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Recharging boat battery's while towing....
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texasair



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PostPosted: Tue Jan 05, 2016 4:24 pm    Post subject: Reply with quote

I used 12 gage wire from the charging circuit at the trailer connector.

At first charged the boat batteries through a cigarette lighter plug near the helm.

After I later installed a trolling motor, I was able to shorten the charging wire by connecting through the trolling motor plug located on the front deck.

Works well, keeps boat batteries charged, Engel cold.

No discharging of truck battery when parked overnight.
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DBBRanger



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PostPosted: Tue Jan 05, 2016 7:20 pm    Post subject: Reply with quote

hardee wrote:


On the Pickup Battery/Alternator to 7 pin plug 22 ft
7 Pin plug on truck to 12VDC Cig Plug at helm 18 ft
12VDC Cig plug at helm to Battery on Boat 14ft
Total length of run ================= 54 feet.

Is the amps figured on the amount of amp out put from the truck alternator? 100 amp I think. Or the amount of charge I want into the battery 5 amp I think to go to the Cig lighter? My on board charger is 10 amp, a 5/5 Guest, if that makes any difference.

Harvey


When calculating the size of a wire needed for a given current, you must use the round trip length, ie there and back. In your example, it would be 2 x54 or 108 total feet. Then use the resistance for the size and type of wire you expect to use (can get that online). Once you have that, then use Ohm's Law to calculate the expected current. So using V=IR, V is actually the voltage drop. So if the truck is charging at 14.5V, say, and the boat battery is at 12.2V, the drop is 2.3V. If 12 gauge wire has a resistance of .00162 Ohms per foot (total of .175 ohms in your case), then expected current is 2.3V/.175 = 13 amps. That's the most you can expect - I got a lot less. Your alternator will supply what ever is possible (it's not a constant current generator so it will not supply 100 amps unless the resistance allows it).

I'd be careful about using a cigarette lighter to pass that amperage - I'm not sure the wires to the lighter are adequate.

Hope that helps!

Doug
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colbysmith



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PostPosted: Tue Jan 05, 2016 8:18 pm    Post subject: Reply with quote

Ok, something I've wondered, but have not tried. What would happen if you plug a 110 vt. inverter into your 12 vdc accessory plug, and plugged your shore power battery charger into the inverter? I'm assuming the battery charger doesn't draw much amperage, and that if you have enough battery voltage to power the inverter, it would recharge the battery. (I'm thinking of a 100 watt inverter.) Maybe a stupid idea, but still, something I keep forgetting to try just to see. Colby
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Captain Bella



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PostPosted: Tue Jan 05, 2016 9:25 pm    Post subject: Reply with quote

This is a little off track, and others have probably done the same...

We occasionally run our Honda EU1000i from the storage site to the boat ramp. This allows us to begin warming the cabin for a long weekend, or cools the fridge, and charges the batteries. It also keeps our generator exercised.

We secure the generator with a dock line, and connect to the boat via the 30amp - 15 amp adapter...no traditional power cord.

Might be worth a try for those of you already hauling around a generator.

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hardee



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PostPosted: Tue Jan 05, 2016 11:38 pm    Post subject: Reply with quote

colbysmith wrote:
Ok, something I've wondered, but have not tried. What would happen if you plug a 110 vt. inverter into your 12 vdc accessory plug, and plugged your shore power battery charger into the inverter? I'm assuming the battery charger doesn't draw much amperage, and that if you have enough battery voltage to power the inverter, it would recharge the battery. (I'm thinking of a 100 watt inverter.) Maybe a stupid idea, but still, something I keep forgetting to try just to see. Colby


Colby, I'm not sure if I get what you are wanting to charge `shore power battery charger` into an inverter. I routinely run a 300watt inverter off my 12VDC Cig plug to charge a camera battery and have charged a lap top battery off that inverter.

I am wanting to charge my #1 and 2 batteries as I am towing though, so I can use off them at night.

If you are thinking of running the shore power charger off the inverter in the truck while I am towing, I don't think the 300 watt inverter is up to that.

Harvey
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hardee



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PostPosted: Tue Jan 05, 2016 11:43 pm    Post subject: Reply with quote

Jeff, I don't carry a portable generator with me, well not a dedicated one anyway. If I need to (and I have on some occasions,) I will run one of the twin 40's for a bit.

Harvey
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colbysmith



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PostPosted: Wed Jan 06, 2016 9:50 am    Post subject: Reply with quote

Harvey, I think you sort of followed my thought, but I'm speaking of running a closed circuit, plugging an inverter into the boats 12 vt accessory plug, and then hooking the boats shore power up to that inverter, so that it would be driving the boat's charger... I don't know if my 100 watt inverter would handle it, but it handles my cpap without the humidifier at night. I think I do have a larger, 300 watt, inverter as well I could try out. I just wonder if running a "closed" circuit like that would work... I'm assuming that the electronics inside the inverter and battery charger are putting out more current than they take in. (I'm only thinking about doing this when dry camping on the boat while it's on the trailer, without AC power available to recharge the boat's batteries after a night of cpap use (and some cabin lighting.) On the lake, no problem, as I can just run the engine for a while to charge the batteries.

