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Thread: RV 12V and 24V

  1. #1

    Default RV 12V and 24V

    I'm doing an RV conversion that I want to wire in 12V for the house. The vehicle runs on 24V (2 x 12V starter batteries). I'm just at the planning stage (haven't bought anything yet or decided on panel or battery bank size etc).

    I'd like to set up the solar batteries and starter batteries so that the RV engine will charge the solar batteries. My current RV does this via a VSR and controller, but it's 12V to 12V. Does anyone know if I can do this 24V to 12V?

    I'm also not clear if such a system charges the starter battery from the solar panel (via the solar battery). I assume it does but I've not tested it.

  2. #2
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    Default Re: RV 12V and 24V

    Anything is possible these days! If you have a 24V RV then you should design for a 24V inverter/chager in the home system. In my offgrid business we only design homes fore 24V or 48V inverter/chargers. Good Luck and keep it simple!

    Are you sure the RV is 24vdc? Most are not!
    http://members.sti.net/offgridsolar/
    "we go where the power lines don't"

  3. #3
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    Default Re: RV 12V and 24V

    There are lots of ways to address the questions you raise--But some of them get a bit expensive and any voltage conversions (like 12 to 24 or 24 to 12) give you more losses and more things to go wrong.

    Like Dave asked--Are you sure the RV is 24 volt? The two 12 volt batteries are typically installed in parallel to give you more current / Amp*Hour capacity, but still at 12 volts.

    Most folks use a switch/relay to keep the vehicle battery system separate from the RV system unless the vehicle engine is running. That way you will always be able to start your engine even if somebody left the lights on the whole day by accident.

    In the end, gather your information and do a couple paper designs before you start paying the big bucks.

    -Bill
    20x BP 4175B panels (replacement) + Xantrex GT 3.3 inverter for 3kW Grid Tied system + Honda eu2000i Inverter/Generator for emergency backup.

  4. #4
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    Default Re: RV 12V and 24V

    Quote Originally Posted by leaf View Post
    I'm doing an RV conversion that I want to wire in 12V for the house. The vehicle runs on 24V (2 x 12V starter batteries). I'm just at the planning stage (haven't bought anything yet or decided on panel or battery bank size etc).

    I'd like to set up the solar batteries and starter batteries so that the RV engine will charge the solar batteries. My current RV does this via a VSR and controller, but it's 12V to 12V. Does anyone know if I can do this 24V to 12V?
    You should be able to. You couldn't go 12v up to 24v without a "voltage converter", but you can go 24v down to 12v.

    To charge a battery you have to apply more voltage than what it is at now. Most "12v" solar panels actually put out more like 18v, which is a high enough voltage difference for current to flow toward the battery.

    Thus, 24v is certainly a high enough differential that voltage will flow into the battery.

    The issue is of course, overcharging. But that's an issue even with an 18v (12v nominal) solar panel and that's why you need a controller - to turn off the flow when the battery is full.

    You need a controller that is rated for that voltage on the input side though, so your current controller (12v-12v) may not be suitable.


    I'm also not clear if such a system charges the starter battery from the solar panel (via the solar battery). I assume it does but I've not tested it.
    Usually no. The solar panels feed into a charge controller, which charges the batteries. There are some charge controllers that can charge two battery banks, such as the MorningStar SunSaver Duo, but I don't think it will do two banks of different voltage - I think it only charges two 12v banks.

    Good luck finding one that will do two banks of different voltages. There may be one, but I haven't seen it.


    It's generally not a good idea to tap multiple charge controllers off of a single PV array, so you wouldn't want to have one charge controller for the 12v bank and one for the 24v bank. (FYI - You can however have two arrays and two charge controllers charging the same battery bank.)


    Personally, I would go with the KISS principle. You will have an inverter on the 12v side, so keep a 24v battery charger on hand just *in case* you need it to charge the starting batteries (left the headlights on or whatever). If you aren't drawing anything from the starting batteries while at camp, then you should rarely, if ever, need to charge them anyway - normal driving will take care of them just fine.

  5. #5

    Default Re: RV 12V and 24V

    Thanks everyone, and I appreciate the reminder to keep it simple.

    Dave, the truck is 24V, the house will be 12V.

    It's not a commercial RV, it's a truck that will have a house built on it. I want 12V in the house because I'm already set up to run off 12V (laptop, tv, stereo, ph chargers etc).


