Electrical plan - dual batteries >20 feet apart

Hunky Dory

Petty Officer 2nd Class
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Jul 20, 2003
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102
So I installed a trolling motor near the front of pontoon's deck, primarily for limp-home capability. It draws 50 amps so the battery is collocated. Anyway, I park the trailer at a storage lot so keeping the dedicated battery charged will be a problem, even if unused. I want to fix that and to fully exploit the presence of the deep cycle battery for both common and emergency situations.

My thought is to install a battery selector switch at the helm just for lights and accessories. Trolling would be direct wired and fused at the front. The outboard would be direct connected at the rear. Both batteries would be tied to the selector switch with fuses at each positive terminal to protect the interconnecting cables. This would allow for absolute shutdown of lights and accessories, and in the 'both' position, would parallel the batteries for charging.

Trouble is that the two batteries are >20 feet apart and big cable gets costly. Maximum Evinrude charging current appears to be 25 amps which 10 gauge should handle but I am concerned about the implications of someday switching a fully discharged trolling battery into parallel with a fully charged starting battery. Additionally, it would be nice to have the deep cycle as cranking backup of last resort.

It turns out that an affordable solution might be to use #4 jumper cable to tie the batteries to the switch. I can easily swap clamps for lugs and route the cable through the deck within PVC conduit along the top of a pontoon to the helm but I am very uncomfortable with using cranking-capacity fuses in this extended lashup. And there may be other "marine" considerations I have missed.

What do you think?
 

BobGinCO

Chief Petty Officer
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May 22, 2012
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539
Re: Electrical plan - dual batteries >20 feet apart

What do you think?

That 20 feet of 4 guage wire isn't THAT expensive. I would use a Blue Sea Add-A-Battery to keep them both charged, and to be able to select between them.

Shoot, I just checked, and for 25 Amps, at 20 feet, you can get away with 10 Ga wire! Oh, I see you already figured that out.

I see you're also in COlorado. Are you in the Front Range?
 

Rancherlee

Chief Petty Officer
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Jun 6, 2006
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621
Re: Electrical plan - dual batteries >20 feet apart

I've been doing what you first described for years. I have a 10 gauge wire with a 30amp fuse ON EACH END running from the rear battery to the front battery with a switch that allows me to charge the front battery. ALL accessories are run off the front battery including a 55# minnkota. I've never popped the 30amp fuse even if I forgot the switch on before starting the engine. A fully drained lead acid deep cycle still only accepts about 15-20amps max while charging so 10gauge/30amp is plenty. I've taken a full drained group 27 deep cycle and hooked it to my battery charger which has a 50amp quick charge mode and the amp gauge only shows a 20 amp charge and that's with the charger pushing 15v+ at it so no need to worry about flipping to switch on a dead front battery. If you have an Orbital, Optima, or other fancy deep cycle then I don't know as I don't know how much charging amperage they draw.
 

BobGinCO

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May 22, 2012
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539
Re: Electrical plan - dual batteries >20 feet apart

a 30amp fuse ON EACH END

As a former electronic engineer, I would say that's redundant, and unnecessary. As a practical guy, I realize that it's a really good idea, because if you pinch the wire, and short it in the middle, BOTH fuses would blow, protecting both sides of the circuit.
 

Rancherlee

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Jun 6, 2006
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Re: Electrical plan - dual batteries >20 feet apart

Not redundant as you have a Power source on both ends of the cable or so I was taught in Auto Mechanics. When you have remote mounted batteries such as RVs and Car Audio installations. Wires can rub through or crack on a long run like that and could possibly start a fire if not protected on both ends such as this with batteries linked by 20ft of wire. 3$ for a 10gauge fuse holder is worth it IMO.
 

jamoke

Cadet
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Apr 15, 2008
Messages
22
Re: Electrical plan - dual batteries >20 feet apart

Tooned Out - Wiring Mains Diagram.jpg

I've wired my boat exactly as suggested, attached is a diagram.
Used 6 gauge wire and 40 amp breakers at each battery.
 

