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Old 07-19-2014, 11:08 AM   #1
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Solar charge controller question

Planning on a solar system (for our MH, not like the original one). We notice some charge controllers offer a connection for “Where to utilize the extra solar-generated electricity after the batteries are happy” and my head is spinning at the thought – what to do with the extra electricity?
If you have such an option, what do you do with it? Our best thought so far is to heat the water in the water heater. We are thinking 400 Watts of PV panels, so it won't be a bunch of extra electrons, but what to do?
Sorry but we won't be online for the rest of the day to reply as we are going “camping” and doing a stargazing event, this time at Tippecanoe River State Park near Winamac, Indiana. Saturn is quite lovely these evenings, BTW.
Thanks for all the help you all provide!
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Old 07-19-2014, 04:11 PM   #2
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Alot of good reading here --- >> HandyBob's Blog « Making off grid RV electrical systems work

There are two types of charge controllers PWM and MPPT. PWM you have to connect you solar to the same voltage as your battery bank. So if you plan on 4 panels at 12VDC you need to put them all in parallel. The MPPT you can hook all your panels in series have have a higher voltage into the charge controller and then the controller will match your battery bank.

Read Bob's stuff there and it may help you out a little also.
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Old 07-19-2014, 04:16 PM   #3
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You could always buy another battery and store the energy so during the times when it's overcast or raining you won't have to depend on a generator to bring the batteries back up to full charge.
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Old 07-19-2014, 05:41 PM   #4
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Isn't that a nice problem to have
I turn the hot water tank on a sunny morning on electric and have a shower for free.
We have six panels connected in parallel (PWM controller), didn't want to spend the money for an MPPT controller plus a shadow on one panel would affect the performance of the whole system. On the other hand with panels connected in series with an MPPT controller you could reach usable voltage earlier in the morning and later at night.
My personal opinion is go with PWM and use the money saved to buy battery capacity.
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Old 07-20-2014, 08:03 AM   #5
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The extra power capacity you have could be used with a grid-tie inverter. They take your dc solar power and converter it to 120vac which is then put back into the shore power. THIS IS NOT A REGULAR INVERTER! They aren't cheap. Although cheap ones are available, they are not UL recognized and may cause a fire. Google "grid-tie inverter"

I thought about the wasted capacity of my system as well. But the price of the grid tie inverters and the minimal output makes them almost a waste.

I bought a 12v dc-dc battery charger. Yes they make them. I haven't hooked it up yet, but I planned on using the extra capacity to charge a spare battery. Then I realized my MPPT charger HAS an auxiliary output for charging another battery once the main battery is charged.
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Old 07-20-2014, 02:45 PM   #6
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Ken, thanks for the link. Handy Bob's material is very helpful. In considering MPPT vs. PWM for our installation, today I am leaning PWM, as I only lose about 1% less of the collected power using a higher voltage feed from the roof.
Murphie, another battery will likely be needed anyway as we are using the original 2 group 27's. The coffee maker is okay but the microwave may push us over that edge. My plan thus far is 400 Watts PV, a bit of over-kill for our current (sorry) battery capacity.
Cypress, I am wondering about a heat pad on the water heater, with temperature sensing and dry operation prevention, so we don't need to use LP to heat water; I haven't measured the water heater's 120V wattage and don't want to lose another 10% through an inverter. Agree, would go MPPT only if I had a compelling reason and I haven't thought of one yet. I am planning less than half your PV input and hoping to use the microwave + coffee maker!
RP, most of our camping doesn't involve hook ups anyway, and we can't use the power at home, so we can't go that route. I am still wrestling with the concept & price of a large pure sine wave inverter, let alone grid-tie capable.
Thanks to you all!
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Old 07-20-2014, 05:28 PM   #7
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We have this much Solar because of our Frigidaire res. fridge and when we have electricity to spare we just run the water heater on electric in the morning. Important part is to have the batteries full by late afternoon.
I don't think a heat pad would work for you on the water heater because of the insulation but there might be some other solution which I'm not aware off.
When you install your panels try to use the space on the roof so that you have room for expansion if you decide to upgrade in the future.
With our setup we only need the generator to run the AC/s when the temp. gets above 85 deg. inside. With two ceiling fans, windows open and three MaxxAir fans running at 50% it's actually very nice.
Caveat is, we can not camp under trees because of shading and tree sap.
FYI.: we have 6 160Watt 12Volt panels, a 60AMP PWM charge controller, 6 6Volt AGM batteries with 230 AMP/h capacity each which leaves us with ~350AMP/h usable, a 1800Watt Xantrex Pro msw inverter which came with the trailer to power the fridge and TV and we also installed a Xantrex Freedom 3012 psw inverter/charger. We have it wired so that either inverter can be used (why toss the one that's already there). The lights don't even blink when we turn on the microwave/convection oven and the water heater at the same time.
I understand that msw inverter are more efficient than psw inverters but the cleaner electricity allows you to operate all appliances and they run more efficiently.
Are we crazy, yes.
Do we need (want) full hookup's, no.
This truly is roughing it in the comforts of home
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Old 07-20-2014, 05:36 PM   #8
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I'm just thinking, our water heater draws ~11AMP AC, you could wire a msw inverter through a 3 way switch to your water heater. Just make sure you're not backfeeding into the system, or you could plug water heater into the inverter if it's not hardwired.
What could go wrong if the water heater only gets - lets say 5AMP from a smaller inverter - probably nothing, but it would always be an option to assist the LP burner.
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Old 07-20-2014, 07:42 PM   #9
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If you plug a larger load into an inverter than what it can handle, the inverter will go into overload. No other possibility. Likely just shut down and alarm. An inverter won't limit the current on its load, as the voltage is fixed (120+/-) and the resistance of the load (in this case a water heater) is fixed. So the inverter must be able to supply the same or more currently than is drawn.

Now if you could somehow add resistance to the circuit to lower the current draw.....you might have something there.
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Old 07-20-2014, 08:53 PM   #10
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Quote:
Originally Posted by RPAspey View Post
If you plug a larger load into an inverter than what it can handle, the inverter will go into overload. No other possibility. Likely just shut down and alarm. An inverter won't limit the current on its load, as the voltage is fixed (120+/-) and the resistance of the load (in this case a water heater) is fixed. So the inverter must be able to supply the same or more currently than is drawn.

Now if you could somehow add resistance to the circuit to lower the current draw.....you might have something there.

Okay, but then he'd be pretty much back to square one from a cost standpoint - a large enough inverter.
In an RV solar setup situation you can't say economical but least painful would be a matching solar/battery setup with room to grow (adequate wiring).
Aren't we all geniuses in spending someone else's money, so to speak
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