Off Grid newb. Ideas?

renji
renji Registered Users Posts: 2
in Off Grid Solar & Battery Systems #1
I was researching ideas for my camper when I stumbled on this site and decided to just ask. I need ideas for how to approach my system. Help?

Info
I own a slide-in truck bed pop-up camper.
I have two batteries at my disposal:
(1)Interstate Marine/RV Deep Cycle Part SRM-24, Cranking amps 690, Reserve capacity 140
(2)Kirkland Starting/Deep Cycle Marine/RV 750 MCA, 115 Amp Hours
(They both sit right next to each other in the camper)
The camper is rigged so that my truck's alternator will charge my battery in the camper (I think.. How to check the connection?).
I need to run: Small water pump, low amp LED lights (not running all the time), possibly a small heater, and lastly an inverter for using a laptop and other 120v appliances at times.

I plan on getting a small solar panel as well, but right now my main question is this: Is it feasible to get a battery switch to switch between batteries for charging by the alternator and usage? So if battery 1 got used all night, switch to battery 2 for usage the next day, and then back to 1 when driving again? I am pretty sure I have seen said switch in a nice offshore boat I used to go out on. It had a switch that switched between batteries for starting and charging.
Is a 24v system more sensible? From what I've read so far, and again I am a newb, for my purposes the answer is no.

Now say I get the solar panel. Can I use the same switching system to change between charging the batteries by solar? Is it possible to have it charge both without it being a 24v system? I am familiar with the usual setup of solar, so no need to explain that in full.

Hope this explains enough! Any thoughts are appreciated.

Jake
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Comments

  • BB.
    BB. Super Moderators, Administrators Posts: 33,613 admin
    #2
    Re: Off Grid newb. Ideas?

    Welcome to the Forum Jake.

    I would suggest a DC Current Clamp DMM (this one from Sears is "good enough" for our needs). You simply set for DC Current, zero the meter, and clip the meter on the wire you want to measure the current in.

    Marine batteries do not make good deep cycle batteries... Their cycle life will not be very good if you discharge them towards 50% state of charge. You would be better off replacing them with a true Deep Cycle Battery (some people do have pretty good luck with Marine batteries in RVs--But we don't recommend them).

    And a car alternator can help charge the battery bank (proper battery isolator switch, pretty heavy/short cables from alternator to battery bank, etc)... But it is not ideal--An alternator tends to be set for ~13.8 to 14.2 volts charging. For a flooded cell deep cycle battery, you really want closer in the range of 14.5 to 14.8 volts or so as the charging set point--And when equalization, you want >15 volts (roughly).

    For solar, there is a neat smallish charge controller that is designed to charge a house and vehicle battery at the same time:
    wind-sun_2272_2667310.gif
     SunSaver Dual Battery 25 Amp 12 Volt Solar Charge Controller

    Sizing the design is always an issue... Ideally, I would like to know how much power (AH or Watt*Hours per day) you plan on using.... How many solar panels you can fit on the roof, how big of battery bank, etc...

    Can you tell us more about your loads (dry camping weeks at a time, weekend camping, summer/winter, where, etc.)? What is you expect from a solar powered system on your RV?

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • renji
    renji Registered Users Posts: 2
    #3
    Re: Off Grid newb. Ideas?

    Thanks for the help! It is most likely that I will not be dry camping weeks at a time. So far I see myself staying 5 days or less dry. I am in Montana and the winter is fast approaching. I will spend many nights in the ski resort's parking lot :D. After this winter I plan on travelling the north west and staying in as many free places as I can. Some may be near civilization like walmart's overnight parking, and some may be 15-40 miles out in national forest land.

    Usage: The water pump will be used very intermittently as I usually bring separate water with me for drinking. In the winter I will not be using it at all.. I will use my laptop no more than 4 hours at a time and the AC adapter says the input is 100-240V ~ 2.5A. I am using very low amp LED overhead lights that I usually use no more than 2 hours a night. I usually only turn on 1. I may occasionally charge DSLR camera batteries with the inverter, but that will not be often at all. I will most likely charge my iphone every day. That all that I can think of.

    With that being said, I do not really want to spend the money for different batteries. 1 is brand new, the other is very new as well. I can really only see 1, maybe 2 panels fitting on the roof. I have plans for a small roof rack that will take up some space, and there is a small sun hatch smack in the middle of the entire roof. The roof is roughly 7ftx11.5ft. the sun hatch is 1.5ftx1.5ft.

    Thanks a lot!
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  • Plowman
    Plowman Solar Expert Posts: 203 ✭✭✭✭✭
    #4
    Re: Off Grid newb. Ideas?
    Usage: The water pump will be used very intermittently as I usually bring separate water with me for drinking. In the winter I will not be using it at all.. I will use my laptop no more than 4 hours at a time and the AC adapter says the input is 100-240V ~ 2.5A. I am using very low amp LED overhead lights that I usually use no more than 2 hours a night. I usually only turn on 1. I may occasionally charge DSLR camera batteries with the inverter, but that will not be often at all. I will most likely charge my iphone every day. That all that I can think of.

    Some example numbers to help you size your system, based on measurements of my RV system:

    RV water pump -- 4.5 amps @ 12V x 15 mins/day = 1.13 amp-hours/day
    Laptop (older 15” HP) – 2.5 amps @ 12V x 4 hrs/day = 10 amp-hours/day
    LED 1141 lamp – 0.13 amps @ 12V x 4 hrs/day = 0.52 amp-hours/day
    Inverter (400W Cobra) – 0.8 amps @ 12V x 12hrs/day = 9.6 amp-hours/day
    Smartphone (charging and used as hotspot) – 0.96 amps @ 12V x 4 hrs/day = 3.84 amp-hours/day

    TOTAL = 25.09 amp-hours/day @ 12V

    Round it up to say 30 ah/day. Pretty minimal. To provide this, including running battery down to 50% and with 2 days autonomy, you'd need:

    30 / 0.5 x 2 = 120 ah 12V battery

    For a 120 ah 12V battery, you'd need roughly 225W of 12V solar panel for the oft-recommended 10% charge rate. Might be able to get by with a lower charge rate if you use the truck alternator or shore power regularly. Could achieve this with one big grid-tied panel (and MPPT controller) or a couple smaller 12V panels (with cheaper PWM controller). I'd round everything up to the biggest you can afford to account for inefficiencies and load growth over time.

    As for your marine batteries, they'll work fine for a while, then they won't. At which point replace them with true deep cycle batteries. Try to find their 20-hour amp-hour capacity rating and see how they compare to the numbers above.

    If I wanted to build this system from scratch, I'd consider:
    --$300 Two Renogy 100W panels (preferably 3)
    --$200 Trojan SCS225 (12V 130 ah deep cycle battery)
    --$95 Morningstar PS-15 charge controller

    Say $1000 or less total when you include the panel mount, wire, fuses/breakers, a meter or two, etc.

    For switches you might check out the Blue Seas line of products. They have some dual circuit switches, primarily for the boat market. I have one of their single circuit switches, it's a solid little unit.
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  • DavidOH
    DavidOH Solar Expert Posts: 112 ✭✭✭
    #5
    Re: Off Grid newb. Ideas?

    In order to get more out of your small system ( ..or any system ) increase your efficiency.
    Look for 12 volt appliances so you won't have to take losses using an inverter.
    Converting 12v to power a laptop ( often 12 volts changed to 19 volts ) is better than converting power to AC then BACK to DC.
    Many items can run on 12 volts. Computers, Televisions, digital recorders, wireless modems. Keep looking for ways to NOT use the inverter. The losses on those can be 20 -25 % on some.
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