Charging while towing

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Re: Charging while towing

Postby capnTelescope » Thu Apr 30, 2015 6:23 pm

Wishno wrote:Is 80amps going to be enough? Or do I need to match it to my alts output? (140)

You need to size the isolator to the maximum amount of current the trailer will draw, which must be less than the alternator can put out. 80 amps should be plenty for most purposes. 20-30 amps should be sufficient, I would think. Other than spending more money, no problem with with a higher rated isolator.

I'm using this $10 one.
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Re: Charging while towing

Postby ukewarrior » Thu Apr 30, 2015 6:42 pm

Is there a reference hookup diagram for these units?
capnTelescope wrote:
Wishno wrote:Is 80amps going to be enough? Or do I need to match it to my alts output? (140)

You need to size the isolator to the maximum amount of current the trailer will draw, which must be less than the alternator can put out. 80 amps should be plenty for most purposes. 20-30 amps should be sufficient, I would think. Other than spending more money, no problem with with a higher rated isolator.

I'm using this $10 one.
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Re: Charging while towing

Postby capnTelescope » Thu Apr 30, 2015 7:17 pm

It's not too difficult. One small terminal goes to ground, the other small one to a wire that's hot when running. The large terminals have one through a fuse to battery positive and one to the trailer.
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Re: Charging while towing

Postby Hal W » Tue Jun 09, 2015 3:37 am

First post here, and I know this is an ancient thread, but I read through the 13 pages and want to add something that has been missed here. I have 30 years experience in auto tech so I have a little background with this subject, at least as far as auxiliary batteries on vehicles. Two very different batteries will have a slightly different charging requirement. That being said, if they are in parallel the alternator will see them as a single load, or in effect, a single battery. Think about it this way; if you connect a fully charged battery with jumper cables to a dead battery the 2 batteries will equalize, the dead battery will come up and the charged battery goes down. The gauge of wire that the jumper cables are made from prescribes the amount of amps that will flow between the batteries, (or, the speed that it takes to equalize them). This is why a set of cheap jumper cables get hot when you are trying to jump your neighbors stone dead battery with your fully charged car, the cable size is too small and they become a resistor as too many amps are trying to flow at once. I could go much deeper into this but if you're still with me...

The "Simple" RV system talked about here is fine, but there needs to be a couple of things in the system. There does need to be a heavy relay and a MANUAL switch. You do not want to have a dead battery on the TD and a full one on the TV that connect at the moment that you turn on the ignition to start the TV. Remember the cheap jumper cables. The cable size should be a minimum of 6 gauge and 4 would be better. (Think; full-sized battery cables are best). So now you are thinking "that is waaayy overkill". Yes, unless you have the above condition happen... Then it is properly sized to not smoke the 10 gauge typical charge wire running through your TV and TD. The way to avoid this problem is to install a resistor on the TV battery end to "slow down" the rate of charge to meet a smaller cable's capacity.

OK, so a "Simple RV system" set up like this will work great and be safe: TV Batt + to a Mega Fuse to a heavy relay run from a switched ignition source THROUGH a manual switch (to switch it off for starting the TV) to fire the relay, Then through a heavy resistor (50 amp 2 ohm? You need to wire it and check the voltage at the TD batt as you are wanting about 14.5 when charging). This is the only way I can see this type of system working properly and safely.

I think a Battery isolator system would be better in the long run if the cable size to the TD doesn't drop the voltage too low. I like the inverter to smart charger even better as the voltage step up eliminates the cable size problem and the charger is on-board the TD to plug into shore power anytime. There is no worries about efficiency here, a typical drive to and from your camp will take way more time then it will take to charge your TD battery fully. Think of that like "I have a 50 gallon bucket to fill up a 5 gallon bucket, who cares if I spill some?"
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Re: Charging while towing

Postby capnTelescope » Tue Jun 09, 2015 9:04 am

Hal W wrote:I like the inverter to smart charger even better as the voltage step up eliminates the cable size problem and the charger is on-board the TD to plug into shore power anytime. There is no worries about efficiency here, a typical drive to and from your camp will take way more time then it will take to charge your TD battery fully. Think of that like "I have a 50 gallon bucket to fill up a 5 gallon bucket, who cares if I spill some?"


Well said, Hal. :thumbsup: :applause:

As the original proponent of the inverter to smart charger method, thank you. :beer:
I'll burn that bridge when I come to it.