I also have a 2000 watt Honda Generator I sometimes travel with, that I have used to recharge my boats batteries when dry camping without electricity available. Colby
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JamesTXSD



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PostPosted: Wed Jan 06, 2016 4:19 pm    Post subject: Reply with quote

So, if I am understanding this right (and I'm probably not), you are going to run the inverter off the boat's 12v plug, then use that inverter to charge the boat's batteries??

Question Crook Question

You'd be using the boat's batteries to charge the boat's batteries? Yeah, I am sure I am missing something in that explanation.
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colbysmith



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PostPosted: Wed Jan 06, 2016 4:47 pm    Post subject: Reply with quote

I said it might be a crazy thought James! Mr. Green I think there is a lot of PFM going on inside that inverter, but maybe I'm giving it a lot more credit for what it can do... (Even I come up with some off thoughts... Rolling Eyes )
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thataway



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PostPosted: Wed Jan 06, 2016 8:43 pm    Post subject: Reply with quote

Sounds like the perpetual motion machine--and none of those work! No, do not run a battery charger off an inverter either using the truck or boat 12 volt charter.

It is easy to hook up a the batteries to the truck circuit. See my post on page one, where we use #10 wire. Ours from the RV is shorter than most trucks. We use simple connectors, inside of the boat--and run the wire thru the anchor deck pipe--no need for external hull fittings.

We use a version of this on our Yukon, which has a platform for a second battery under the hood. We run #8 wire (fused) from the start battery to a Voltage Sensitive Relay, on a bracket I fabricated out of sheet metal next to the second battery (Group 29) There is also an "on and off" switch, there to take this all out of the circuit. This second battery has a single output (fused) to # 12 wire to a cig lighter plug under the driver's seat of the truck--and this powers our refer/freezer.

This same concept (using a VSR on the start battery) to power a circuit going to the boat works very well. You don't have to worry about running down the trucks battery over night, but will charge the boat battery when you run the truck engine.

I am not sure I am keen on running an EU 1000 on the boat, when unattended going down the road! Where do you put the EU 1000? What about CO? Fire hazard, power issues--what do you use to "heat the boat" with--small electric heater? I won't leave a boat with a generator running--I have seen too many bad issues, ranging from run-away generator, to cooling issues, short circuits etc which can happen with any type of generator...let alone on the road.

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colbysmith



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PostPosted: Thu Jan 07, 2016 5:32 pm    Post subject: Reply with quote

Ok James and Bob, my younger brother got his laugh of the night I'm sure.... He is quite the Electronics guru, so even though he is my younger brother, I think he is one smart cookie when it comes to anything electrical, so I defer to him. FWIW, I'll just copy and paste his reply (without the laughter that I'm sure was going on as he typed....)

It sounds like your trying to make electricity out of thin air. Let me re confirm your question. You want to be driving down the road with your 12 volt inverter plugged into you boat 12 volt accessory outlet, to power your battery charger, to charge the boat battery's that's currently powering the inverter. This will absolutely not work. First off, there are power losses. Your inverter will use more juice than the battery charger will replace. Second, there would possibly be a ground loop type issue that could damage either the battery charger or inverter. You have to use a separate power source for your inverter. If you plug your inverter into your car accessory outlet, run the battery charger on that to charge your boat battery, that will work as long as you do not have your boat batteries tied/connected in any way to your car/towing electrical system, and the current rating of your accessory outlet is high enough to run your inverter, and the battery charger power requirement does not exceed the output of the inverter.

Example: If your accessory outlet has a 10 amp fuse, it will deliver up to 120 watts of power. If your inverter is 100 watt inverter, the accessory outlet will handle that ok. But, your battery charger will need to require no more than 100 watts of AC power, or the inverter won't be able to handle it.

So, lets look at an example. I have a Sears 12 volt 2 amp battery charger. It's input requires 120 vac at 0.79 amps. That equates to 120 volts X 0.79 amps = 94.8 watts of power (ohms law). Your 100 watt inverter will power my 2 amp battery charger. You have to run it on your car battery. Remember, your drawing just under 10 amps of current off your car battery to produce a measly 2 amps of charge on your boat battery, but that will work. You want to be driving the vehicle though to keep from running the car battery down, as your drawing about 8.3 amps in the above example.

To be clear. You can not run a power inverter on a battery, power a battery charger with that inverter, and then use that battery charger to charge the same battery your running the inverter on. If that was possible, we'd all have free power and no one would need an electric company/utility.

You could also install a battery isolator or simply a disconnect between your tow vehicle's electrical system (battery) and the boat electrical system (battery) that would allow the boat batteries to charge along with the car battery while your driving. Only problem, you'd have to use large gauge wire, because the current flow through the wire would be high if the boat battery is low. Current flow would gradually go down as the battery charged and became matched in charge to the auto battery.

If you had a current limiting device that simply limited the current to say 5 amps, you could use small wire or plug into your accessory outlet. The time required to charge the batteries would be longer.

