Second battery, choosing a solenoid

SuperDave

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Attention shade tree electrical engineers:
I got a chunk of change for my birthday & decided to invest it into Project Turd, my 73 More-Door Newport (called that because it doggy doo brown & it gets crappy gas mileage). I ordered carpet, dead-liner insulation, cables, amplifiers, & speaker wires to complete the sound system I'm making for it. I'll be pushing close to 2000 watts thru 12 speakers (2 dash mounted tweeters, 4 5" midrange door speakers, 4 6x9 woofers on the rear package tray, and a pair of 10" subs in the trunk). I'll also employ tactile transducers in the seats, bass shakers, to supplement the subs for Bass you can Feel. I want my stereo capable of giving you CPR, but I don't want to be arrested or piss off my neighbors doing it.

That much power is sure to make the headlights dim and pulse in time with the music, undesirable. SO, I will be augmenting and fortifying my sound system with a 2 fared capacitor for the amps, a 2nd battery in the trunk (secured inside a marine type battery box), with a battery isolator between the main battery & the second battery. Cables to connect the 2 will be a 4 gauge cable, both positive & negative. Having a negative cable to ground out to besides the chassis ground eliminates lots of noise issues. The isolator has a diode built in, allowing charging current to pass only when the alternator pushes 13v, but blocking discharge thru the main system. This will prevent the stereo from draining the main battery, dimming the headlamps or cause issues with anything electrical.

All that being said, having a 2nd battery in the trunk allows for the possibility of being able to give yourself a jump start when the primary battery fails to crank over that high compression big block that is designed for a muscle car. But rather than drag out the jumper cables & stretch them from the trunk to the hood, why not introduce a solenoid into the circuit, bypassing the isolator and bridging the 2 systems and wiring it to a switch on or under the dash? Plans call for 4 gauge oxygen-free copper cables ($60 for 2 20' lengths!) rated for 500 amps, plenty to provide a boost in cranking amperage. A 3 position toggle switch, up for starter assist (wired thru the crank trigger) so the solenoid only activates when the key is twisted to the crank position, center for off - isolator circuit only connects when the motor is running, and down for full time connection, for "charging" the primary battery or running both batteries simultaneously (in the event the main battery is too pooped to do the job) so I can limp to the auto parts store and buy a new battery.

Sounds good right? Here comes the part where I want to pick your brains: What solenoid do I choose? Initially I thought a standard Ford type fender mounted solenoid should do the trick until I started looking to purchase one. We have your standard garden variety versions (cheap, under $10), heavy duty for high amp pulls, like on a diesel motor, but then there are "Continuous Duty" versions that are set up for long term, low amp draws, rated 300 amps max. The intermittent versions (cheap) can handle 750 amps but only short term usage, I'm not sure if they wouldn't overheat and hang open, "locking" the 2 batteries together. The heavy duty versions can withstand en excess of 1000 amps, but they too are designed for intermittent use. The continuous use versions I see online aren't rated higher than 300 amps max, and the battery I want is a Group 24 battery rated for 600 Cold Cranking Amps, double the rated capacity of the solenoid. So which do I choose? Is there another option? Or do I use 2? One high amp, intermittent use AND a low amp continuous use?

I surrender to the collective wisdom and experience of the forum. Thanks in advance for your reply.
 
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Big_John

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You have to look at how much the starter draws rather than the capacity of the battery.

Probably 300 amps at the worst case and that's really only momentary to start it turning and that's a overheated starter or a cold engine (below zero). If you google it, you'll probably find a more accurate figure and I'll bet it less.

For example.... A 75 watt light bulb will draw less than 1 amp. It doesn't matter that the power plant that it's hooked to 20 miles away is capable of putting out 2000 megawatts. The 15 amp circuit in your house that the light is plugged into is more than you need because the light bulb is only drawing 1 amp.
 

SuperDave

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The starter is one of those modern mini starters you would find on a late model V8 Durango. In fact, that's what I told the parts guy when I ordered it so that's what I got. I knew (from watching those gearhead shows on the Motor Trend channel on cable) that they are a direct bolt on replacement for the mega-sized starters my 400 big block took. It seemed like a good idea because of the headers, plus (I think) it's a gear reduction unit so it'll handle a high compression motor. Not sure how much it draws, and I don't have a meter that'll read amperage draw for something that high. But it has no problem cranking a 400 big block with shaved motorhome heads & flat top Pistons.

Anyways, I did find a heavy duty marine solenoid rated at 65 amp continuous duty with a max intermittent surge rating of 750 amps. It's $40+ on Amazon so I'll probably get that one.

It's a little more than I wanted to spend, but that could be a long term savings compared to replacing several underpowered/overwhelmed solenoids not suited for the task, not to mention time, hassles & headaches because I took the cheap route.
 