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Re: Charging while towing

Postby Gunguy05 » Tue Jun 09, 2015 9:20 am

I might have missed it so excuse me if it was mentioned. What is a descent smart charger for the dc-ac-DC method?
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Re: Charging while towing

Postby capnTelescope » Tue Jun 09, 2015 9:56 am

I'm quite happy with the NOCO Genius G7200 12V/24V 7.2A UltraSafe Smart Battery Charger, which is 7.2 amps, but I'm not running a fridge, just LED lights & a FanFan. Generally, any AC charger will work. I'd recommend one that has a float/maintenance mode, as you can use it to keep the TD battery charged between camping trips. 5 amps is probably enough for most purposes. Definately get one that is more than a .75A maintainer-only.
Last edited by capnTelescope on Tue Jun 09, 2015 10:07 am, edited 1 time in total.
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Re: Charging while towing

Postby Gunguy05 » Tue Jun 09, 2015 9:58 am

capnTelescope wrote:I'm quite happy with this charger, which is 7.2 amps, but I'm not running a fridge, just LED lights & a FanFan. Generally, any AC charger will work. I'd recommend one that has a float/maintenance mode, as you can use it to keep the TD battery charged between camping trips. 5 amps is probably enough for most purposes. Definately get one that is more than a .75A maintainer-only.


What about inverter? What would be the minimum needed there?

I'm starting to see the beginning of a solar system here. I'm just weighing do I hook this up, or save up fora 100W solar panel as my trickle charger.

Thanks!

Brian
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Re: Charging while towing

Postby capnTelescope » Tue Jun 09, 2015 10:19 am

I'm using a 300 watt inverter. 7.2 Amps * 12 volts = 86 watts. About 40 bucks. Understand that you'll pull more than 86 watts due to charger inefficiencies. I couldn't say how much actual watts, but a 3X margin should be plenty.

This leaves a coupla hunnert watts reserve for other tow power uses. You don't need a big'un. :D
I'll burn that bridge when I come to it.

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Re: Charging while towing

Postby capnTelescope » Tue Jun 09, 2015 12:40 pm

Gunguy05 wrote:I'm just weighing do I hook this up, or save up fora 100W solar panel as my trickle charger.


Brian, my take is that if you usually boondock for more than a couple of days in the same spot, solar will give you the "staying power" you need without burning a bunch of gasoline.

OTOH, if you mostly drive-camp-drive, I think the dc-ac-dc setup will better meet your need.

:beer:
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Re: Charging while towing

Postby capnTelescope » Wed Jun 10, 2015 3:56 pm

Just for giggles, here's a fresh post in my build thread showing how I did it. It's all Plug & Play. No switches to flip or "Don't forget to"'s.

Enjoy! :beer:
I'll burn that bridge when I come to it.

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Re: Charging while towing

Postby bdosborn » Wed Jun 10, 2015 6:58 pm

Is there a reason you didn't just bring the 12V DC straight to your battery?

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Re: Charging while towing

Postby tony.latham » Wed Jun 10, 2015 8:17 pm

bdosborn wrote:Is there a reason you didn't just bring the 12V DC straight to your battery?

Bruce


Sheeeeesh. I just read through this entire thread. Forty-five minutes? I dunno. Maybe an hour. It's kind of a fog. Maybe it was a dream. Perhaps Einstein could 'xplain to me how time is relative and I could look back and understand what just happened to me. Maybe he could warp-drive string theory into my mushed brain while he's at it. And I thought AC-DC was a band. :thinking:

And then you, Bruce, lay the one-liner down like (Dirty) Harry Callahan back in '71: "Do I feel lucky?" Well, do ya, punk? 8)

I need another beer. :beer: I think I'll stick to my "factory disconnect when the engine is off" charging wire.

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Re: Charging while towing

Postby capnTelescope » Wed Jun 10, 2015 8:57 pm

bdosborn wrote:Is there a reason you didn't just bring the 12V DC straight to your battery?

Well, Bruce, you know me. Gotta do it the dc-ac-dc way. It's the same old "is the voltage drop too much/is the wire big enough/is it right for the battery" thing. I'm somewhat surprised you had to ask. :)
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Re: Charging while towing

Postby bdosborn » Wed Jun 10, 2015 9:04 pm

Ummm, wow. I just went back and re-read the thread too. I must be getting old as I forgot all about the discussion previously. :?

I know there are a lot of different ways to skin this cat but lets don't loose sight of the fact that you can charge your trailer battery straight off the TV without using an inverter/smart charger. I think newbies read threads like this and decide it's too complicated to charge your trailer battery while driving, when it actually can be really simple.

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