In your case. Plug the inverter into your car accessory outlet and use a small battery charger on the boat battery. Probably have to run a small extension cord from car inverter to the boat charger. My guess is that your shore powered battery charger is going to put out about 10 amps of current or more, and will require more input power than your inverter can provide. Give me the info on your shore power battery charger and I'll let you know, but I'm guessing you inverter won't power it.

I do have a 1500 watt inverter I use to power my house on batteries when I don't want to run my generator, but it draws about 130 amps of current. I use 3 big ass deep cycle batteries wired in parallel. I'm sure it will run your shore power battery charger. lol
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gulfcoast john



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PostPosted: Thu Jan 07, 2016 5:40 pm    Post subject: charging on the road Reply with quote

My particular wrinkle would be this...I already have a small battery on the trailer tongue to power the EOH brakes if the breakaway switch is activated (in which case a Tom Cat's least concern is whether the fridge stays cold that day). It would be nice to simply tap that +/- to a HD wire down the windlass hole across the berth and plug into the helm 12 lighter socket with fuses. My fridge draws 3A with a 10A fuse. If I forgot to unplug it at night it might drain both the truck batteries and even larger boat House battery, but I bet that would take a couple of days. Or maybe a VSR...hmmm. Any thoughts appreciated.
Cheers!
John

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thataway



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PostPosted: Fri Jan 08, 2016 7:36 pm    Post subject: Reply with quote

John, the break away batteries only have a very small amperage charging circuit. You can use the #4 spade connector on the 7 pin plug, but need to be sure that there is at least #10 wire from the fuse on this circuit and #10 wire into the boat.

Is the 3 amps an average, or when running? Generally the C Dory refers draw 2.5 to 3 amps an hour an average, and draw 5.5 to 6 amps when running. A Group 24 battery (usual battery which is used on most cars and trucks, (except diesels) is 60 to 70 amp hours, and will be at 50% discharge in 10 to 12 hours with 3 amp per hour draw. The group 31 batteries I use for the freezer/refer are 110 amp hours, and I recharge about every 12 hours when using both refer and freezer. If just one alone, then every 24 hours to avoid more than 50% discharge of the battery.

If left to the truck batteries then def a VSR to avoid damage to the truck batteries.

Better to use dedicated wires, directly to the batteries, rather than to a cig plug, because of wire size and current draw. Also the fusing requirement is important.

Colby, get one of the "Kill-a-watt" to measure what a battery charger draws, if it is not on label. It is better to actually measure the draw. I use a 30 amp charger (temp and battery type adjusted) to charge my refer batteries, with the Honda EU 1000 I. The 30 amp charger plus a few other minor accessories, are near what the max capacity of the generator. Battery chargers are generally not particularly efficient, which is one of the reasons not to run them off an inverter--better to have a regulated 12 volt charge directly from the vehicle's alternator.
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thataway



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PostPosted: Fri Jan 08, 2016 9:13 pm    Post subject: Reply with quote

I came across this device which was designed to keep the battery on a "TOAD" or "Dinghy" --vehicle towed behind a RV. It limits the charge to 10 amps, has all of the cable and plugs, easy to install, and disconnects the towing vehicle battery from the "Toad". Would work well for those who do not want to make their own setups with a VSR etc. to charge the battery in their boat while it was towed behind the truck. Cost starts at about $110.

http://www.lslproducts.net/ToadChargePage.html
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Marco Flamingo



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PostPosted: Sat Jan 09, 2016 11:56 am    Post subject: Reply with quote

DBBRanger wrote:
So if the truck is charging at 14.5V, say, and the boat battery is at 12.2V, the drop is 2.3V. If 12 gauge wire has a resistance of .00162 Ohms per foot (total of .175 ohms in your case), then expected current is 2.3V/.175 = 13 amps. That's the most you can expect - I got a lot less. Your alternator will supply what ever is possible (it's not a constant current generator so it will not supply 100 amps unless the resistance allows it).


I haven't followed this thread too closely, in part because I hope to never need to charge a 16' boat while towing. If it were simple and effective, I might think about it.

What I don't get is how one determines or controls the amount of juice going to the remote batteries. If the batteries are close to dead for some reason, and your vehicle's alternator can put out 100 amps, what limits the amperage to the batteries? 50 amps over 10 AWG is a problem. Over 12 AWG the wire becomes a heating element. Is it simply fused to protect the wire and if the battery truly needs a major recharge the towing charge won't work?

The other issue is voltage loss. Won't running 40 feet of 12 AWG (80' total) at any meaningful amperage result in too much voltage drop? Just 10 amps through 80' of 12 AWG would be about a 15% drop in voltage, maybe more. So at it's best, the 14.5V "charging" system could only ensure that the remote battery doesn't drop below 12.3V. Not really a charging system, but an over-discharge protection system. Or am I missing something about how this can be done with small gauge wire?

I just went though some of this when relocating the batteries on my CD16. Moving the batteries an extra 5 feet meant upgrading to 2 AWG cables, running both grounds to a common buss near the outboard (so that a single ground would never carry the current from both batteries). The calculation had to be done with the starter motor amperage as the biggest draw, so it's different than a trickle charge to a battery. But just running 12 AWG wire from the tow to the "toad" and back seems too easy.

Mark
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