Charging boat battery with truck while driving

[Wave34]

Sometimes we tow for several hours, like 8 hours, and have to sleep en route in a WM parking lot, so, the fridges in the boat are on batteries for a total of 18 hours, and at destination voltage is low and fridges a bit warm.

I was thinking, since I have electric brakes on my trailer with a safety battery, that is charged by the truck, would it be ok to just install a DC plug on the trailer, and run a temporary cable on the bow, and under the cover to a cigarette lighter plug to keep my battery topped up?

I don't think there is a problem, but prefer to ask first.

 

[Scott Danforth]

If you have a 7-pin connector, you have 12 volts. Use a dc/dc charger to isolate

 

[fishrdan]

I used to "charge on the go" while towing my fishing boat, to re-charge the trolling motor batteries, so I don't see why it wouldn't work for your application. I don't think the cigarette lighter outlet is going to carry enough current for continuous use of running the fridge, I'd rig something with more current capacity.

You're going to want to use fairly heavy wire for the jumper wire, to avoid voltage drop. I used 12ga wire for the 10' run from to 7-way outlet to the battery. That wire was undersized, but worked...

 

[Wave34]

If you have a 7-pin connector, you have 12 volts. Use a dc/dc charger to isolate

Yes I have 12V.
Since when the truck is running on the road, there will be like 13.8V at the outlet, that should do the job without a 12V-12V charger. (except when I stop to sleep at night)

I was more worried about the boat side, the alternator, 120V-12V charger and so on.
May be using the cigarette lighter plug is not a good idea if the wiring is not big enough.

 

[Wave34]

I used to "charge on the go" while towing my fishing boat, to re-charge the trolling motor batteries, so I don't see why it wouldn't work for your application. I don't think the cigarette lighter outlet is going to carry enough current for continuous use of running the fridge, I'd rig something with more current capacity.

You're going to want to use fairly heavy wire for the jumper wire, to avoid voltage drop. I used 12ga wire for the 10' run from to 7-way outlet to the battery. That wire was undersized, but worked...

LOL, you were answering my concern about the cigar lighter while I was typing.

 

[Scott Danforth]

get something like this. https://rvibrake.com/products/towed-battery-charger-plus

that way it charges and current only flows when needed.

 

[ahicks]

This is not a lot different than what you see used on most travel trailers going down the road. You seem aware regarding what needs to be done. I would just suggest you consider a relay (if there isn't one already) installed in the engine compartment of the tow vehicle. Purpose would be to avoid the boats battery bank from draining the truck batteries when the truck isn't running. Make sure the relay is either of the latching type, or constant duty. Something like a starter relay will over heat and burn out in very short order.

 

[Scott Danforth]

or use a dc/dc charger.......

 

[dingbat]

The 7 pin works for the breakaway but you’ll need a dedicated connection to charge a larger battery with an unregulated connection

To power my wife’s horse trailer (fans, lighting and small fridge), I installed an dual battery isolator in the trailer and use my power winch (60 amp) connection to power it.

 

[Lowlysubaruguy]

Two ways to do this the right way requires a trailer share circuit powered on or off via a continuous duty solenoid or isolation module. I prefer the solenoid it’s cheaper easier to hook up part is sold at NAPA and other shops for $35 v a $100 plus dollar isolation module that overheat fail stick on or off and requires more complex install.

The cheap way run a 10 gauge wire from the tow vehicles battery route it accordingly down the frame along the hitch tongue and into the boat put a 30 amp fuse right at the battery and disconnect it or put a HD switch there. This is simple if you are not skilled enough to wire a solenoid or switch too accessory power. Use alligator clips at the battery on the boat if you want to disconnect them without tools but I would put eyelet connectors on them so there not falling off in route. This is actually fine for long term towing however it will not disconnect at the trailer if the need arises or it comes apart when towing but I think you have more to worry about than that one wire thats fused if it actually came loose towing.

Obviously your going to have to hang this wire over the edge of the boat any way unless you planned on mounting a plug at the boat for it so I dont think you have to get exotic. Meaning routing this through your tow rigs trailer plug isnt really necc and if your running a 4 or 5 wire flat rubber plug any way that plug isnt heavy enough for long term charging they will burn out.

How to wire a solenoid. Mount it close to the battery but in a safe place youll have two powered points of contract supply fused battery power to it with 10 gauge wire. Then run the output wire neatly to the back of the truck. You could make a connector here or leave it one piece roll it up tightly put it in a zip lock bag and stuff it int he back of the tow vehicle with the positive terminal securely covered. To turn on the solenoid youll need too locate an accessory power circuit. If you have a late model SUV or truck many have open places on the fuse block that you can buy a pre made pig tail for make sure it’s wired for the right spot and use this to turn on the solenoid it needs to be fused as welll usually 7 to 10 amps or even less. Do not hook to an ignition circuit on powered in accessory. Very simple last a long time. If you put a connector at the back of the truck it needs to be one that can handle a higher amperage and continues load charging a dead battery on a long drive can repack havoc on small wires and light connectors. Myself I prefer 8 gauge but wiring has increased in price a lot 10 gauge is adequate.