    Bill, the two 12V truck starter batteries are in series (connected to each other by positive to negative, not pos to pos and neg to neg). The VSR is the relay that will disconnect the starter batteries from the house ones.


    Dwh, ok, that makes sense, I'll look around and see if I can find a controller that will handle 24V and 12V. The manual of the controller model I have currently says it is rated for 12V and 24V but I'm not clear yet if that's two different models or one that can handle both voltages.

    However it does say the input shouldn't be higher than 25V. Would the truck batteries put out more than that?

    The main thing I want to do is have extra charging capacity from the engine as a back up for the solar eg for in winter. If I get enough panels and battery this shouldn't be such an issue, but it's more about future proofing. I tend to park up in out of the way places so I want that additional back up if something goes wrong.

  6. #6
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    Default Re: RV 12V and 24V

    Regarding 12/24 volt controllers... Probably 99.999% of the time, that controller can only do one voltage (12 OR 24) volts at a time... It cannot charge two "parallel" 12/24 volt battery banks at the same time.

    There is at least one (that I know of) charge controller from MorningStar that can charge two separate 12 volt battery banks from one array/controller.

    And for charging a 24 volt battery bank, your controller would have to have an output of ~29 volts and Vpanel-input would be ~Vmp=35 volts for the array Vmp, and the Voc rating would be at least ~45-50 volts (voltage open circuit).

    What you may be looking at for your 12 / 24 volt controller--The V=25 volts may be for when charging a 12 volt battery bank, and they have another value when charging 24 volt banks.

    So, that leaves you with an issue...

    • Two sets of arrays--one to your 12 volt controller and a second to your 24 volt controller;
    • One controller that you can flip between 12 and 24 volt bank (be careful--controllers many times have to go through a "boot process" to identify the battery bank voltage in question--Or have a 12/24 volt switch, etc.).
    • Variations of above
    • Or you charge one bank with solar (primary bank), and then use a second charge controller to go from 12 volts to 24 volts; or from 24 volts to 12 volts... Both are doable--Both add cost, complexity, and some additional losses (another ~5-10% or so).

    There are "silly things" you can do in an emergency--Use your 12 volt AC inverter, plug a an AC 24 volt battery charger onto your truck battery for emergency/long term parking use.

    Lots of options--probably comes down to "How Automatic" you want your setup to be and how much are you willing to spend.

    A drawback with the automatic approach is--For example, you leave some truck lights on and go hiking for a couple days--You now have discharged both your truck and "house" batteries and everything is still dead when you return. The manual charging connection reduces the chances those types of Oops will happen.

    -Bill
    20x BP 4175B panels (replacement) + Xantrex GT 3.3 inverter for 3kW Grid Tied system + Honda eu2000i Inverter/Generator for emergency backup.

  7. #7

    Default Re: RV 12V and 24V

    Let's see ...

    You could wire your solar array for "24 Volts" and run it to the truck batteries via a PWM controller, then pick up the 24 Volts (probably 27-ish) and feed that through an MPPT controller to keep the 12 Volt "house" batteries up.

    Just an idea. Probably too expensive.
    1220 Watts of PV, OB MX60, 232 Amp hrs, OB 3524, Honda eu2000.

    Ohm's Law: Amps = Volts / Ohms
    Power Formula: Watts = Volts * Amps

  8. #8
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    Default Re: RV 12V and 24V

    Quote Originally Posted by leaf View Post
    However it does say the input shouldn't be higher than 25V. Would the truck batteries put out more than that?
    Yea. 24v nominal battery bank would be more like 28v when fully charged. But the batteries aren't the real problem - the alternator has to put out over 30v to charge the 24v battery bank up to 28v.

    The main thing I want to do is have extra charging capacity from the engine as a back up for the solar eg for in winter. If I get enough panels and battery this shouldn't be such an issue, but it's more about future proofing. I tend to park up in out of the way places so I want that additional back up if something goes wrong.
    Best way to would probably be a programmable split-charge relay (SmartBank) from SmartGuage:

    http://www.smartgauge.co.uk/


    Easiest might be to use a charge controller connected 24v on the one side and 12 on the other. BUT you need to make sure it's turned off when the engine is off - otherwise it might (probably will) drain the engine battery trying to charge the house batteries when the engine's off.
    Last edited by dwh; September 16th, 2010 at 15:27 PDT.

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