Rv solar system questions

System
System Posts: 2,511 admin
in Solar Beginners Corner #1
I have been intrested in solar power for several years and want to hook up and install a solar system on a camper but I have several questions about buying and installing a solar charging system and could use advice before buying. What I currently have is 3 interstate Deep cycle batteries that are not mached in age,3 to 7 years old and a Honda eu3000si generator. I use the generator and a charger to keep the batteries charged. The batteries have gotten to the point I have to run the generator several hours a day to keep then charged.I need to repace batteries and I have several questions before doing that. Everything in the camper is 12v except the microwave,a/c and tv. I have a 12v 1000w inverter for the tv and to charge laptop battery.If I run the micowave I use the generator. When the batteries were newer I could go two to three days before running the generator to charge, dropping battery voltage to 11- 11.5 volts.When camper is not in use it is plugged in to 120 power and batteries maintained thru the camper converter. After a lot of research I have several questions before buying anything.
I am looking at getting a 30a mppt charge controller, two 185 - 200w solar panels and batteries. Questions are
(1)Should I buy 6 or 2v batteries?
(2)hook up solar panels in parallel or series
(3)hook up batteries in parallel or series 12 or 24 volt. If I hook up in 24 volt, can I hook the camper load directy to battery instead of thru the controller. Will the batteries equalize or will it kill the battery the load is pulling from.
I also want the panels to maintain batteries when the camper is not in use.I plan to keep the panels portable, I dont want to have to park the camper in the sun to charge the batteries.
I was looking at the 2 of the Trojan T105 6V 225ah batteries, or two of the Trojan 12 batteries hooked in series.
Any help, advise or ideas would be appreciated. We like to dry camp and don't like to listen to a generator running. We also plan on full time camping in a couple of years and would like to be able to add to the system later if needed
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Comments

  • BB.
    BB. Super Moderators, Administrators Posts: 33,622 admin
    #2
    Re: Rv solar system questions

    In general, "we" (at least a lot of us ;)) recommend that you measure your loads first and figure out how much power you will use in a day (amp*hours at what voltage, or Watt*Hour/kWH). Here are a couple relatively inexpensive ways of measuring your power usage:

    Kill-a-Watt Meter (120 VAC Watt*Hour meter)
    DC Amp*Hour / Watt*Hour meter (for DC applications)

    If you already have your battery bank/system setup--You can use a Battery Monitor to see what you are using today from your battery bank... Very nice for managing your battery state of charge (don't need to use a hydrometer as often, and for sealed/AGM type batteries--you cannot even use a hydrometer).

    Solar power tends to be very expensive, and you want to first conserve, second measure, and third to plan out your system.

    Regarding batteries--I personally like to recommend that you have one series string of batteries to give you the voltage/AH rating you need... And in any case, I would recommend you avoid more than 3 parallel strings of batteries (this is a personal recommendation--there are people with lots of parallel connected 12 volt batteries that are happy with them). Basically, it becomes a pain to get current sharing among the batteries, and each string needs an (expensive) fuse or breaker per parallel connection to protect against short circuits in the bank (very few people use parallel fuses per string on battery banks it seems--But all you need is one short to make you a believer).

    So--what the means is--for example you want 450 AH at 12 volts... You could put two 225 AH 12 volt batteries in parallel or two 450 AH 6 volt batteries in series.

    In theory, both battery sets will cost you about the same. But with one you will have 6 cells to water/check and one series string (2x 6 volt series batteries) vs 12 cells to water/check and two parallel "strings" (2x 12 volt parallel batteries).

    You can look at this link for how to best series/parallel connect a battery bank to balance current flow (and resistance) among the individual batteries.

    www.smartgauge.co.uk/batt_con.html

    Generally, if your inverter/peak loads are around 1,200 watts or less, a 12 volt battery bank will probably be OK... 1,200 watts and 12 volt battery bank, you are looking at 100+ amps to run your inverter with a maximum of 1.0 volts or so of wiring drop for everything to work correctly.

    If your loads are 2,400 watts or less, a 24 volt battery bank is a better choice.

    And above 2,400 watts, look at a 48 volt battery bank.

    While you can use a microwave on a battery bank+inverter (a hand full of minutes per day--not cooking 3x meals a day)--You are better off to conserve as much power as you can and use alternative fuels (propane, etc.) for cooking, hot water, and heating as these loads from a solar system can drive the costs way up.

    Roughly, we use a 5-13% rate of charge for a battery bank (i.e., 10% of a 500 AH battery bank is 50 amp charging) as a starting point.

