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Old 02-09-2014, 11:00 PM   #1
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Battery charging through the umbilical

I had the opportunity to test the ability of the TV to recharge coach battery through the umbilical this weekend. The battery hadn't been charged for approximately 1 month and had been used once since then to run the furnace for about an 3 hours and some interior lights for a few minutes. Prior to placing any loads on the battery, the resting voltage was 12.51V (~80%).

Before connecting the vehicle, I ran the furnace for about 1 1/2 hrs. No other significant loads were active. The voltage was at 12.3V and climbing before the umbilical was connected.

I connected the umbilical and measured the charging current at the battery @ 4.56A and 13.6V with the vehicle idling. The open circuit voltage at the battery connections was 14.7V.

Since batteries will only accept charge relative to their state of discharge, I pulled the battery and put it on my charger to compare. On the 10A setting of the charger, the battery was charging at 5.20A and 13.6V.

The TV charge rate appears to be roughly equivalent to a 10A charger. Not too bad considering the battery in my Overlander is almost as far as it possibly can be from the alternator of the Yukon.
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Old 02-09-2014, 11:19 PM   #2
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Your charge current of 4 to 5 amps is about what I have measured on several combinations of Airstreams, Argosy's and tow vehicles. Once in a while on my Argosy I have hit 6 amps.

The numbers on my 2014 Airstream FC 20 and 2012 Jeep Grand Cherokee are a little better, in the range of 10 amps as the max charge. I attribute that to the fact that the batteries are on the front of the trailer in the tongue, and that the wires, cords, connectors and TV things are virtually new and in top shape.
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Old 02-10-2014, 04:48 AM   #3
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The vehicle charge line is little more than a trickle charger. The wire gauge is on the small side. IIRC mine maxes around 6 amp at road speed rpm. YMMV. I did the calculations once and I seem to recall that it would take 20 hours of driving at speed to bring a fully discharged (~50%) battery back to full charge.

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Old 02-10-2014, 06:23 AM   #4
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One thing you are over looking. Regardless of the amperage that your battery is getting from your TV, it is a constant voltage source. Deep cycle batteries will show a drastic decrease in longevity when exposed to only a single stage charging voltage.

Think about all of the owners, both new and old, removing the Univolts and single stage Parallax converters to replace them with 3-stage 'smart' chargers. What you really need to properly charge your trailer's batteries is a 12VDC to 12VDC smart charger system using the TV's alternator as the voltage source, and cable heavy enough to not show any voltage drop at the trailer's batteries.

I do not have a recommendation for this device, but I'm sure they are out there. When we add solar charging systems to RVs, we disconnect the charge line in the umbilical (especially when new Lifeline AGM batteries are installed) to protect them from over/under charging (that is not temperature compensated either) by the TV alternator.
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Old 02-10-2014, 06:46 AM   #5
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I have charged my trailer battery from the TV through the umbilical. More so in my early days of camping because it was the only way. It will get you through a few more days of camping but it's more of a "bandage than a cure.
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Old 02-10-2014, 06:59 PM   #6
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Quote:
Originally Posted by lewster View Post

When we add solar charging systems to RVs, we disconnect the charge line in the umbilical (especially when new Lifeline AGM batteries are installed) to protect them from over/under charging (that is not temperature compensated either) by the TV alternator.

Would it be advantageous at all to reconnect the charge line to the charge controller in order to provide a controlled charge to the batteries. Jim
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Old 02-10-2014, 07:30 PM   #7
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Lewster: I just read your post and think I need to follow your advice on this, since I followed your advice for my solar install and put the new Lifeline AGM's in as well. Simple for me to do this as I am doing an interior project and have ready access to the black box where the charge line comes into the trailer from the 7 pin connector. I think I might put in a switch that will allow me to use the TV alternator if I'm in a situation without sunshine and where I'll be towing all day and might want to keep pumping juice into the batteries. Pretty obvious that you're not going to pump a lot of volts/amps through that puny little wire, though....especially when you compare it to my heavy duty copper from the panels to the Lifelines! I don't want to overcharge them and I can sure see that if I'm towing on a sunny day for 6-7 hours and the panels and the TV alternator are both charging....well, that could be more current than the Lifelines are going to be happy with.
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Old 02-10-2014, 08:24 PM   #8
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Would it be advantageous at all to reconnect the charge line to the charge controller in order to provide a controlled charge to the batteries. Jim
Jim,

Perhaps a second smaller charge controller might do, but there is no second input on the Blue Sky and Morningstar controllers that we use.
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Old 02-10-2014, 09:26 PM   #9
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Originally Posted by wahoonc View Post
The vehicle charge line is little more than a trickle charger.
This conventional wisdom is what prompted me to check the charging current. I had heard this before, but have observed significant improvement in the house battery's charge after a tow.

