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Continuous Duty Solenoid, Two Batteries

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Forum Name: Car Audio
Forum Discription: Car Stereos, Amplifiers, Crossovers, Processors, Speakers, Subwoofers, etc.
Printed Date: September 26, 2021 at 10:09 PM

Topic: Continuous Duty Solenoid, Two Batteries

Posted By: kenwood_nut
Subject: Continuous Duty Solenoid, Two Batteries
Date Posted: August 07, 2020 at 9:04 AM

My 2006 Saturn Ion 2 has the battery in the trunk, which can be a pain sometimes when your trunk is full, but when it comes to car stereo stuff, that's the perfect place for the battery! Now I only have to run my 1/0 gauge cable from the battery to each amp just a few feet instead of all the way from the engine compartment to the trunk! And since my spare tire compartment is where my battery is, I found there is more than enough room for a second battery! So I want/plan to run a second auxiliary battery to power my stereo and use the main battery for what it was designed for and mainly starting the car. I purchased a 100 amp continuous duty solenoid (assuming that's high enough of an amp rating). I've watched tons of videos and looked a million diagrams on how to hook up a 4-wire continuous duty solenoid, but found NOTHING that tells how to connect your stereo power to it! Connecting the batteries to the solenoid seems like a simple task, but some make it complicated by suggesting a rocker switch to the solenoid, most just say run a wire that is hot when the car is running to the one small post and a ground wire to the other small post, then a main cable from each large post to each battery's positive terminal. Makes sense so far! But what NOT ONE diagram or video tells me is how you power your stereo off it the solenoid. This is where my confusion and frustration starts.

I'm going to assume you simply run the power wire to your power distribution block off the positive post of your auxiliary battery, and ground everything correctly. Is it THAT simple? I've noticed that the 4-wire continuous duty solenoids get their ground to a small terminal on top, where the 3-wire solenoids are grounded to the body of the solenoid. The grounding part isn't my question. Nor is the main battery connections.

Again, what's confusing me is the cables AFTER the solenoid. I plan to run my 1/0g power cable to my system through a distribution/fuse block, and even a second distribution block for my grounds (something I started doing many years ago so you only have to make ONE ground for all components instead of a separate ground for each).

As for the rocker switch shown in some diagrams, would anyone suggest this connection? Or would it be just as good to run a wire to my solenoid that is hot with the engine running and avoid the switch? And should that wire be on that is hot whenever the key is in the ON position, or only when the car is RUNNING?

I never imagined something as simple as a continuous duty solenoid could be so complicated, but then a battery isolator is even worse!

Have any of YOU used a continuous duty solenoid so you can sit and listen to music when the car is not running and not drain your main battery? Was it a simple process to connect? Like I said, the wiring TO the solenoid and batteries is quite clear, it's the wiring FROM the solenoid to your stereo that confuses me!

Thanks for any advice! (And to think I'm 62 years old and haven't figured this out on my own!)


Posted By: eguru
Date Posted: August 07, 2020 at 6:30 PM
Some clarification please.
1. The secondary battery is hooked in parallel with the cars battery when the car is running to charge it from the alternator?
2. The secondary battery is disconnected from the main car battery when you want to run your gear and the car is not running?

Posted By: kenwood_nut
Date Posted: August 07, 2020 at 10:32 PM
I don't know how much more clear or specific I could have been! NO, it's NOT run in parallel or I wouldn't have posted my question! YES, it's a separate battery only used to power my stereo when the car is not running. I thought I spelled that out quite clearly over and over in my original post.

After reading posts in another thread about this same thing, I'm now leaning toward actually running the batteries in parallel, but in my opinion, this does NOT serve the purpose I need. I want to KNOW that my main battery is only there to start the car, and my secondary battery is only there to power my stereo when the car is not running. Seems like connecting two batteries in parallel is NOT isolating each battery for one purpose, so I'm skeptical.

I'm pretty sure I'll figure this out on my own, especially when I get questions like yours after being WAY overly specific about my plans here. I got my answers in another thread, so I won't be replying back to this one. I was looking for someone who actually UNDERSTOOD what I was saying instead of having to confirm anything. Did you EVER even ONCE read the word "Parallel" in my original post? I didn't think so. But thanks for being the only person to offer input anyway.


