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Old 04-15-2015, 11:36 PM   #1
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Electricians: help me thru this project

Hi all. I have a trailerable tug that we tow between FL and NY. Takes about 3 days. During the days while she is in tow, I want to keep the batteries charged up. There is always battery drainage from the CO detector, fridge, etc and I sleep on her at night while underway.

So, here is an idea. I'm thinking of running a lead from the truck's alternator back to the boat and allow the truck's alternator to charge the boat's battery banks. The alternator is running anyway and there shouldn't be much draw on the truck as I drive those 10 hours each day. I have accessory switches / connections under the hood that I could tie into on the truck side and I'm thinking the easiest place to tie into on the boat side would be at the alternator connection on the boat (single engine inboard).

Would this work? Are there any diodes or check valves I need to think about with this type installation? Really, just brainstorming right now so I'm open to all ideas.
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Old 04-16-2015, 12:30 AM   #2
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Yes it would

You'll need battery and ground.

I would install an ACR

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Old 04-16-2015, 12:49 AM   #3
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You might need some pretty large cables. What kind of amp load are you looking at?
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Old 04-16-2015, 12:56 AM   #4
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Does your truck have a factory installed trailer tow package? If so there may already be provision for that function. If you are unsure then check it out and be sure as if it is present it will not take much to finish the job to the boat.

It is doable either way but hopefully the tow pkg. is present.
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Old 04-16-2015, 01:06 AM   #5
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The truck has a single 180 amp alternator and it also has a 7 pin trailer connector and the boat is already equipped with ACRs. We are a Ram 3500 towing a 30 ft boat. Since it is just twice a year we do this, I'm thinking a 50' 10 or 12 gauge extension cord with the ends modified to tie into each end of the DC systems.
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Old 04-16-2015, 01:12 AM   #6
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External amp load on batteries while charging them and required gauge of long cables should be calculated even with a tow package ready setup.

Here is a video that might help you start evaluating your tow vehicle ... ... there is more and others videos, just google it for some ideas.
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Old 04-16-2015, 06:38 AM   #7
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IF you have the 7 pin connector you are already able to power the boat.

Just get the schematic for the 7 pin and hook it up.
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Old 04-16-2015, 07:21 AM   #8
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Does anyone know if the power on pin 7 is current limited by something other than just a fuse? After a night on the "hook", you boat's batteries will be drawn down in some measurable way, and wanting to accept a lot of current. If the truck doesn't have some sort of current limiting circuit, you may find that you blow the fuse.

I know a number of people here have RVs as well as boats. Anyone got any experience with this? I image the same situation occurs when you park your RV overnight and are not connected to dock power. And how much current can you typically draw from pin 7?
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Old 04-16-2015, 07:49 AM   #9
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As others have noted this is done all the time w/ 5th wheels & motohomes w/ toads...
knowing amperage draw is a good starting point.
if you want to protect the "systems' from overloads here's a sample of a product designed to do what you are considering... TOAD CHARGE

Lots of other similar products out there...
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Old 04-16-2015, 07:55 AM   #10
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Another option may be to use a 12 volt to 12 volt battery charger. These are used to take power from one system to charge another. The advantage to doing it this way is that the charger which would be mounted in the boat, will boost the voltage that is lost in the long cable run to properly charge the boat battery. Secondly, if you used one with a 6 amp charge rate, it becomes the amperage limiting device. So it might draw only a constant 7 amps until the boat battery is completely recharged.

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Old 04-16-2015, 08:09 AM   #11
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This....

20145DDC


Wired from truck battery to boat battery (with a plug for disconnection) wired with wire size for up to 30 amp (max output of Duo-charge).

Will only charge when truck is running and automatically disconnected when shut off, has several smart charge profiles for all types of boat battery's.

It is a 12 volt to 12 volt smart charger.

Plenty of power available for the boat needs while trailing.

Easy 2 wire hook up, boat battery will need to be connected to tow vehicle ground with same size wire used for positive.

Fuse within 6 inches of pos. battery connection with recommended (30amp) size fuse.
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Old 04-16-2015, 01:00 PM   #12
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The truck has a single 180 amp alternator and it also has a 7 pin trailer connector and the boat is already equipped with ACRs. We are a Ram 3500 towing a 30 ft boat. Since it is just twice a year we do this, I'm thinking a 50' 10 or 12 gauge extension cord with the ends modified to tie into each end of the DC systems.
------------------------------------------------------------------------------------

Good to know. You should have no trouble rigging up the final connections to the boat batteries using/adapting what you have now through that 7 pin plug/receptacle.

