12 volt power while trailering

[Longbranch Louie]

Capt Macs comment on his batteries almost keeping up with the frig while trailering made me wonder if anyone has ever figured out a way to charge their batteries while the boat is in tow by tieing into the line on the trailer that is used for the tugs stern marker lights? I dont know what would be needed or if it can be done. Dont RVs do this? Or perhaps a switch on the frig line so tow rig power could be routed to the frig while on the road or at a no services camp site ???

 

[Todd]

You can defiantly power the boat's 12 volt system from the tow vehicle's electrical system but some caution is needed. You need to install a resistor in the wire to limit the maximum current to a level that the wire can handle. If you don't when you make the connection the battery with the greater voltage will send a large amount of current to the battery with the lower voltage. That is why jumper cables are made from real thick wire!

The actual connection may be simpler than you imagine. The 7 way plug has a wire for this purpose. It is normally used to charge the break away battery, if one exists, as well as other accessories that may be installed in the trailer.

 

[RProffer]

This link shows the wiring diagrams http://www.etrailer.com/faq-wiring.aspx

 

[knotflying]

I would be very careful about using the trailer wiring to charge your boat batteries while trailering. I remember starting a thread on this a while back when I had the same thoughts. After doing some investigation on this, the correct thing to do would be to have an appropriate sized cable run straight off your vehicle battery to a connection by the hitch then have the line run to the rear of the trailer and then you can always have clamps or make a positive quick release connection on the wire ends and attach them to your house batteries. The point here is that once you connect the batteries directly they will also be attached to the vehicle’s charging system. This is important because the voltage regulator will disengage charging once the batteries are topped off and avoid overcharging. Additionally having the appropriate size cables to the batteries will avoid a definite overheating and even a possible catastrophic even to the wiring.

 

[Newtugnut]

Knotflying has the right idea. Further, when stopped for the night disconnect the charging cables so that the boat house set is not in parallel with the vehicle engine - otherwise circulating currents, possible vehichle battery discharge.

 

[Osprey]

Charging travel trailer and fifth wheels trailers via the tow vehicle’s charging system is standard. The truck’s outlet should already to wired to allow such charging, but the boat & trailer will not be. One would need to modify the trailer’s power cord wiring and make a connection to the boat’s batteries. Travel trailers generally use something like AWD 8 or 10. The truck’s regulator will prevent any over charging. One item that might have to be added to the truck’s wiring is a solenoid in the charge line to the trailer. The solenoid only connects the truck’s battery/charge system to the trailer’s when then the engine is running. Without the solenoid the trailer could draw down the truck’s batteries, if connected, with the engine off – say overnight. Again this is standard for RV applications. Speaking of RVs, any RV dealer would be able to help you on this matter.

 

[Longbranch Louie]

Ya my fords 7 prong has a 12v supply line for charging Rvs. I think it has a 30 amp fuse and a relay and only connects when the engine is on. I dont know how it knows which batteries need a charge and at what rate.
My trailer came with a 5 flat so I would have to rewire the connector and run an appropriate guage line back. Id only have it done by a qualified person. Maybe a charging wire could be included on the marker line harness by the factory at time of build. Seems alot of people use their boats as an RV while trailering and it would be nice to charge via a tow rig.
Speaking of connectors I was having problems with my surge brakes not releasing while backing and learned that the center prong on my trucks connecter had corroded just enough to not allow constant power to the blue wire (backup lights) that deactivates the surge brakes when backing. The brakes kept randomly locking while backing. By cleaning the contacts and reseeding the lawn I was able to correct the problem.

 

[Osprey]

Following up on Longbranch Louie’s comment …“I don’t know how it knows which batteries need a charge and at what rate…”. The truck’s alternator is connected the truck’s battery via fairly short, HD wires. The regulator monitors the truck battery voltage and therefore the charge rate. With the RV setup, a long wire of about AWG 10 runs from the truck battery, via a fuse and solenoid, all the way back to the boat batteries. Due to this long run back to the boat batteries the voltage to the boat batteries is a bit less than the charge voltage to the truck batteries. In effect the truck battery is not overcharged and neither are the boat (or RV) batteries.
For the last 15 years I have towed a fifth wheel over much of western North America as far south as Mexico and as far north as the North West Territories. Overcharging of the trailer batteries has never been an issue.