Posted By: geepherder
Date Posted: August 08, 2020 at 8:58 AM
After re-reading your original post, what you have described is having your batteries connected in parallel when the key is turned to ignition. The negatives should both be grounded, and even though it's through a battery isolator, the positives are common when the key is on. This is by definition a parallel connection. There is nothing wrong with this. When the key is off, they are isolated. You can only either connect them in parallel, or series (and series would be the wrong way in a 12 volt system/vehicle.

To power your amps, you want to connect them to your second battery (or you could connect them to the battery isolator on the second battery side). The stereo constant (yellow on most aftermarket units) should also be run to the second battery. You can connect your stereo accessory wire (red on most aftermarket units) behind the dash so that it powers on with the key, but make sure you install a diode with the striped side towards the stereo. Now, run a wire from the second battery to the stereo accessory wire and connect it after the diode (on the stereo side). Make sure you fuse these wires appropriately, otherwise you risk a fire in case of a short. The diode you install will prevent backfeeding voltage into the vehicle's accessory circuit (HVAC, power windows, etc.). Also install a switch on the accessory wire run to the second battery so you can run the stereo without the key and not drain the primary battery.

My ex once told me I have a perfect face for radio.

Posted By: kenwood_nut
Date Posted: August 20, 2020 at 10:37 AM
Your last sentence really perked my interest! Because one of the diagrams I've seen online showed a rocker switch running to the solenoid. I'm still a bit confused on why you would need a switch. From everything I've learned about continuous duty solenoids, you simply run a wire that's hot when the car is running to either top (small) terminal of the solenoid, so when the engine is running and that wire carries voltage, the solenoid is charging both batteries. When that wire loses power, the internal plunger moves and disconnects the primary battery, allowing you to run your stereo off the secondary battery. So would the switch really help that much?

After I finally got the last of the stereo system installed, I'm finding that my upgrade to the 150 amp alternator was a wise investment! The voltage doesn't change a bit when the stereo is powered up or even turned up! That's what I was hoping for! BUT, when I shut off the car while the stereo is on, of course it slowly lowers the voltage of the battery. I don't run it without the engine running yet, only a few times to watch the amount of voltage drop to help determine if I need to isolate a second battery or just not listen to the stereo when the engine isn't running.

I'm still up in the air about installing a secondary battery, but it sure would be nice. I've read several posts about running two batteries in parallel, but that makes me wonder WHY? You're still going to drain power since the car isn't running, so why kill TWO batteries instead of just one? This is all too confusing for me. I'm still learning as much as I can about isolators, solenoids and secondary batteries.


Posted By: geepherder
Date Posted: August 21, 2020 at 1:12 PM
I guess you could power it down manually. The switch just simulates you turning the key.

My ex once told me I have a perfect face for radio.

Posted By: davep.
Date Posted: November 05, 2020 at 10:33 AM
This is 2-1/2 months later, but this is important to add:

The high-current solenoids used for "battery paralleling" have the potential to generate massive voltage spikes when the coils are de-energized. The use of anti-spike diodes across the coil terminals to prevent damage to vehicle systems is imperative.

Posted By: kenwood_nut
Date Posted: November 06, 2020 at 10:32 PM
Thanks davep.! I'll do that! I appreciate the info! All I'm doing is the typical two batteries, one charging while I the engine is running, the other powering the stereo when the engine is NOT running. All my friends tell me this is a super simple hook-up, but I'm still worrying. I'm just up in the air about if I should use a battery isolator OR a continuous duty solenoid. I have both, ready and eager to be installed!


Posted By: davep.
Date Posted: December 06, 2020 at 10:49 AM
kenwood_nut wrote:

I'm just up in the air about if I should use a battery isolator OR a continuous duty solenoid. I have both, ready and eager to be installed!

Use the solenoid with anti-spike diode across the coil terminals.
Battery isolators have voltage drop across the diodes. This results in chronic battery under-charging which has a whole host of problems. This can be overcome by tricking the alternator sensing to read after the isolator, but generally requires dissecting the alternator and modifying the regulator circuit. Most internally regulated alternators I've worked with, this modification is really fussy, and I know how to do it.

Keep it simple. Use the solenoid. Don't create problems where none exist now.

Posted By: kenwood_nut
Date Posted: December 06, 2020 at 1:21 PM
Thanks, davep! That's useful information! Yeah, I've read on other websites and watched videos about the voltage drop present with isolators. I've pretty much decided it's going to the the solenoid. Just need to actually DO this! I've had the solenoid for months, just haven't cleaned out my trunk to do this simple job.


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