I am assuming that the 7 pin was part of the Dodge package, CORRECT?

I do not have close at hand the plug/receptacle configuration but you already have the output for the battery charge function. The dwg. may be in your truck manual. If not then go online and find it.

I have a Ford with the trailer towing package and mine is the 7 pin also and again with the battery charge option.

Don't reinvent the wheel. You may cause more trouble with additional wiring and stuff if you go straight from the truck. If you do that you will need an isolation relay, finding an ignition switch + source that is on only when the truck runs, fusing and much heavier wire as there will be no current limiting when gone direct.

Other than the need to get from that 7 pin plug your boat/trailer will be no different that the thousands of travel trailers and 5th wheel trailers already using that setup.

All you should need to do is open up the trailer side plug and add the wiring to the boat batteries from there. If that cable runs to a jucton box then open that up and add the extra wire from there. From that plug use a dedicated pos. and neg lead of #10 AWG wire. and use a plug/receptacle setup at the batteries. NOT alligator or bulldog clips. If they fall off you will lose the battery charging and may cause a short cct.

I also suggest you go directly to the boat batteries, not to the boat alternator. The fact that you have ACR already is good. Let them do their job and all batteries should be recharged as you travel.

Your towing package will also limit the charge current to about 30A so you do not have to fool with any more fusing, relays, or extra heavy wire. It's already present. The towing package also isolates the truck from the trailer when the truck is shutdown.

If you go direct you will need heavier wire, you open the possibility of flattening the truck batteries for a no start by bypassing the built in isolation feature.
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Old 04-16-2015, 03:34 PM   #13
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It still boils down to the max amps you're going to draw, for how long and what the distance the current has to travel.

If it's say 30A for 60 minutes you're looking at #6 wire based on a 20 foot round trip.
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Old 04-16-2015, 11:36 PM   #14
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Normally I would agree with that yet he is asking about a setup that is used in literally thousands of various types of trailers successfully everyday.

The voltage drop will reduce as the batteries recharge while travelling. The truck will itself limit the current to ~ 30A. I do not know specifically about the Dodge but I believe 30A is typical for Dodge, Ford, Chev.

It will work for him without altering the system except where he has to go to the boat itself.
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Old 04-17-2015, 06:52 AM   #15
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Old 04-17-2015, 08:24 AM   #16
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If he has electric brakes then the battery charge output from the 7 pin plug is already wired to the breakaway battery on the trailer to keep it charged.

Saying that I don't know of any boat trailers using electric brakes, salt water would not be kind to them.
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Old 04-17-2015, 02:40 PM   #17
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I agree about the electric brakes on a boat trailer. Since I am NOT boat trailer friendly
I will bow out on that. He can confirm how the trailer brakes operate.
On a travel trailer/5th wheel with electrics that would be the case.
He may have hydraulic brakes which are electrically operated [the pu mp] then the battery and safety pull switch could still be present.



CAPT. BILL 11 You would be absolutely correct if he cobbles something on his own as that would bypass the truck control/safety which is built in.

The current could then be much higher untill the boat batteries gain some charge. May not last long but could be high enough and long enough duration to be trouble.
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Old 04-17-2015, 06:28 PM   #18
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The solution I presented would not care what the state of the boat batteries were and they would be charged properly and would not be over charged or be dependent on the trucks charging system regulation.

The batteries would be charged by a 12/12 battery charger providing up to 30 amps.

Wiring size would be simple just look at a chart for 30 amps and adjust for length then select wire gauge.

When the truck was shut off there would be no back-flow so no worry about depleting the truck battery when engine was not running, charging would stop when the engine was turned off and resume when started again, completely automatically.
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Old 04-17-2015, 07:14 PM   #19
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If it's a Ranger Tug, look in the anchor locker. There is another hook up connector. I believe (call the factory) that if you connect that one, not only will the lights on the boat work as extra running lights, it will also charge up. But, call the factory to make sure.
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Old 04-18-2015, 12:22 AM   #20
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Great discussion guys. Thanks for the ideas. The 7 Pin connector is Ram standard equipment and they also provide a 5 pin connector. I pick up the truck on Monday so maybe I can talk to the mechanics there to see what insight they have. Nice if I could not mess with the 7 Pin and let it do it's thing with the brakes (electric / hydraulic) on the trailer. Meanwhile, maybe I can use the 5 pin for the battery charge function. I didn't know there were 12v-12v chargers. I knew I did not want to go thru the 12v-120v-12v charger option for sure. It is a R31 Ranger Tug so I'll look in the anchor locker to see what surprises I may find in there. Keep it coming....
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