Annoying stereo shutoff when starting

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I like to shut my engine off when I'm stopped for very long (I'm a bit of a hypermiler) and when I do so it shuts my stereo off. The police department who had the car before me installed a switch under the dash that allows them to turn the ignition switch off and have the car remain running. This works to prevent the stereo from shutting off when I kill the engine (flip switch on, turn ignition to "acc", then flip switch off to kill engine) but when I start the engine again, all of my electrical devices have power cut to them. My stereo has a built in MP3 player that reads files off of a USB drive. It takes about 20 seconds for it to boot up, read the files, and start playing again... Now I've come up with the below idea that would involve only one switch on my dash that I could turn on just before killing the engine, or even for passengers in the vehicle to turn on when they want to listen to the music and they don't have the key (say I've gone into a store or whatever). I figured that a secondary power wire run directly from the battery to the stereo, with a switch in the line for manual activation would be the easiest and cheapest. The problem with that would be that if I do not turn the switch off when I start the vehicle, power will run through that wire to all other electrical devices in the car that have their power cut when the cars starter is engaged. I figured that the solution would be a large diode installed in the factory stereo power line to prevent power from feeding back through that line to anything else. Will this work? If so, how large of a diode will I need to use?
 
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Why not use a Relay between Battery Positive and the Ignition switch. This will power your device without needing a diode, and you can use the toggle-switch to bypass the relay should you want to keep it running. PM me if you need a diagram.
 
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This can be achieved with a relay feeding power from the battery to the stereo, triggered by the ignition switch. These are more reliable than a Diode IMO. You may need a huge diode whereas a simple relay will get the job done just the same without drawing a ton of current through the ignition switch, but drawing it directly from the battery instead. This will save your ignition switch which probably wasn't rated for much more than it is doing right now... ;\)
 

chevrofreak

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I just don't see how it would work, unless the relay also effectively cuts the power going from the ignition switch to the stereo. My headlights, fans, and pretty much all other electrical devices are turned off when the starter is engaged, and I thought without a diode they would try to draw power through the added wire. Ad far as large diodes, I have seen 40 amp 600v diodes on Ebay that are used with solar power generation, but they seem like major overkill.
 

chevrofreak

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The power for the stereo when the switch is turned off would be coming straight from the battery, rather than through the ignition switch. Still, these cars have some seriously heavy duty wiring on the ignition switches.
 
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 Originally Posted By: chevrofreak
I just don't see how it would work, unless the relay also effectively cuts the power going from the ignition switch to the stereo. My headlights, fans, and pretty much all other electrical devices are turned off when the starter is engaged, and I thought without a diode they would try to draw power through the added wire. Ad far as large diodes, I have seen 40 amp 600v diodes on Ebay that are used with solar power generation, but they seem like major overkill.
Yeah, I downloaded your diagram and was playing with it and realized the problem. Diode it is... Or forget about the Ignition switch and just have a toggle switch.
 

chevrofreak

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Yeah, not even having the power for the stereo come through the ignition switch would be easiest. If I could somehow combine everything with a timer to make sure that I don't accidentally leave the toggle switch on for more than say 5 minutes after the engine is killed, that would be ideal, but now we're getting into some complicated stuff that I've never played with.
 

chevrofreak

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My other idea was a decent sized capacitor thrown into the existing power line, with a diode to prevent it from powering anything but the stereo. That would keep me from having to do any manual intervention, but would cost the most and take up the most space since it would be a large cap.
 
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I wouldn't do the diode the way you have it, because that will draw lots of current through the ignition switch. I would do a smaller diode on the wire that trigers a relay to feed power directly from the battery. This will save your ignition switch and the diode from mega current draws... Want me to diagram it for you?
 
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This takes the load of your "Head Unit" off the ignition switch and toggle switch and feeds is directly from the battery versus through the ignition switch.
 

chevrofreak

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That would work, but is more complicated, and my ignition switch is seriously heavy duty as it is (I believe it is 8 gauge wire)
 
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It's easier too hook up than it looks, doesn't overload your toggle/ignition switches and is protects your other electrical devices on the accessory line, and your ignition switch should the "Head Unit" malfunction... You would also needs a small diode versus a big one because it takes almost no current to drive the relay. ;\)
 

chevrofreak

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I do have a 12v hot in the cabin that the police used to power their equipment. I'm not 100% that it doesn't turn off with the starter, so I'm going to go test it. If it stays hot, that would make things easier.
 

chevrofreak

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It does stay hot when starting, but the voltage drops to around 11v. The manual says working voltage is 11-16v, so I hope the stereo can deal with 11v for about half a second without cutting out.
 

chevrofreak

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Now, I just hope Walmart sells relays since Radio Shack is closed, and I'm one of those instant gratification/ADD "I thought of it now, so I'm going to do it now" types.
 
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