Any ideas on where to get a wire with +12V onlywhen engine running?

**RFMan** · 06-04-2005, 01:31 AM

Hi,

I'm looking for a wire, somewhere, that provides +12V only when the engine is running (not cranking), but actually started.

I will be using a very large battery (Like an optima deep-cell) as part of my tank circuit so I can run the CarPC and amplifiers (1000W+) for a few hours while sitting at a show or something.

I want that battery to be connected to the main battery (via a relay) only when the car is actually running, to avoid computer shut-downs during cranking and also to avoid killing my main battery when demoing for long periods.

I've read almost every thread on tank circuits, and I've found nobody that's tried to do exactly this.

The really "ghetto" solution for me would be to use a switch on the parking brake lever, since I always start me car before putting the e-brake on and have it on before I stop the car (It's a 6-speed, and I've taken the habit

) That solution doesn't help when the car is parked and idling (which is sometimes needed just to charge up the battery after playing the tunes loud for a while)

The really, really hi-tech solution is to use an OBDII reader connected to the PC, with a program that will send out a control voltage (through the serial port) to the relay only once the RPMs exceen 500 or so.

But there has got to be a solution between those 2 that would work.

Any suggestions appreciated.

Luc a.k.a RFMan

**mushin** · 06-04-2005, 02:14 AM

How about a circuit that detects when the system goes above 13V?

**Geking** · 06-04-2005, 02:31 AM

How about useing the Headunit +12v as a trigger to turn on a relay (electromagneticaly operated switch) to connect your main batt to your tank batt?

**Jahntassa** · 06-04-2005, 07:26 AM

How about taking off the bottom of the dash under the steering column, finding the ignition harness (the one with the thick wiring coming from the steering wheel), and pulling the 12v Ignition power off the Ignition wire which is Black /w Red stripe. That only shows +12v when the key is on Ignition.

Sorry, just wanted to follow the 'how about' theme from the other responses.

the12volt.com is your friend. http://www.the12volt.com/installbay/...s.asp?TID=7165

**organasised** · 06-04-2005, 07:37 AM

Join a car forum for your make and ask s often needed for stuff.

eg for audi`s drivers side footwell have some bolts you can screw onto.

**NotchbackVW** · 06-04-2005, 08:14 AM

The simplest solution is to find an output wire from your altenator. If you have a 'charging warning light' on your dash this is easy. Usually there is a warning light on your dash which illuminates when ignition is on, once altenator begins charging (engine running) this goes out.

Then you just need a simple circuit, even a couple of relays so that when ignition is live and feed to that light is off, then your have an output for your required use.

tell me if i have made no sense!

**NotchbackVW** · 06-04-2005, 08:16 AM

BTW, this is how most dual charging circuits do this, so cranking (assuming you dont hold your key turned once engine has fired!) is from your car battery only. Then once charging light goes out, it connects the two together and charges both batteries.

**RFMan** · 06-04-2005, 10:16 AM

Thanks for the replies guys,

Using the +12 remote from the HU would have been an easy way, if I was actually going to have one in my car. The power supply I will use for the Car PC will have a similar wire, but I cannot use it because if the secondary battery goes completely dead, the connection between batteries would never be established.

Has anyone actually done the ignition switch trigger. My concern as that when you turn the key to start the car, the switch momentarily goes through the ON position on it's way to the start position, and its possible that this brief moment of contact would be enough to trigger the system before the crank (I'm not really worried about the computer surviving the cranking, the power supply will handle that, but I don't want to be powering the starter motor from the secondary battery through 16-10ft of wire all the way down the car.)

The charging light idea sounds good. I would need to make a small microcontroller board to detect the signal going up (whenever the ignition is set to on and then down again (once the car is actually running for a few seconds) before triggering the switch. I could probably also do it with the oil ressure warning light if its easier to access.

I'll probably have to get a service manual for my car. Nissan has some very weird color coding for their electrical stuff (I've seen a pink ground wire in the OEM HU wiring harness... that's jusr weird)

**NotchbackVW** · 06-04-2005, 11:32 AM

Originally Posted by RFMan

...but I don't want to be powering the starter motor from the secondary battery through 16-10ft of wire all the way down the car.))

If that is all your worried about then add in a sufficiantly rated relay in the chunky wire that connects your two batteries. Use a double pole relay so that at rest (no current in coil) it is closed. Have the cranking trigger wire (the one which goes to starte solenoid) open this relay.

So when ever your engine is cranking your two batteries are seperated!

**zootjeff** · 06-06-2005, 05:29 PM

Originally Posted by mushin

How about a circuit that detects when the system goes above 13V?

The uSDC20D will do this.

www.mpegbox.com

-Jeff

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