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Thread: Running 12v appliances off a 24v pv system

  1. #1

    Default Running 12v appliances off a 24v pv system

    Planning a SHTF backup/emergency solar power system (pv-cc-battery bank-inverter)... call me a "prepper" if you want, imo i just think we are going to see power outages in the near future ranging from 2 weeks to 3 months at a time and this being "normal" for quite some time... i know, call me a nut job like my wife does :) anyway, i have some questions i am not sure about:


    1) What are the problems of running low amp 12v appliances like lights or fans off of a 24v system? i assume you would need a 24v/12v dc stepdown converter?


    2) Any problems with using a 24v dc to 120 ac inverter to run low amp a/c appliances like a laptop, recharge cell phones, a tv, or a fan?


    3) If the 24v battery bank dies and can not be recharged, can dc or ac appliances still be run off of the charge controller?

  2. #2
    Join Date
    Jun 2003
    Location
    pittsburgh, pa
    Posts
    10,278

    Default Re: Running 12v appliances off a 24v pv system

    1> none as long as the converter and battery can deliver the current properly.

    2> no, but just as i stated in #1 it has to be sized properly.

    3> no.
    NIEL

  3. #3

    Default Re: Running 12v appliances off a 24v pv system

    Just make sure you don't tap one 12 volt battery (or 2 6 volt batteries) in a string to get 12 volts - that will cause an imbalance... i.e. problems.

    There is one exception to #3. It is possible to run a water pump with a properly sized PV array and specialized pump controller, with no battery. Not the system you have, but it is possible to do something w/o batteries.
    12 kw Generac, Honda EU3000is, 4.2 kw APC UPS powered either by battery or a Prius. Really.

  4. #4

    Default Re: Running 12v appliances off a 24v pv system

    what about using some type of 120v grid tie inverter, like the Sunny Boy 700, could you use this inverter from the PVs to use the 120v ac output?

  5. #5

    Default Re: Running 12v appliances off a 24v pv system

    Quote Originally Posted by Caneman View Post
    what about using some type of 120v grid tie inverter, like the Sunny Boy 700, could you use this inverter from the PVs to use the 120v ac output?
    Nope.
    GT inverters shut down without an AC sync signal.
    With no batteries to provide consistently available power you can't run any inverter alone. Panels put out a wide variety of current and Voltage and need the batteries to stabilize this output to provide a consistently available power source. The pumps techntrek mentioned are designed to work off this varying DC source; their speed changes with the power available. This won't work with an inverter.
    1220 Watts of PV, OB MX60, 232 Amp hrs, OB 3524, Honda eu2000.

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

  6. #6
    Join Date
    Jun 2010
    Location
    Sterling, Alaska
    Posts
    105

    Default Re: Running 12v appliances off a 24v pv system

    Technically, I suspect you could get away with running small loads off of an inverter or directly off of DC with no batteries if you replaced the batteries with a large supercap or bank of supercaps. You would need a voltage regulator to insure that the input voltage to the inverter or DC devices never exceeded it's max input voltage.
    The advantage to the supercap is that it will last far longer (probably the rest of your life) than a battery but the downside is that it can not store much energy long term. So most of the power would need to be used as it is being made available in real time.
    I suspect it will act sufficiently as a resevoir for power to be useful.
    Also for long term plans you might look into NiFe ( Nickel Iron) batteries. They have been known to last for 50+ years with just a replacement of the electrolyte (typically a potassium hydroxide solution available as a powder) which can be purchased and stored.
    6 SolarWorld 175 (1 KW), 14 Arco Solar 16-2000 (~1.5KW total), 15 REC 250 (3.75 KW) 2 KW 48V Whirlwind turbine, Outback 3648, 8 Dekka Solar L-16's, Outback FM80, Midnite Classic 150, Honda 2000i backup, & Generac 5KW.

  7. #7

    Default Re: Running 12v appliances off a 24v pv system

    I call you an "end of the worlder".

