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Rewiring for Dry Camping

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Can you give me a sanity check on my plans to convert my Fifth Wheel for use off the grid? I'll be full-timing as soon as my S&B home sells.


The previous owner obviously did little or no dry camping. Everything appears to be exactly as it came from the factory. The RV systems are 12VDC. The fridge and hot water are switchable between LP and 110VAC. The furnace, range and stove are propane (with 12VDC thermostats). Everything else is 110VAC (TVs, stereo, A/C. microwave) with no inverter.


I've added a generator (Cummins Onan LP – 3600 Watt). My plans are to add solar panels with MPPT charge controller and an extra line from my truck's alternator (180 amp).


My goal is have to the critical systems, in my office area, available 24/7 whether or not I have shore power. I intend to manually prevent overloads, such as not running the A/C unless the generator is running with everything else off or I'm on 50A shore power.


Attached is a JPG file showing how everything is currently configured and other JPG file showing my plans. The large dots on the 2nd diagram indicate isolators (diodes for the DC lines and automatic transfer switches for the AC lines).


Does this look like it will work? Does anyone else have a similar system?





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Right off the bat... you'll want your MPPT controller upstream of your charger and alternator... so instead of "solar panel"/"alternator", you would be looking at "MPPT controller"/"alternator" legs. Your charger leads I would run directly into your battery bank on a dedicated leg. I don't know how you were planning to isolate selected 110v outlets, but, personally, I would use a subpanel as opposed to a transfer switch.


Is there a reason for not supplying all 110v outlets with power from your inverter?


That's just off the cuff.. I'll look a little closer after supper . :)

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Better yet.. Your MPPT controller on the dedicated leg and your charger and alternator on their own leg feeding directly to your battery bank. You'll also want to be sketching in disconnects, fuses, breakers, shunts.. possibly a terminal post and/or a bus bar. The more detail the better.

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Thanks for your feedback!


Why should the MPPT controller be upstream? It seems harmful to me to have several devices feeding directly into the battery bank.


One goal is to fully charge the batteries whenever I'm driving several hours from one site to another. I'm assuming that the long DC feed from my truck to my battery pack will have significant voltage loss. Also, I don't think the puny wires in the standard 7-pin connector can provide sufficient current (even though the alternator can); so, I'm planning to add a wide gauge connection. My biggest concern is that it will not adapt to the current battery charge state. Shouldn't the MPPT be able to adapt the voltage to the "maximum power point"?


Similarly, if the 110V charge is directly connected to the battery pack, wouldn't it confuse the MPPT controller regarding the active charge state. The charger now in the RV doesn't appear to be a "smart" multi-state charger. Do I really need to manually cut off the solar feed when connected to a shore line?


By making the controller the master, I was figuring it could optimize best. From everything I've read, it just selects the optimal output voltage for the current charge state of the configured battery type. Do you believe the controller only likes working with Solar?


Yes, I'll need disconnects and fuses and terminal posts. I was simplifying for graphics clarity. I haven't planned them all yet.


Although it's possible to power the entire 110 VAC with an inverter, I was thinking that I wouldn't run the high-powered appliances (A/C, microwave) from the batteries. This would mean a smaller, less expensive inverter, a smaller solar array, and protect my battery power longer. All I really care about is my computer and TV, which have a relatively insignificant draw and happen to feed from the same outlet on the same slide-out.


For the low amperage I'm considering, the Xantrex transfer switches are small, automatic and not very expensive. I like automatic! One less thing for me to remember. <grin> I would mount them inside a sub-panel for protection but without a manual switch.

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Here's a revised drawing with both the 110VAC and the 12VDC feeding the same battery bank.


It's safe to assume that, if plugged into shore power overnight, the batteries will be completely charged overnight. Hopefully the MPPT controller can compensate if the generator is only run for an hour.



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Your truck alternator is supplying the correct charge voltage for the batteries - no need to go through the MPPT controller. MPPT controllers are meant to convert high(er) voltage (18-48 or more) down to 12 (or whatever your system is set up for).

Regardless of how you connect all of your charging sources, each will tend to "confuse" the other when they all are "energized". For example if your shore power charger is pumping 14 volts into the batteries the MPPT controller may "think" the batteries are fully charged because it sees the 14 volts as being a fully charged battery so it begins to throttle back it's charge rate. Likewise, if the MPPT controller is putting out 14 volts, the other charge sources will throttle back. You could almost say the strongest source will end up charging the batteries but that may not always be the case.


Kind of like trying to charge a battery with two chargers - really only one is going to do the work - especially if they are both "smart" chargers.



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