Hunky Dory

Petty Officer 2nd Class
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Jul 20, 2003
Messages
102
Re: Electrical plan - dual batteries >20 feet apart

View attachment 151250

I've wired my boat exactly as suggested, attached is a diagram.
Used 6 gauge wire and 40 amp breakers at each battery.

Wow, nice diagram, great post. Nice to know that it is a viable scheme. Have you ever tried jump starting through those 40 amp fuses?

Sorry for the delay, a minor surprise kept me offline. And yes, I am on the front range, boating primarily in Horsetooth Grand Lake or Granby (via trail ridge). Wonder how long we will have water this year?

And thanks for all the other counsel. I was surprised nobody zinged me on the idea of #4 jumper cables. I'm guessing that it would be unrated cable but am presuming that the jacketing would be tough.
 

gpfishingdude

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May 1, 2012
Messages
538
Re: Electrical plan - dual batteries >20 feet apart

I was trying to ask but my question seems to be going into cyberspace somewhere. Is the negative buss bar grounded to the frame of the boat? I always thought that in a 12V circuit that the negative just returned to the battery. Could the part of the circuit with the shunt-meter-and negative buss bar be eliminated without changing the integrity of the circuit? I have to tell you that I think that is one of the neatest ways of showing a circuit that I have ever seen.
 

5150abf

Vice Admiral
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Aug 12, 2007
Messages
5,808
Re: Electrical plan - dual batteries >20 feet apart

NO, never ever ground to the boat, they always run a closed system not an open ground.

I worked on a boat that the guy did that to and it caused terrible electrolosis, little white pillars all over his tubes, it was ugly.
 

Silvertip

Supreme Mariner
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Sep 22, 2003
Messages
28,758
Re: Electrical plan - dual batteries >20 feet apart

That 20 feet of 4 guage wire isn't THAT expensive. I would use a Blue Sea Add-A-Battery to keep them both charged, and to be able to select between them.

Shoot, I just checked, and for 25 Amps, at 20 feet, you can get away with 10 Ga wire! Oh, I see you already figured that out.

I see you're also in COlorado. Are you in the Front Range?

10 gauge wire won't cut it if you intend to start the engine from the HOUSE (troller) battery). That scenario is very real with a dual battery switch. Those switches are designed to handle 300 - 350 amps so they allow powering everything from either or both batteries, starting the engine from either or both batteries, and charging either or both batteries. Consequently the interconnecting cables need to be battery cable size -- not 10 gauge.
 

Rancherlee

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621
Re: Electrical plan - dual batteries >20 feet apart

10 gauge wire won't cut it if you intend to start the engine from the HOUSE (troller) battery). That scenario is very real with a dual battery switch. Those switches are designed to handle 300 - 350 amps so they allow powering everything from either or both batteries, starting the engine from either or both batteries, and charging either or both batteries. Consequently the interconnecting cables need to be battery cable size -- not 10 gauge.

If he plans on using it that way, yes. For charging and general usage a 10 gauge with 30amp fuse is more than enough. 8 gauge and a 40amp if he has 2 batteries up front to charge. If he forgets the switch the fuse will pop if the starting battery is dead. If the only thing on the starting battery is the engine/gauges then that battery should never go dead during an excursion out on the lake, and in the rare case that it does he SHOULD have enough tools with him to move a trolling battery to the back to start the engine with. If the starting battery died while you were out on the lake then either ALL the batteries are dead because you left the switch to both, or the starting battery had an internal failure and needs to be disconnected before flipping the switch anyway. A 10 gauge is alot easier to route/use then a 2 or 0 wire and you can still use small fuses. In the end he needs to set it up for his particular situation, just like I have. I'm rebuilding my pontoon right now and will be upgrading to 8 gauge on mine since I plan on putting buss bars inside my new console and tapping power for the radios, fish finder, lights, and pumps from it instead of running extra wires to draw off the front battery and 10 gauge with a 30amp might not be enough to handle charging the trolling battery up front and running several accessories at the same time. This also allows me to put the Battery switch on the console instead of the doghouse.
 

thebobbykey

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Apr 30, 2012
Messages
9
Re: Electrical plan - dual batteries >20 feet apart

Maybe while its on the lot you can connect a small solar panel directly to the batteries.
 

jamoke

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Apr 15, 2008
Messages
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Re: Electrical plan - dual batteries >20 feet apart

Wow, nice diagram, great post. Nice to know that it is a viable scheme. Have you ever tried jump starting through those 40 amp fuses?

