RV upgrade from 12 volt to 24 volt??

opticalmikeopticalmike Registered Users Posts: 20 ✭✭
Is a 24 volt system a good upgrade from a 12 volt system?
I am looking to upgrade my RV solar setup. Currently I have 4- 85 watt 12 volt panels, a midnite combiner box with 4 DC breakers and a 30 amp CC(not mppt). 4 T-105's. I have a 3000 watt pure sine inverter(overkill but it was cheap) to plug the shore power cord into. I have installed a battery disconnect that I use as a manual transfer switch, thats installed on the positive lead coming from the trailer converter.This prevents the trailer converter from trying to charge the batteries.
I was hoping to add just one more panel, but say a 235 watt or higher. These bigger panels are mostly 24 volt.The one I had my eye on was a Hanwha 235 watt.I guess my main question was is the 24 volt system a good upgrade. I would need a step down converter thats only 85% efficient.I was hoping by getting a big panel to add to my 4 smaller panels that I could see better charging on cloudy days and or spring and fall days, as I live in edmonton alberta canada...not exactly great solar area.We do get a lot of sunny days tho.
Maybe I should just add the biggest 12 volt panel I can get and forget the whole 24 volt switch. I had thought now would also be a good time to add a MidNite kid mppt controller and whizbang to give us a better sense of whats in the tank for reserve minutes.
any help appreciated,
regards
opticalmike

Comments

  • BB.BB. Super Moderators, Administrators Posts: 30,177 admin
    Re: RV upgrade from 12 volt to 24 volt??

    Welcome to the forum O'Mike.

    Lets backup up a moment... 12 vs 24 ( or 48 )volt battery bus. That really depends on the size of your loads (peak watts, Watt*Hours/Amp*Hour per day) and the size of the battery bank (larger AH batteries do better with higher voltages to keep the current lower -- P=I*V, double the voltage, 1/2 the current).

    Higher voltages allow you to use the same size charge controller with a 2x (or 4x) larger array:

    30 amp MPPT controller * 14.5 volts charging * 1/0.77 panel+controller derating = 565 Watt array "cost effective" maximum
    30 amp MPPT controller * 29.0 volts charging * 1/0.77 panel+controller derating = 1,130 Watt array "cost effective" maximum

    More or less, if your loads are 1,200 watts or less, you can usually stay with a 12 volt battery bank.

    If your loads are in the 1,200 to 2,400 watt range--You should think about a 24 volt battery bank.

    Then there is the question of "24 volt" solar panels... Most of these panels are not really good for charging a 24 volt battery bank. They tend to have Vmp~30 VDC. A "true" 24 volt panel would have Vmp~35-39 volts (typically a 72 cell panel). (a "12 volt" panel has Vmp~17.5 to 18.6 volts or so).

    You have to "match" panel voltage with battery bank voltage when using the less expensive PWM controllers. For MPPT controllers, you still have to meet the minimum panel voltage, but can go up to ~100 Volts Vmp-array. This allows you to use smaller gauge wire for longer runs/less money spent on copper wiring.

    So--you could use these higher voltage/larger size/less expensive solar panels with a MPPT type charge controller (more expensive) and it will efficiently take the high voltage/low current from the solar array to the low voltage/high current needed to charge the battery bank.

    Lots of options, and some questions--Don't buy any hardware just yet. You should do a couple "paper designs" and see what works out best for you.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • vtmapsvtmaps Solar Expert Posts: 3,738 ✭✭✭✭
    Re: RV upgrade from 12 volt to 24 volt??
    Is a 24 volt system a good upgrade from a 12 volt system?

    Welcome to the forum,
    Usually when we refer to 12 and 24 volt systems, we are referring to the battery bank nominal voltage. From your questions here and at the Midnite forum, you are referring to the array configuration, not the system's battery voltage.

    Bill (BB.) explained that most "24 volt" panels are NOT true 24 volt panels because their voltage is not high enough to charge a 24 volt battery. They work great with 12 volt batteries if you have an MPPT type controller. If you can find a true 24 volt panel, it's voltage will be double that of your 12 volt panels and you could put two of your 12 volt panels in series and parallel that with a true 24 volt panel. You would still need a MPPT type controller.

    As I mentioned over at Midnite, you can add the new panel and kid controller and keep your present system.
    I would need a step down converter thats only 85% efficient.

    I'm not sure what you mean by this... are you saying that the kid is only 85% efficient?

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
  • opticalmikeopticalmike Registered Users Posts: 20 ✭✭
    Re: RV upgrade from 12 volt to 24 volt??

    The Pyle step down converter says it's only 85% efficient. I see I won't need one though as the kid will do t he job.
  • BB.BB. Super Moderators, Administrators Posts: 30,177 admin
    Re: RV upgrade from 12 volt to 24 volt??

    More or less, most DC to DC and AC inverters average around 85% efficiency (check the power curves--Usually most efficient "somewhere in the middle", much less efficient at the ends--And of course, if your loads are off and the converter/inverter is running, that is 0% efficiency or "tare" losses).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • TucsonAZTucsonAZ Solar Expert Posts: 136 ✭✭
    Re: RV upgrade from 12 volt to 24 volt??

    24v versus 12v comes with a lot of plusses and is the way I'm going but it sounds like you're wanting to incorporate your existing equipment which I think may become problematic instead of doing a larger upgrade. Either way, you will benefit from an MPPT and I would suggest Rogue charge controllers for the value personally. If you can sell your current panels and buy two 24v panels that would be ideal and may not cost you much more out of pocket.
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: RV upgrade from 12 volt to 24 volt??

    This point may have been missed:

    There is a differences between upgrading to a 24 Volts system and using "24 Volt" panels to charge a 12 Volt system through an MPPT controller. The latter is an easy change to make, but not necessarily any better than using "12 Volt" panels & a PWM controller. The former is a more difficult change and involves installing a lot of new equipment and possibly dealing with 12 VDC loads which will no longer have a direct power source.
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