It may be that there are trickle chargers that are a bit more robust than those that I am familiar with. All the ones that I have used are capable of only about 1A maximum regardless of the battery's state of charge.

You are correct that a significantly discharged battery would take a long time to fully recharge using the umbilical. However, I would say the charging capacity is significantly higher than a trickle charge. Maybe just splitting hairs, but hey, it's winter. It's certainly not enough to expediently charge a considerably discharged battery.
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Old 02-10-2014, 09:54 PM   #10
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Quote:
Originally Posted by lewster View Post
What you really need to properly charge your trailer's batteries is a 12VDC to 12VDC smart charger system using the TV's alternator as the voltage source, and cable heavy enough to not show any voltage drop at the trailer's batteries.

I do not have a recommendation for this device, but I'm sure they are out there.
http://www.airforums.com/forums/f37/...ml#post1386639

This thread details installing a DC to DC charging system. The author attributed most of the charging capacity to the increased cable size. I'm not interested in adding a similar system since I have limited need for significant recharging after a single overnight stay. I was mostly curious about what kind of charging the original '69 wiring and OEM GMC was capable of.
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Old 02-11-2014, 05:28 AM   #11
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Quote:
Originally Posted by Kyle401 View Post
This conventional wisdom is what prompted me to check the charging current. I had heard this before, but have observed significant improvement in the house battery's charge after a tow.

It may be that there are trickle chargers that are a bit more robust than those that I am familiar with. All the ones that I have used are capable of only about 1A maximum regardless of the battery's state of charge.

You are correct that a significantly discharged battery would take a long time to fully recharge using the umbilical. However, I would say the charging capacity is significantly higher than a trickle charge. Maybe just splitting hairs, but hey, it's winter. It's certainly not enough to expediently charge a considerably discharged battery.
I consider anything under 10 amps to be a trickle charger, but then again I am quite often dealing with 200+ amp charging systems. We run a lot of heavy equipment here on the farm and use 10 amp chargers in the winter to keep things topped up.

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Old 02-11-2014, 09:29 AM   #12
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Jim,



Perhaps a second smaller charge controller might do, but there is no second input on the Blue Sky and Morningstar controllers that we use.

Thank you for jogging my marbles. I do have a little spare controller left over from a 15watt panel that I used to use that may work nicely. Wee bit of insurance. I'll try it in the spring. Jim
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Old 02-11-2014, 01:48 PM   #13
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Originally Posted by lewster View Post
I do not have a recommendation for this device, but I'm sure they are out there. When we add solar charging systems to RVs, we disconnect the charge line in the umbilical (especially when new Lifeline AGM batteries are installed) to protect them from over/under charging (that is not temperature compensated either) by the TV alternator.
Most if not all modern tow vehicles have temperature compensated voltage regulation. In Minnesota (where it was -20 F this morning), it's an easily measurable difference that you can see with an ordinary voltmeter. Typically alternator output is controlled by the ECM. That's the situation on both my 1997 truck and my 2004 Suburban.

It is my observation, based on watching the dash-mount voltmeter, that there is some sort of multi-stage algorithm at work, also, since the charge voltage usually drops a little after a couple of hours on long trips.
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Old 02-11-2014, 02:00 PM   #14
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I connected the umbilical and measured the charging current at the battery @ 4.56A and 13.6V with the vehicle idling. The open circuit voltage at the battery connections was 14.7V.
I have a setup designed to maximize the value of the vehicle alternator as a power source. I have an extra group 24 deep cycle battery under the hood of my TV, which is connected to the trailer socket using 6 gauge cable (+ and -). It is also connected to the alternator through an isolator relay which closes when the ignition is on. There is overcurrent protection at various points. There are the two stock group 27 deep cycle batteries in the trailer.

The engine is equipped with an AD244 large-frame alternator which is capable of producing around 80 amps at idle, and 120 amps at highway speed.

When the batteries are at 50% charge, I get around 10-15 amps into each of the batteries in the trailer and around 20 amps into the battery in the engine compartment.

The setup allows the battery in the engine compartment to charge when we leave the trailer at a camp site and travel in the tow vehicle. When we return to the campsite and reconnect the umbilical, the engine compartment battery shoulders a good deal of the electrical load.

This system works for us and allows us to run the furnace while boondocking without the expense of solar. I would like to augment it with some solar panels at some future point.
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