Off griding in a travel trailer

System
System Posts: 2,511 admin
in Off Grid Solar & Battery Systems #1
I am off gridding in a travel trailer and my electrical needs are for 12 volt lighting, 12 volt furnace blower motor and 12 volt water pump.

Right now I have a very crude system set up. I have 2 Kyocera 135 watt 12 volt panels on a home made rack that I use to manually track the sun.

These panels go into a home made combiner box and then to a Tristar TS-60 and then to my two low budget 115 AH Group 27 marine deep cycle batteries that I bought new at Auto Zone. They are wired in parallel to keep the system at 12 volts.

Right now I am using the TS-60 as a charge controller. It seems that I am often capable of using more power than my batteries can store.

Should I be using the TS-60 as a diversion / dump load and heating some water or doing something else productive with the excess power???

My six gallon propane water heater also has a 120 volt heating element. What would happen if I dumped 12 volts into that 120 volt element??? Or do I need to buy a 12 volt element?

I have also noticed that my system only produces 230 watts / 16 amps. I have never seen it reach its full 270 watt potential. Is this because the charge controller doesnt allow it based on the size of my battery bank and or battery bank DOD or using 6 awg wire for 25 feet to battery bank (resistance in wire) or angle of the sun early in the morning??? How does one go about diagnosing something like this?

The plan is to eventually run a conventional 120 volt 12 cubic foot refrigerator off this system once I build it up some more.
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #2
    Re: Off griding in a travel trailer

    I can see you're a bit confused about how it all works. The only thing wrong with your charge controller is that i is very large for the amount of panel. Panels rarely put out their nameplate rating. Conditions have to be ideal for that, and during the course of a day they almost never are. In general we use a "typical" derating factor of 77% What you could expect from a pair of Kyocera135's:

    two 135 Watt panels = 270 Watts. @ 77% = 207 Watts. If you do better than that, you're doing well. The Kyocera's Imp is around 7 Amps, so an output over 14 Amps is unlikely. An MPPT controller can boost output, but isn't a good investment on such a small system. Cold temps will also increase output.

    You have a total of 230 Amp hours of battery (if those ratings are correct - RV/Marine batteries not the best choice). The panels should be able to charge that.
    Should I be using the TS-60 as a diversion / dump load and heating some water or doing something else productive with the excess power???

    I doubt you have any excess power. See:
    Right now I am using the TS-60 as a charge controller. It seems that I am often capable of using more power than my batteries can store.

    That is correct: the batteries are the limit for power storage. Panels' limitation is how much you can "replace". At 50% DOD the most you could use from the batteries is 1380 Watt hours (approx.). The panels over four hours of "equivalent good sun" would only harvest about 800.

    So the question is how much power are you using each day? If you are taking the batteries down 50% you would probably have trouble fully recharging them. If you're only using 25% you would have a small amount of "surplus" power available from the panels. A battery monitor would be a good idea here: http://www.solar-electric.com/metersmonitors.html
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,613 admin
    #3
    Re: Off griding in a travel trailer
    Eric R. wrote: »
    My six gallon propane water heater also has a 120 volt heating element. What would happen if I dumped 12 volts into that 120 volt element??? Or do I need to buy a 12 volt element?

    First--The safety warning... If you make this a 120 VAC / 12 VDC switchable function, you need to use a two pole transfer switch--You don't want one leg of a 120 VAC utility circuit powering part of your DC battery system.

    Second, heating elements can be defined by V^2 / R = Power... The difference in power between 120 VAC and 12 VDC on such an element:
    • 12^2 / 120^2 = 1/100 when operated at 12 volts
    So for example, a 1,000 watt 120 VAC element will only consume around 10 watts at 12 VDC... If you could find a "higher power" element that would fit the heater it would something to think about...

    Now we get into "thinking about" and Safety... There is are thermostats and limit switches... placing 10 Amps of 120 VAC through a limit switch vs 10 amps of 12 VDC is not the same thing. DC current is much more difficult to "break" for a switch. Running the same level of current at DC will probably result in a short switch life and possibly even welded contacts (not a safe thing).

    Lastly, you have to review how your thermostat works... It may be an electronic relay which will not function correctly on DC.

    All in all, very little heat for a whole bunch of trouble.

    I probably would not recommend it unless you setup a separate temperature controller that is rated for the DC voltage and current you wish to support. May not be cheap.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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