feel comfortable with the outcome.
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Old 06-11-2020, 05:05 AM   #6
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I have not looked at 1996 van, but my assumption is that the solenoid is engaged when it is the run position only. It is not engaged during start. So sure leaving in run would transfer some power to discharged starter, but as is it would be disconnected during start.

You could certainly put a momentary switch in my diagram, it means you don't have to worry about turning off.

You could also wire a large current capacity switch in parallel to the solenoid.

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Old 06-11-2020, 07:06 AM   #7
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The Battery Doctor 12v Battery Isolator has a button to achieve what you want. It replaces your existing solenoid:

https://www.wirthco.com/battery-doctor/isolator
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Old 06-11-2020, 07:14 AM   #8
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Thank you Very much Greg!

I’m having trouble following your description diagram...
If you could clarify more. I mean I’m concerned that I may introduce voltage into the system that may damage the solenoid energizing system.

Also in your diagram it looks like the manual switch would energize and close the relay then send power to terminal 87 which would send power to 85 which would permanently keep the relay closed..
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Old 06-11-2020, 07:46 AM   #9
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Thanks but I can’t imagine replacing something I already have with a solenoid that seem much lower amp rated just because it has a built in button and if placed where the factory solenoid is under the battery, I wouldn’t be able to get to the button anyways.
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Old 06-11-2020, 07:54 AM   #10
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Quote:
Originally Posted by jimtmcdaniels View Post
Thank you Very much Greg!

I’m having trouble following your description diagram...
If you could clarify more. I mean I’m concerned that I may introduce voltage into the system that may damage the solenoid energizing system.

Also in your diagram it looks like the manual switch would energize and close the relay then send power to terminal 87 which would send power to 85 which would permanently keep the relay closed..
The switch sends power 85 to close the relay, it also feeds 87 (the normally open throw of relay), so when switch is on, the relay feeds 12 volts from aux battery to your solenoid, connecting for jump start. Turn the switch off and power goes away from both 85 and 87, relay goes to Normally Connected throw and passes existing solenoid wiring to solenoid, work just as before. As I said you could use a momentary switch.


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