    1. Many 12 volt appliances will "let their smoke out" if you connect them to voltages much higher than about 15v. 24VDC to 12VDC converters are available. Be sure to check for high effciency and low "phantom" loss.

    2. No problem. You might want to check out and test your inverter's "Idle current" draw. Some inverters consume as much as 90 Watts just to be on, even with no loads. Most decent quality inverters will draw 7-22 watts constantly just to be "on".

    3. If you have a battery in you system and it "dies", it's voltage will still rise as long as the sun is on your PV panels and your charge controller will allow the solar to pass thru. Some charge controllers need a minimum battery DC voltage to allow PV to pass to the battery. The inverter will turn on if it has the proper voltage present. As long as your inverter and loads do not draw more than your PV is producing, you will still be able to power loads during the day, even with "dead" batteries.

    Alex Aragon
    Last edited by SolaRevolution; April 16th, 2012 at 8:41 PDT.

  8. #8

    Default Re: Running 12v appliances off a 24v pv system

    At work we have M-113 Armored Personnel Carriers that have a 24V electrical system. Our communication equipment is 12V. Our solution was to use a Samlex SDC-20 DC-DC converter. Does a nice job of converting 24VDC down to 12VDC (13.8 VDC +/- 0.1 V).

    These are well built, and hold up to the hot, humid, and sometimes wet conditions inside this APC. The guts of the converter are all potted.





    This one is rated at 15A output. They have a 20A and 60A version as well.





    I avoided the issues with my setup at home. I stuck with a 12V setup. I do have to use fatter cables to the batteries and inverters, but all my 12V items are compatible. My "opinion" is the cost savings of using a 24V system due to thinner wires is negated by the extra cost of 24V inverters & appliances or converters. JMHO.
    Robert
    Central FL
    http://www.2manytoyz.com
    900W of solar panels, 860 AH 12V battery bank (8 6V Golf Cart Batts), Xantrex Prosine 1800W inverter, Xantrex Prowatt SW2000 inverter, 2 Prostar 30M controllers, 1 Morningstar Sunlight 10L controller, Iota 75A charger (for rainy days), Furrion 30A automatic transfer switch, Yamaha EF2400iS generator.

  9. #9

    Default Re: Running 12v appliances off a 24v pv system

    ^^^ cool pics, that looks fun :)

    thanks for you insights... so with dead batteries the inverter will still work during daylight hours so long as the load is not greater than what the panels are drawing...

    24v vs. 12v, that is my big decision right now... I can double the power pumped by the cc if I use 24v and it runs at lower amperages (not just smaller wire, but for me it is safer as well)... but the 12v system is compatible with all the 12v appliances... i am leaning at this point toward the $75 cost of the 24/12 converter being much cheaper in the long run by being able to have much more power available to the battery bank when running at 24v and being able to expand the system, the 12v seems too limiting and not as resource efficient as the 24v system.... still not sure though, and thinking about it...

  10. #10
    Join Date
    Jun 2007
    Location
    Williams Lake, BC
    Posts
    3,086

    Default Re: Running 12v appliances off a 24v pv system

    take an extreme example, same 4 batteries in 12 & 24 v config. in 24v there are 2 "batteries" in parallel , in 12 v there are 4 batteries in parallel, now think of how you will run the wires to connect them to the CC and inverter etc??? anmd keep all the cables the same length...

    remember KISS!

    hth
    100% Off Grid @ 51* 46' N lat 124* 44' W long

    New House system: coming - 2 arrays @ 2240W
    CL150&Epanel 2 @140 W 12v PVs , 24V C&D AT-15P AGM @ 950Ah
    + CL150 2@120W 12 V panels, 24V C&D AT-15P AGM @ 950Ah
    Cotek ST1500W inverter, TBS 30a-24v Omni-charger, Honda Eu3000is,

    Guest cabin system: 3 - 70W panels to SB 2000e CC, with 2 - 100 ah 12v SAFT wet NiCd's , 600W TSW Inverter

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