 

[Mischief Managed]

Best solution is a DC to DC charger and a automatic charging relay (Blue Sea makes nice ones). You don't want to directly connect the batteries together unless you use wires that can handle the starting current of the truck, and you don't want the boat to discharge the truck battery while the truck is not running.

Running your boat battery below 50% state of charge is really bad for it. Also, the typical trailer boat alternator is pretty weak and will take many hours of continuous running to bring a discharged battery back, even longer if you have more than one battery. May want to avoid the whole situation and pack the cold stuff in a cooler for the road part of the trip.

 

[H20Rat]

Do not underestimate the size of wiring you will need! DC power doesn't like being run long distances at high current. The largest 7 pin connector I could find was 40 amps, so using that as the absolute max, you will need 2 gauge wire to safely run that with 5% voltage drop. Don't forget that the distance for the ground counts also, and do not use the trailer ball connection as a ground. I'm just using a swag that you are going to have at least 100 feet of battery to battery distance, including ground. (30*2 for in-boat wiring, 20 ft for positive in truck, 10*2 for the length between boat and truck)

Run the numbers yourself before doing anything.

https://www.wirebarn.com/Wire-Calculator-_ep_41.html

 

[AShipShow]

I'll repeat what others have said, the safest, cleanest, and BEST way to do this is with a dc-dc charger. It would be a fairly simple thing to build yourself to accomplish basically the same thing without the integrated safety/charge control circuitry, but, your towing something that my guess is, is worth more than the 30 dollars you would save to make something yourself....

 

[Wave34]

Thanks guys for the suggestions.
I just found that my 2016 F150 with towing package has a relay that cut power to the 7 pin plug when the truck is not running.
So, that eliminates the problem of draining the truck battery when not driving.

What I also found is that even if the F150 has a 25A fuse on that circuit, the 12V wire is not that big compared to the ground. (normal since the ground is common for the brakes and lights also.)

Some F150 owners replaced the 12V wire with a bigger gauge one. I will have to crawl under the truck to see with my eyes.

I agree that it will take a fairly big and long ($$$) wire from the trailer plug to the whole length of the boat where the batteries are located. The boat is 26' so 2 wires of minimum 10AWG X 30'; total of 60' of 10 AWG multi strand wire, ouch!

I will have to calculate all that to see if it makes sense since we do this long trip only once a year.

 

[Scott Danforth]

100 feet of 10 gage wire is under $25
a DC to DC charger is about $30 for low end ones and $150 for really good ones

to simply maintain a charge of your on-board batteries while you tow, the existing wiring and 25 amp circuit are more than adequate to handle the intermittent 10 amp in-rush and 3 amp running usage of the refrigerators

tie into the 7 pin, run to a deutsch connector (IP67 rated). make a pigtail harness to plug into a bow-mounted Deutsch receptacle. wire the DC-DC charger to that receptacle.

so now, when you tow your boat, and you have the little extension plugged in between your boats bow and the hitch on the truck, your batteries are being maintained while you drive.

the DC-DC charger takes care of maintaining the battery, protecting the up-stream circuitry, tempering battery charge current, etc.

remember, maintaining a battery with a small draw is very small amount of current required every once in a while vs re-charging a dead battery which takes several amps for a long duration of time

 

[bruceb58]

This is a popular DC to DC charger used for RVs. You can get the 20A or 40A model. You want this mounted as close to your boat batteries as possible.

https://www.amazon.com/Renogy-Batter...s%2C212&sr=8-1

Put in a dedicated wire with an Anderson connector. I would wire up with at least 8 awg wire. The DC to DC converter will allow some voltage drop but you still want it as low as possible. 40A on a long run can have a lot of voltage drop if you chose the 40A model. Use one of these:
https://powerwerx.com/anderson-power...0&pageNumber=1

 

[dingbat]

It would be easier and a lot cheaper to buy a pound of dry ice for the trip.

Have used dry ice for years to store and transport fish home after a trip.

 

[fishrdan]

It would be easier and a lot cheaper to buy a pound of dry ice for the trip.

Have used dry ice for years to store and transport fish home after a trip.

Not a bad idea, I use dry ice keeping bait frozen in 100*+ weather, and it will keep anchovies rock solid for a day. I don't keep the dry ice cooler inside the boat while sleeping, as it will give off CO2. I toss the cooler out onto the swim platform, so (hopefully) CO2 goes overboard.

The OP said he was stopping over at WM parking lots for the night, and sleeping in the boat? That and dry ice in the fridge could be a bad combination...

 

[Old Ironmaker]

As someone that is Electrically challenged to the point I need an Electrician to change the batteries in my flashlight is all sounds Greek to me. However what I love about this site is one can learn something everyday. A DC to DC charger is a great advantage for trolling motor batteries forguys I know that do very long isolated Northern Ontario fishing trips where there isn't any Hydro locally. Nothing for these young lads to drive 12 hrs. from one lake to the next. They need to use a generator in the back country to charge the trolling batteries. However if they are 24V not 12V, still doable? Charge 1 at a time perhaps?

 

[H20Rat]

That and dry ice in the fridge could be a bad combination...

CO2 isn't CO. CO2 isn't the silent killer like CO, you will wake up gasping and coughing if the concentration gets high enough. Did some really quick envelope math, but a couple of pounds of dry ice in a boat wouldn't be an issue. 10+ pounds would be if the boat was well sealed.