    For pure storage, you can look at 1-2% rate of charge to keep the batteries charged over winter OK. But, you would not be able to draw much power for your TV/lights/etc. without using the generator to recharge the bank.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    #3
    Re: Rv solar system questions

    if your rv has lots of stuff that runs on 12v, and i'm guessing it does, then stay with the 12v battery system or you will have the expense of a converter and a loss of power due to the idle of the converter and the efficiency of the converter. higher voltage systems have their place, but most rvs and campers seem geared to 12v.
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  • dmiller
    dmiller Solar Expert Posts: 68 ✭✭✭✭
    #4
    Re: Rv solar system questions

    1) 2 or 6v will largely depend upon the amp hours of your design. Figure that out and then shop for batteries. 6x L16 2v cells is a classic larger boat configuration, but very large for most RV configurations.

    2) Short answer on an RV panels is Parallel. Partial shading of a panel, especially in good light, is the reason. Roof top design is very important.

    3) Short answer is 12v. With hindsight 24v would probably have been a better standard. But it's not the standard, and in a mobile app you are dealing with 12v consumers.

    You can certainly do your bank in both parallel and series, But you probably want each physical cell to be the same. I'm sure there are many people here who can specifically advise you. I don't know that much about specific wet batteries brands.

    Also note in wiring batteries that electrically correct and safe does not automatically mean the configuration is optimal. "Optimal" is the wiring that best pulls the same AH from each battery. Not an issue with 2v cells.

    I used 12v 8D AGM batteries in a bus parallel configuration due the rough and remote places I go. I can simply take a suspect battery out of service by disconnecting a cable and keep going. I would not use 12v batteries in a typical RV needing large capacity. So there is no one answer.

    Edit: I recently became aware of "inverter microwaves" to use with undersized inverters. You may want one of those.
    You're aware you don't want to go under 12v? You batteries failed early due to a degenerative process caused by too infrequent charging. It's expensive to have too little battery.
    The good news is that batteries in RVs with solar panels tend to last a long time. The frequent gentle charge and the chance to bring the batteries up to 100% is the reason. As a rule of thumb use the generator to bring the batteries from 50-80% charge. And then let solar (or plugged in) do 80 to 100%.
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  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    #5
    Re: Rv solar system questions

    often times the weight of the battery in question plays a role in a decision too. if identical high capacity 6v batteries are too heavy for you to manage then it is possible to get identical individual 2v cells and place 6 of them in series to get the 12v rather than 2 6v batteries in series.
    use quality interconnections to avoid failure or unacceptable voltage drops. i always advocate using larger wires for battery interconnects, but at a minimum they should be as large as the largest wire size connected to them and this is often the wires, of proper size, to the inverter.
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  • dwh
    dwh Solar Expert Posts: 1,341 ✭✭✭
    #6
    Re: Rv solar system questions

    Some thoughts...

    Check the camper's converter and see what voltage it puts out. I went and grabbed some parts for my old camper from a Winnebago that someone was parting out, and was surprised to see that the converter's output was rated at 12.6v - which is NOT enough to fully top off batteries. Chronic undercharging will shorten the batteries' life.

    Some battery chargers are 3-stage, but if you are running loads from the batteries at the same time you are charging them, 2-stage is recommended (converter/charger).

    6v or 12v batteries doesn't matter, since either way you'll probably hook them up to get 12v. I would go with 12v AGM batteries. AGMs normally don't need to be equalized (and often they recommend NOT EQing because over-voltage can create pressure and blow the seals). If you are charging with a 2-stage converter/charger the batteries won't get EQ'd anyway, so AGMs are perfect for that application, as long as the converter puts out a high enough voltage to charge them fully. (And turn off the EQ function on the solar charge controller.)

    For full-timing, you might want more battery capacity. Battery life is rated in cycles, so if you discharge only 20% you might get 3000 cycles out of them, but discharging them 50% they might only last 500 cycles and taking them down to 11v like you have been they won't last nearly as long.

    You can't hook up 12v loads directly to a 24v battery bank - you would need a 24v-12v converter.

    You don't hook up any loads to the solar charge controller- that is purely to charge the batteries from the solar panels. Basically, it goes like this:

    charger (solar or converter or both) -> battery -> loads
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  • bryanl
    bryanl Solar Expert Posts: 175 ✭✭✭✭✭
    #7
    Re: Rv solar system questions

    The first place to spend money when it comes to RV battery systems is your converter, charger, and maintainer. You need a charger that will provide a prompt and vigorous charge and then transition to a maintenance mode that will inhibit sulfation and keep your batteries in shape. (the ChargeWizard based converters are a good example)

    The limitations on how much battery you can carry mean that the composition of your battery bank is more a matter of convenience than anything else. The differences between series or parallel or battery brand or battery type tend towards insignificance.

    Get batteries from a reputable retailer who sells a lot to folks who use them like you do, offers a good warranty, and will stand behind what he sells.

    Plan on at least a watt of solar for each pound of battery. Don't expect an RV solar system to support and extravagant lifestyle and do not expect it to properly keep your batteries in top shape.

    Keep in mind that batteries slowly degrade over time. Like the car owner who only finds out he needs a new battery after the first cold snap of the season, you really shouldn't see an immediate need to replace your RV batteries. Your charging needs observation is probably a good indicator of time to replace batteries.
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