No, never have run down the engine battery but I have run down the trolling/house battery many times.

Don't think the trolling battery would start the engine if the engine battery was flat. The 40A breaker(s) would probably trip 1st. Don't plan on trying this.
I keep a Jump-Start battery at the ready in case of emergency. So far, never needed it.
Would also be easy to swap the two batteries in a pinch.

Did a lot of calculations of load/distance/voltage drop and settled on 6 gauge wire with 40amp breakers.
Can run every light and electrical gizmo on board, stereo at full tilt, and the lights do not dim. Voltage drop is minimal.
I've never popped a breaker or fuse in two years. Very happy with this setup.
If I had to start again, I wouldn't change a thing.
 

Hunky Dory

Petty Officer 2nd Class
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Jul 20, 2003
Messages
102
Re: Electrical plan - dual batteries >20 feet apart

Well, thanks for participating -- all the great advice has left me paralyzed with indecision but I have ordered some of the parts and it will eventually match the diagram (except for shunt and meter -- I might forgo amperage measurement).

Cable size is the issue. I get enamored with 'do everything' solutions even when obviously ridiculous. But in this case, the #4 jumper cable also wins the cheapo contest, since the manufacturers can buy that stuff, cut it up and eBay it to me at so much better a price than I can acquire custom cut #10 marine wire. Wish I could touch and feel the cable in advance, it needs both flexibility and abrasion resistance which are features of some jumpers, but certainly not all.

"This life's hard, man, but it's harder if you're stupid!" All of this is about the dread fear of leaving some tiny load switched on and draining the batteries to zero while just sitting on the trailer. But I cannot conjure a solution that my selfsame scatterbrain cannot defeat. It has occurred to me to add a timer controlled relay between the 1/2/both/off selector switch and the front battery so that it will automatically shutdown in the likely event of a ID10T failure. But the relay itself would certainly impact the half-life of the battery under normal use. Sigh.

This forecast of yet another stupid human trick might seem too personal, but I'll bet that somewhere this very day, there is a boat about to be launched with not just one, but two batteries -- both dead.

Likely, I will keep carrying a booster pack and hope I've remembered to charge it...
 

gpfishingdude

Chief Petty Officer
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May 1, 2012
Messages
538
Re: Electrical plan - dual batteries >20 feet apart

Can't you just run two separate circuits? One for the starter battery and one for the trolling motor batteries? That way you don't have to worry which way the switch is turned. I figure on doing that and charging them with a dual bank charger or charging each battery separately. I have never had a starter battery run down yet.
 

Hunky Dory

Petty Officer 2nd Class
Joined
Jul 20, 2003
Messages
102
Re: Electrical plan - dual batteries >20 feet apart

Can't you just run two separate circuits? One for the starter battery and one for the trolling motor batteries? That way you don't have to worry which way the switch is turned. I figure on doing that and charging them with a dual bank charger or charging each battery separately. I have never had a starter battery run down yet.

I'm not certain that I follow but I seem to recall dire warnings about running the engine with the battery disconnected. If I want to be sure that I don't do that accidentally, then I have to make sure that I can't do that accidentally. Murphy is always aboard.
 

gpfishingdude

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Messages
538
Re: Electrical plan - dual batteries >20 feet apart

I just wired mine with one switch block for the starter-running battery. The only things I have connected to this switch block are lights and fish finder- low amp items. I have two deep cycle batteries in the front. One is for the trolling motor and the other for the anchor winch- they both are designed to hook right up to the power source type of battery boxes. I decided that it is much easier to run a live well aerator off of flash light batteries. I don't have a big stereo amp to worry about drawing a lot of amperage so it is easy for me to get by on the set up I have. I don't need a big power switch to complicate things. They make battery chargers that will charge both of your batteries without having an isolator switch in your circuit- they are called dual bank chargers I bought one of them instead of the isolator switch.
 
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