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 > Charging trailer battery while towing and smart charging

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time2roll

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Posted: 12/04/19 12:09pm Link  |  Quote  |  Print  |  Notify Moderator

opnspaces wrote:

Thanks Time2roll and GaryM, Yes I do get that a DC to DC converter should do what I need. But those seem to be hundreds of dollars. So before I consider putting down that kind of money I really want to understand why a factory setup is not longer applicable. I've read your and the other posts multiple times and something still eludes me. Please tell me if I have it correct below.

I think what I'm gathering is that the alternator will see the sag as I plug the trailer in. But it doesn't see the additional battery as a battery. So all the electronics see is an additional load as the starter battery and the trailer battery try to equalize to each other. So the alternator only bumps up the output to get the whole system back up to 13.x volts and not into any kind of useable charging mode. Or put another way, even though the trailer battery is needing a strong charge, the alternator is only putting out enough to float charge?
Thanks
You have it right. Just missing the fact that even if the alternator was pushing the proper 14.8 volts and the trailer battery wanted 20+ amps... the power would be going through a 25' #14 wire and the voltage is still not where you need it.

Yes larger wire will reduce the issue. But for the money (Renogy 20a $130) I would get the DC-DC converter and save yourself the cost and time and hassle to do a half solution with large wire.

Solar on the trailer certainly works too.


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theoldwizard1

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Posted: 12/04/19 12:33pm Link  |  Quote  |  Print  |  Notify Moderator

time2roll wrote:

The point of the DC-DC converter (charger) is such that you can ignore the voltage gyrations of the alternator and any voltage drop in the wire and get proper charging voltage on the trailer battery. You no longer care what the vehicle is doing or how it works.

DING DING DING ! We have a winner !

Add in, most DC-DC battery chargers also do a "proper" 3 stage charge on the house battery.
opnspaces wrote:

Or put another way, even though the trailer battery is needing a strong charge, the alternator is only putting out enough to float charge?

Basically, yes.

The house battery would charge but at that rate it might take more than 24 hours.

theoldwizard1

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Posted: 12/04/19 12:38pm Link  |  Quote  |  Print  |  Notify Moderator

Bedlam wrote:

Start by running a heavy dedicated charge line between the truck and trailer. It will help in trickle and maintenance charging. If you want more, later you can add a DC to DC charger off this new run line.

The wire that is currently there (probably about 12 AWG) is adequate. It is not worth the cost and labor to try and install a heavier wire.

opnspaces

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Posted: 12/04/19 01:37pm Link  |  Quote  |  Print  |  Notify Moderator

Thinking about wiring gauge and voltage drop, do I then have to mount the dc to dc charger at the back of the Suburban so it's close to the trailer? Or can I mount it up front and run a 12 or 10 gauge wire back to the plug? If I mount the charger in the back, do I need to run a 12 or 10 gauge wire to feed it? I think oldwizard kind of answered that but I want to be sure.


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Bedlam

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Posted: 12/04/19 01:55pm Link  |  Quote  |  Print  |  Notify Moderator

I run a 2 gauge dedicated run between my truck and trailer and use Anderson connectors for the link at the receiver.

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ependydad

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Posted: 12/04/19 02:48pm Link  |  Quote  |  Print  |  Notify Moderator

opnspaces wrote:

Thinking about wiring gauge and voltage drop, do I then have to mount the dc to dc charger at the back of the Suburban so it's close to the trailer? Or can I mount it up front and run a 12 or 10 gauge wire back to the plug? If I mount the charger in the back, do I need to run a 12 or 10 gauge wire to feed it? I think oldwizard kind of answered that but I want to be sure.


The one installation that I've seen of this, the DC-to-DC converter was installed in the camper near the batteries.

btw, thank you for asking this. I'm learning a lot!


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time2roll

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Posted: 12/04/19 03:31pm Link  |  Quote  |  Print  |  Notify Moderator

opnspaces wrote:

Thinking about wiring gauge and voltage drop, do I then have to mount the dc to dc charger at the back of the Suburban so it's close to the trailer? Or can I mount it up front and run a 12 or 10 gauge wire back to the plug? If I mount the charger in the back, do I need to run a 12 or 10 gauge wire to feed it? I think oldwizard kind of answered that but I want to be sure.
Assuming you get the 20 amp you will mount it on the trailer close to the battery. No other modifications needed.

theoldwizard1

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Posted: 12/04/19 05:27pm Link  |  Quote  |  Print  |  Notify Moderator

Bedlam wrote:

I run a 2 gauge dedicated run between my truck and trailer and use Anderson connectors for the link at the receiver.

How about some "real world" feed back !

Next time you are hitched up, check the battery voltage at the truck battery and then the battery voltage at the trailer battery.

For best results for this test, the trailer battery should be at least 10% discharged.

opnspaces

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Posted: 12/05/19 05:39pm Link  |  Quote  |  Print  |  Notify Moderator

Thanks everyone. I'm hopefully going camping (boondocking) soon with a slightly newer tow vehicle, 2001 Suburban. I plan to take a few voltage checks while there. I'm really curious to see if I get any significant charging while driving.

time2roll

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Posted: 12/05/19 09:32pm Link  |  Quote  |  Print  |  Notify Moderator

A DC clamp-on ammeter will tell the real story.

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