 

[Todd]

All the RVs being discussed were designed to have the batteries charged by the tow vehicle. That is they have the proper resister in the line to avoid an over current situation. The problem is not over charging, the truck regulator will handle that. The problem is that if the boats battery is real low, when the connection is made, it will act as a large AMP draw which could blow the fuse, or damage the wiring if not properly fused.

A simple solution would be to install a 12 volt accessory socket, cigarette lighter socket for us older folks, in a convenient location on the trailer. Available from Radio Shack or any automotive store. Fuse it for 10 amp and connect it to the accessory wire on the trailer plug.

Then buy one of these cheep "jumper" wires sold at auto stores designed to charge one cars battery from another. It is a wire with a 12 volt plug at each end. you connect the cars 12 volt accessory sockets together and after an hour or two you can start the car with the previously dead battery. The wire has all the necessary resistors to prevent the over current situation described above.

You could just plug this "jumper" wire into the accessory socket on the trailer and to one of the accessory sockets on the boat. Make sure the boat socket is permanently hot and not switched, and the batteries will be safely recharged from the tow vehicle.

 

[Osprey]

Consider the resistance in the “charge line” between the truck’s and boat’s batteries. For a complete circuit we need to look at the total distance from the truck’s battery to the boat’s batteries and back. For a Ranger Tug this could be perhaps the equivalent of 100 feet or more. i.e. from the front of the truck to the back of the boat and return. Using AWG 10 the wire alone has about 0.1 ohm resistance. (Yes, we only run actual wire one way and rely on a combination of wire and frame conduction along ground paths back to the truck battery, but the resistance of this path is likely to be more than AWG 10 copper wire. Ground contacts are frequent trouble points). We would also need to include contact resistance for the number of contacts along the line. Another 0.1 ohm would be a significant understatement of the total contact resistance. The voltage loss at 30 A (the typical fuse rating) would be 30A x 0.2 ohm = 6 volts. So while the voltage at the alternator may be perhaps 13.8 volts or so, the voltage available to the boat batteries would be only 13.8 - 6V = 7.8V. The obvious conclusion is that this setup is simply not capable of sending anything near 30A to the boat’s batteries. Even if the boat’s batteries were discharged down to 12V this setup would not be capable of delivering anything more than about 10A or even less. This is sufficient to keep the fridge running plus provide only a very modest amount of battery charging. To recover from a near fully discharged boat battery another charging method needs to applied.
Fusing: The 30 A fuse really is present to protect against a faulty short to ground along the charge line.

 

[Todd]

As stated in the previous post, the frame is not a good reliable ground. Especially the trailer ball which can be dirty and rattling around during transit. You need a good ground which the trailer plug should provide. This ground will have to carry the current load of the entire trailer, lights, breaks, accessories, and the boat charging circuit. Therefore the charging circuit needs to be limited to well below the 30 amp capacity of the wire in the connector, Usually #10, rated at 30 amp. To do this additional resistance is added to the charging wire to limit its ability to transmit current.

In the last example we assumed bout 200 feet of #10 wire or 0.2 ohms. My R29 tows at about 60 feet so it may be a bit much. With the vehicle running the voltage is around 14. If the boats batteries are at 10 that is a 4 volt potential. Current = Volts / Resistance. In this case 4 / .2 = 20 amps. Add that to the trailer draw and you may exceed the capacity of the ground, and if that wire fries you have no breaks. This situation would be worse if the resistance is less than the 0.2 ohm estimate or if the boats batteries were at less than 10 volts. Each a possibility. You may want to reconsider this hook up and use one with a dedicated resistor in the line to limit the current to lower levels.