Hey guys

I am considering using the battery charge lead on the 7 pin connection on my 2002 F250 to charge my boat batteries while I am towing.

I have the towing package on this truck and if I understand the way these plugs are wired up, a relay controls the charge lead so no power is there if the truck is off. To me, that says that there should be no connection between the batteries when the truck is off so the truck battery can't discharge into the boat batteries or vice-versa. Does this sound right or do I need to install an isolator somewhere to prevent this? I figure some of you guys that tow RVs around would know this.

Thanks

Scott

 

 

[updated:LAST EDITED ON 01-16-03 AT 12:29 PM (EST)]You are correct the relay is energized in the "run" position to send power to the trailer connector. The power from the truck actually charges the batteries on the trailer, or boat in your case. If both batteries (truck and trailer) are in good conditions and all is well with your electrical system you will have no problems. Now take this scenario, the battery on your truck for some reason becomes weak, old age, lights left on, bad battery connection, etc. You go to start your truck with your boat connected, when you go to start the starter relay clicks, but not enough voltage-current to start the truck. The second you release your ignition switch to "RUN" the relay closes (relays will close with as little as 6-8 volts, they pull very little current) and now the boat battery is powering your truck accessories (blower motor, lights, fuel pumps etc). The wiring for the trailer connector, trailer, boat, etc. is not designed to handle this type of current. So now you have overheated wires with melted insulation. Without getting too deep in electronic theory you can make isolation circuits with fuses, relays, and diodes. Most RV travel trailers have these circuits built in, mine does. In my opinion: Yes, you need a isolator to protect your expensive toys.

ON EDIT: http://www.hellroaring.com/rv.htm#BasicAuxiliary

Scroll down to "Trailer/Camper Mounted Battery Isolation"

GAV10 Mark

 

Good response Mark! Thanks for the link. It looks like it will answer a lot of questions I have in the back of my mind.

 

>
>Good response Mark! Thanks for the link. It looks like it
>will answer a lot of questions I have in the back of my
>mind.

Arco, which sells aftermarket charging and starting parts for boats sells an isolator. Basically a big diode in a heat-sink. You can easily
mount it inside the boat to accept charging from outside, and you could might want to use a battery switch to fully disconnect the boat from the truck before pulling the trailer connector (sparks).

Other companies also sell them. Any big boat supermarket will probably have them in stock.

art k.

 

Cokeman,
You are correct!. No isolator needed. Ford gives it to you free.

Gav10,
Do you have any experience testing your theory? I think it would be interesting to know. I do not believe the relay would energize at anything less than 10.5 volts. Interesting.




99.5 Ford Super Duty F-250 XLT 4x4. S/C L.W.B. PSD Auto loaded.
4 in. Pro-Comp lift. 35 x 12.5 x 16.5 BFG AT/KO. Weld SSII 16.5 x 9.75. Rancho 9000's Dual Front.
Warn M12000 in Semi-hidden mount. Astro-start Supreme 3006.

 

BlueOvalBoy,

A guy I work with had this situation happen to him. He has a 1999 F-150 with the trailer tow package. He had a wire running from the trailer connector to his bass boat to charge the trolling motor battery. He had it rigged up so he could unplug the trolling motor from the plug mounted in the front of the boat and plug in a harness he made from the 7 pin truck connector to the boat mounted connector. This way he charged his trolling motor battery anytime the truck was running. He used this set up for 2 years with no problems. Then one day the harness overheated and burned up. Made a real nasty mess on the astro turf carpet in front of his boat. He had left his dome light on the night before and the truck had to be jump started the next day. The scenario I have listed above is the only thing we could come up with. We spent hours going to wiring diagrams etc, trying to figure out what happened. The odd thing was the fuse never burned out. My theory on that was the wire he used was only 18 ga. so it could not handle the current and it overheated. Since it was not a short to gnd, wire laying on fiberlass and carpet, the fuse did not blow. After this incident he installed a isolator between his 2 boat batteries and never connected back to the truck.

The "pull In" voltage for most relays is 50% to 75% of the control voltage, on large commercial airliners anyway. They do this to prevent relays from dropping out when heavy loads are applied to electrical circuits. I have a variable output DC power supply around here somewhere. If I can find it I'll pull a relay out of my truck and see where it exactly pulls down.

GAV10 Mark

 

Great readin here! But why didn't the fuse in the truck blow when the load from the boat became high enough to melt his boat wiring? Seems like an easy solution would be to use wire rated for the fuse size, like we do in the electrical business. I don't know the size of the fuse Ford used, but it should be easy to find out.

The other alternative, if you still wanted to use #18 wire, would be to put an inline fuse near the 7 pin plug. This should be done anyway, since a short at the boat (pinch the wire in the battery box cover, or something) would give the same unpleasant results - meltdown of the #18 wire, unless the Ford fuse is low enough to pop before meltdown.

Sure is a handy way to keep those trolling motors charged tho!

Oldchap

 

The only thing I can figure out is the fuse is rated at 30 amps and the wire was 18 ga. The wire should have probabaly been a 10 ga. I know the root cause of the problem was his design flaw of using a small wire. The system he had probably would have worked good if he used a 10 amp fuse. To me I'm noy going to run the risk of trashing an electrical system on a 31K truck, or a 22K bass boat for a $77.00 isolator. With a isolator you are safe no matter what happens.

GAV10 Mark

 

Thanks for the info guys. I haven't decided what I am going to do yet. I was originally looking for a way to avoid buying a charger for awhile.

I may just go ahead and buy an onboard charger for the boat. If I just use the truck's charging system, then I have no way of knowing how "full" the batteries are unless I install a battery meter. So, I guess I still have some things to think about on this.

Scott

 

I suggest skipping the isolator. Reason is that they are basically a large diode and a diode will drop some voltage. This will result in insufficient charge to your RV/boat batteries.

Ford has already provided a RV/boat charge circuit. All you need to do is make sure the relay and fuse is installed in the box under the hood and then you'll have power to the aux batteries when the key is turned on. I can't remember the factory wire size, but it's definitely larger than the 18 guage that caused the other guy problems (I'm thinking it's 10 or 12 guage). BTW, you also need to make sure another relay and fuse are installed if you want your backup lights to work on your trailer/camper.