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Old 04-30-2007, 06:48 PM   #1
Join Date: Jan 2007
Posts: 90
SUN #168
Charging The Trailer Battery

This seems like a good way to rapidly charge the trailer battery:
The Meltric connectors are a nice touch http://www.meltric.com/artwork/pdfs/...2007-ws-lc.pdf
An isolation solenoid and relay delay http://www.intellitec.com/PDF/5300362.100.pdf would be nice, too.

Anyone using this method?


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Old 04-30-2007, 08:23 PM   #2
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Join Date: Nov 2006
Location: Michigan
Posts: 854
SUN #115
I know a lot of people ask this question, and while looking in another forum (RVNet) I found this. So far, it is the most through explanation I have found.

I hope this isnt illegal, but since I figure we are all here to help one another, it cant hurt to re-post someone else's very good info.

This was posted originally by smkettner in the RVNet Forum.
Four GC batteries is about 450 amp hours. Down 50% (12.0 to 12.2 volts) will need 225 amp hours to get full. Actually closer to 300 with efficiency losses. The typical charge wire to the trailer will give about 10 amps regardless of the alternator size. Possibly a little more at the start and definitely less near full charge. Even if you figure 10 amps it will take 30 hours of driving to get up close to full charge.

Now you can upgrade the alternator, wires etc. Or wire in an inverter to drive a battery charger etc. And end up reengineering the whole thing. But ultimately you will be better off with a quiet generator and a modern three stage converter. You should be able to get set up for less than $1,000. The truck will still provide some power when traveling but the generator will keep you charged up while stationary.

If you run the batteries to 10.5 volts with any regularity they will loose capacity quickly and need replacement.

2007 Sunline T-2499 4" Lift
Rigged Boondocking & Dry Camping

Pat & Cindy Bonish
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Old 04-30-2007, 11:05 PM   #3
Join Date: Jan 2007
Posts: 90
SUN #168
Our 1950 just has one deep cycle battery with an amp-hour rating of
around 80. If it's drained to the 40% charge level, it would take 48 amp-hours to recharge. The article mentioned a 25 amp charging rate using welding cable. So it might be possible to recharge the battery in about 2 hours.

The downside is that there's no "smart" charger involved. I don't know if the Sunline converters' chargers are 3-stage.

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Old 05-04-2007, 11:50 AM   #4
Join Date: Jan 2007
Posts: 90
SUN #168
It appears that a better way to charge the trailer battery is to install an inverter in the tow vehicle and run a 120 VAC cord with disconnect back to the converter. An isolation relay will prevent the starting battery from attempting to power the inverter unless the engine is running. An automatic transfer switch at the converter will prevent connecting shore power and inverter power at the same time.

Our T-1950 has a WFCO converter with 3-stage battery charging. The same company makes an automatic transfer switch which mounts in the converter box.

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