Connecting audio to 2nd battery

[rockdogz]

So the plan is to have the audio and auxillary electrical (extra outlets and inverter) system connected to a secondary battery. My question is on the head unit, should I connect both the power and accessory power wires to the secondary battery, or will that drain it? I thought I might hook up a relay so that there's only power to the accessory power wire when the key is turned, but the power it draws will be from the secondary battery. Or, maybe make it so that it draws power when the key is turned counter clockwise to the accessory position, so that the fuel pump and other items don't get power when I just want to have the audio system on when parked.
I think it will be a miracle if anyone understands what I'm asking, so let me know when you need clarification...

[68BNUT]

I kinda get what your saying because ive been sitting in the floor upside down holding 3 different power wires going,,,,,,,, uh uh uh. Try and hook up the accesory wires to a switched power preferebly when the key is on and in the acc position. Less battery drain and if you plan on hooking up amps they will only be on when the radio is "supposed" to be on. and you wont turn your car off but leave the radio on for an extended period of time and come back to find BOTH batteries dead.

Just my 2c's

[rockdogz]

Right, I guess what I'm saying is that the secondary battery doesn't have a 'switched' source anywhere, so I was going to create one by installing a relay and didn't know if that makes sense or not.

[Steve N 69 69 69]

I've seen big furd starter type relays used to allow batteries to be charged when the car is running, but switched off from the start battery when the key is off.
there's a company that sells a ready made setup for use on 4x4 trucks, and other multibattery vehicles that I saw at an off road expo, but cant remember the name at all.

I wouldn't worry too much about the head unit drawing too much current when its off, but I've had several power amps that seem to suck the battery dry when they're turned off, but still hooked to power. I think its amps with large capacitor banks. I usually put a resettable circuit breaker on the audio systems power line, and click it off if I park the car for more than a few days.

My buddy is friends with a top flight audio installer( it looks like a trophy store in his store front),who's also a motor head, I can ask him what he suggests if you want.

[rockdogz]

Thanks Steve,
I already have an isolator to charge the secondary battery while the car is on and to not allow the main battery to be discharged when the car is off, but I didn't know if it would be a good idea to keep the switched power to the head unit connected to a battery all the time. I'm leaning toward running a relay...

[68BNUT]

Oh oh oh I see what your talking about now. Thats a good idea and sounds like a good plan. I was thinking inline batteries like on a diesel.

[wickedridz]

rockdogz, if i am understanding what you wrote, it sounds as if you want to have your audio system/accesories running off of your auxillary battery without draining the main battery. what i have done in the past and seemed to work out very well is this, several manufacturers of audio accsesories as well as painless performance have a high amp 100-150 amp battery isolation relay available. painless sells a complete kit with a toggle switch however all you need is the relay. whatever you do do not use a "dual battery isolator" i have seen several of these systems melt down as well as wipe out batteries due to over charging, heres why, an alternator charges while its spinning, the car draws off the battery for its power needs. the problem is in a situation with an isolator, if the auxillary battery is at a lower power level then the main battery the alternator will charge the load, the more dead aux battery. meanwhile the main battery is fully charged and basically getting baked. with a relay system, when the key is off the batteries are disconnected from each other,simmilar to a light switch in your kitchen turning on and off the table light. with the relay wired to the key switch, when its turned on both batteries become connected and equalize each other. at this point the car runs off of both batteries as if they are one battery also charging at the same rate. the isolator is basically just a big double diode setup always keeping the loads separated.another plus side to the relay setup is that there is really no limit to the amount of batteries in your system providing you have an alternator capable of providing the additional current demands. i believe powermaster has units up to 250 amps for these systems.
besides the battery isolation relay you will need a small bosch style 20/30 relay. they seem intimidating at first but basically all it is is an electronic switch. they have 5 terminals on it, 85-86-87-87a-30. looking at the relay 87a is in the center, 85 and 86 or on either side, 30 is on the bottom and 87 is on the top. with the relay at rest 87a and 30 are at rest together when switched power is supplied to 86 and ground to 85, 87a and 30 disconnect and 30 and 87 connect.
now in your system heres what you do, connect the battery back up, typically the yellow wire on your radio to the aux battery with the appropriate fuse rating.ground the radio to a good chassie ground along with terminal 85 of the relay. connect the key switched power from the radio to terminal 87 of the relay. connect terminal 30 of the relay to the auxillary battery also with the appropriate fuse rating. now terminal 86 of the relay should be connected to the auxillary key switched wire from the fuse panel, the factory radio supply wire. the battery relay should get its signal to energize from the cars ignition wire ( only hot when in the run position) with this circuit the cars main battery is disconnected from your audio system. all the main battery is doing is providing the power to energize the relay for the radio.

[wickedridz]

in regaurds to the ford style starter relay used to isolate batteries. i have seen alot of them fail due to the prolonged contact heat. they are pretty much only good for hot start retrofits where only a few seconds of cranking amps are needed

[rockdogz]

Thanks for the info wickedridz. Someone else remarked to me that they use a relay instead of an isolator too - if they overcharge one of the batteries when the other needs charging, then wouldn't they always cause this problem? Why are they so widespread? I will check out the Painless site though, wasn't sure where to buy the relay type.

In regards to the relay, that's exactly what I had in mind. What I was thinking though is to give the relay the signal from the key switch when it is in the accessory position where you turn it counter clockwise - not sure what that's called - just so that when I'm parked all day and want to run the audio, power isn't being supplied to the fuel pump and whatever else I won't be using... Have to check this with a multimeter though first to see if it will work, since that circuit will need to have power while the engine is running too. I figure it would have though.

[rockdogz]

Well it looks like I won't be able to use the isolator after all, and will need that relay setup, wickedridz...
I set up my charging system based on Mark's suggestions at madelectrical, and so my alternator charging wire goes to a junction point in the front and then to the battery. From what it looks like on the isolator instructions, the isolator A post needs to receive the feed directly from the alternator. So, I'm gonna look into the Painless relay.
Anyone need an isolator? Never used!

On the bright side, got the stereo hooked up for the first time... sweet