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fury fan

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Mini-starter is good - I was gonna suggest that first-thing.
If it were me, carry a set of jumper cables - cost will be the same, and how often will you really use them?

For your battery isolator system, check out Intellitec - I used them at a utility truck building place years ago, they were one of the first to use a solenoid to separate the batteries, whereas the prior method was a huge diode-isolator thing. The solenoid system uses electronics to NOT charge the aux battery unless the chassis battery is above a certain voltage. But that was 15-ish years ago, there are likely newer products out there.

Your 4ga cables might be rated for 500 amps, but you should check the voltage drop thru them at the ~15ft cable length you'll have. For a starter motor, voltage drop is important also. Also - you must ensure good fusing at both batteries! Some folks overlook this. The factory did without it due to good history of cable routing and no chafing issues. You could od that as well, but if you ever have an issue it'll be ugly.

I have a smattering of solenoids, fuesblocks and much terminal strip stuff from a factory closeout. If any of this looks interesting LMK and I can make you a package deal?
For the terminal strips - I have oodles of them, for the other things I have small qty.

1661309637120.png

1661310031358.png

1661310055269.png

Here's the white box 120-901:
stancor 120-901 - Google Search

Here's the gray-potted Waytek 77003:
Cole Hersee 24117 Solenoid Continuous Duty

Other items in the pics, I suspect you recognize them.
 

300rag

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...

For your battery isolator system, check out Intellitec - I used them at a utility truck building place years ago, they were one of the first to use a solenoid to separate the batteries
...
If memory serves me correctly, that is the brand that was used in motorhomes to separate the coach and chassis batteries, and with a switch on the dash for an auxiliary start mode.
 

SuperDave

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That grey solenoid Fury Fan posted is an 85A version of the 65A one I have my eye on, same brand too, Cole Hersee. That would work, it'll do what I need it for.

@fury fan what do you want for that grey solenoid, the clear fat fuse & the 6 lug strip? I have to power 4 amps and tie in a big capacitor, that black 6 lug terminal strip would do the trick. What do you want for them? Shipped to Texas, zip code 75686.
 
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fury fan

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If memory serves me correctly, that is the brand that was used in motorhomes to separate the coach and chassis batteries, and with a switch on the dash for an auxiliary start mode.
Could be. I was lookign for something else and the Intellitec name popped up, and it was a throwback for me for sure.
I don't remember the aux start feature, we wouldn't have used it anyway.
 

SuperDave

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If it were me, carry a set of jumper cables - cost will be the same, and how often will you really use them?
When I lived in Florida, they were used every other week. Lots of dumbasses in the sunshine state. It is ground zero for Florida-Man after all. Not to mention all the tourists who forget to pack their brains for their vacation. In the 2 years since my return to Texas, I've only used them maybe 3 times?
 

fury fan

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Shipping will be $17 by UPS (2-3 day delivery) or $12 by US Postal (8-9 day delivery).
How does $17 + shipping sound? Less than Amazon overall and gets you some extra goodies.
Send me a PM and we can work out details.
 

1970FuryConv

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No doubt! Takes a special sort to love them big body Mopar's most others would pass by without a glance, and the fellowship here on this site is above and beyond what one would find for other classes or body styles.
I have taken my 70 Fury convertible and 72 Fury coupe to the local cruise-in. Both cars get a lot more attention than I expected, especially the vert.
 

John Kirby

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Any battery mounted inside the vehicle needs to be vented to the exterior. The fumes from a charging battery can be toxic. Gen 3 Challengers use a battery mounted in the trunk. They have a vent tube exiting through the floor for this reason.
 

'66 Fury I

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Fumes from a battery are also flamable and explosive. As well they are corrosive. When I bought an '06 Charger, the battery was not properly vented. The fumes had seriously corroded the terminals in the trunk mounted fuse box and caused serious problems, but that is another story. Enough to say, venting is very important, just as John said. Lindsay
 

SuperDave

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That's good advise, @John Kirby & @'66 Fury I thank you. I hadn't considered that. I already have large grommets ordered for wiring, a hole saw assortment & I can get a length of 3/4" plastic corrugated conduit I can use to plumb an exhaust port from the trunk battery box to the floor behind the box to vent fumes out & hide it from sight.
 

3175375

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That's good advise, @John Kirby & @'66 Fury I thank you. I hadn't considered that. I already have large grommets ordered for wiring, a hole saw assortment & I can get a length of 3/4" plastic corrugated conduit I can use to plumb an exhaust port from the trunk battery box to the floor behind the box to vent fumes out & hide it from sight.
Or get an Orbital sealed battery…
 
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