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Old 12-15-15, 10:53 AM   #11
oil pan 4
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Buy the time you get any 100 watt panel shipped to you, I think you will be looking at spending at least $130 to $140.

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Originally Posted by doug30293 View Post
To help offset the system cost I was thinking of installing a preheat tank for DHW in my house and powering it with the solar array when the trailer isn't in use. DHW is a big expense for us, even in summer.
This is almost exactly what I do with my suburbans solar roof top.
See:
Vehicle solar panels to grid - Fuel Economy, Hypermiling, EcoModding News and Forum - EcoModder.com

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Old 12-17-15, 06:19 PM   #12
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Home Depot doesn't charge shipping if you pick them up at a store. They had them for about $120 a while back but I procrastinated. Now they are $134 - still not bad. I would rather refuse a broken panel at the store than have a three way fight involving UPS or Fedex.

Oil Pan 4:
I like your thinking. Put the excess PV energy into vampire, or in my case, continuous loads. Around my house there is always water to heat. I will look for an MPPT with the ability to divert the PV energy when the batteries are in float mode.

With a few more panels I could even run a window shaker once the DHW reaches setpoint.
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Old 12-17-15, 09:09 PM   #13
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I know of no MPPT with a load dump. But I have really only looked at genasun and Morningstar.
It turns out the MPPT charger and grid tie work together rather well.
If the batteries are hungry for power the MPPT charger will gobble up almost all of the available PV power. Then when the batteries are full the MPPT charge controller will go into stand by mode where it draws almost no power and then the grid tie will start soaking up that excess power.
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Old 12-19-15, 08:35 AM   #14
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Originally Posted by oil pan 4 View Post
I know of no MPPT with a load dump.
I suppose some variation of this idea is available in all but the least expensive controllers. Otherwise an off grid system would waste a lot of energy once the batteries are up. This is the kind of information I need before buying components.

Doug
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Old 12-19-15, 09:50 AM   #15
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I just run the MPPT charge controller and grid tie inverter together.
If the batteries in the camper sit idle then the solar panels and charge controller form the worlds most expensive battery maintainer.
I hooked the grid tie into the solar panels circuit.
The cool thing about the MPPT charger is it will soak up almost all the power and put it into the batteries when the battery needs power. While the battery is charging the grid tie puts out about 5 watts. Then once the battery's MPPT charge controller tops up the battery and goes into stand by mode it draws very little power, as that happens the grid tie output ramps up. No need to get your self another expensive MPPT charge controller with load dump.
Its almost as if I carefully selected components to do this, but in reality I just recombined old junk that was collecting dust into a new configuration for most of the build.
I also didn't just haphazardly throw all this stuff together. I do have an associates in applied sciences for wind and solar power generation, what I studied didn't exactly cover this application but, as I put it together I had a pretty good idea it was going to do exactly what I wanted. So when it worked, it really wasn't a big surprise.

I think what you have is just going to be a larger version of what I have.
Really the only major component you need to pick up is a 12 to 28 or 14 to 28 volt grid tie inverter.
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Old 12-19-15, 12:44 PM   #16
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I looked at the documentation on the Morningstar MPPT-45 this morning. It has four auxiliary relay control outputs (open collector, I presume) but I can't find much in the way of an explanation how they are programmed. The operator manual refers me to the website which refers me to a data sheet on the relay drivers which refers me to the manual. Morningstar has a good reputation but this is not comforting.

My guess is that an output could be programmed as a load dump. I just haven't found the information.

The Morningstar is about double the price of the Renogy MPPT-40A. This could be justified by the Morningstar's PC connectivity options. Perhaps the software interface manual will tell me more about the output programming features. I like the Modbus interface. If the software is any good it would replace the need for a separate battery monitor.

Grid tie seemed like a good way to utilize otherwise wasted energy. Then I read the power company's requirements. So much for that idea.
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Old 12-23-15, 11:17 AM   #17
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It turns out the relay driver is a $160 option. For a little less money one could instead use a brick PLC with six outputs instead of four and also have eight inputs. The Tri-Star Modbus interface is well documented and shows that it would be a simple matter to interface with a PLC to make a load dump.

I couldn't find a manual for MSView on Morningstar's website and there isn't one in the downloadable zip file. I didn't install the software but I suppose a help file is included in the executable.
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Old 01-07-16, 05:19 PM   #18
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OK, new problem. I have a converter/charger in the RV of the type known to destroy batteries. A new 4 stage Iota 45 amp converter is $187. Not bad but is it necessary?

The MPPT provides four stage charging when the sun is shining and becomes a brick when the sun goes away. Rather than buy the Iota would it not be simpler to provide a backup DC source for the MPPT in lieu of the PV array? A simple current limiting supply can be built for much less than the price of the Iota.

I can provide 120V to the supply from a generator or the pedestal at an RV park. Having one charging device means I only have to monitor one system.
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Old 01-07-16, 09:33 PM   #19
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That's what I did.
Took a PV charge controller and powered it with external non-PV power.
But I built my charger to mimic the output of a PV array, around 21 volts DC open current and then drops down to between 17 and 15 volts while under load.
Just like a solar panel.
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Old 01-12-16, 11:26 AM   #20
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Default supercaps

Buy blemished solar panels for about 50 cents a watt and get enough deep cycle batteries to handle the normal loads and use supercapacitors hooked in series to match the voltage of your solar panels. Most good panels are about 300 watts and 34-37 volts. The caps will handle the heavy loads for about 2 minutes and recharge very fast. By adding caps you do not need as much cranking amps in the batteries.

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