Klayman: 24v to 12v DC system

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Klayman
Klayman Registered Users Posts: 3
in Off Grid Solar & Battery Systems #1
Bill,
A question about using an MPPT to charge a 12v bank from a 24v bank: Will the MPPT continue to draw current from the 24v bank after the 12v bank is charged?

Currently, I have a Morning Star TS-45 that I am using to charge a 24v battery bank from a solar panel array. When the sun is shining bright, and the controller has been showing a steady 27.0 volts for a while, there is usually a high pitched humming coming from the controller. With all those cooling fins on the controller, I just always assumed that it was burning off the excess power from the solar panels. If I connected the 24v batteries to the input side of an additional TS-45 MPPT, and the output to a 12v battery bank, once the 12v bank was fully charged wouldn't the controller sense constant power from the input of the 24v bank and try to burn it off? Hence draining the 24v bank?

Sorry if I did not think through something obvious (first time poster). My goal is to have a substantial 12v battery bank to run a camping trailer, workshop, etc. I also have needs for a 24v bank like a well pump and an inverter.
-Klayton
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  • vtmaps
    vtmaps Solar Expert Posts: 3,741 ✭✭✭✭
    #2
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    Re: Charge 12 volt battery from 24 volt battery bank
    Klayman wrote: »
    Bill,
    A question about using an MPPT to charge a 12v bank from a 24v bank: Will the MPPT continue to draw current from the 24v bank after the 12v bank is charged?

    Welcome to the forum,

    Perhaps this should be a new thread. One problem with your scheme is that many MPPT controllers do not cope well with trying to find the maximum power point of battery.

    There may be other ways to accomplish what you want to do... There are 12 volt DC power supplies that can make 12 volts from the inverter output of your 24 volt system. There are also DC to DC converters (24 volt to 12 volt). Tell us more about your systems and loads.

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
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  • Mountain Don
    Mountain Don Solar Expert Posts: 494 ✭✭✭
    #3
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    Re: Charge 12 volt battery from 24 volt battery bank

    We have a 24 volt battery system. We use 24 to 12 volt converters for our 12 volt needs. There are models with varying amperage capacities. Solving the 12 volt needs with a 24 to 12 converter eliminates the extra batteries. To me that makes more sense than having to maintain another set of batteries, but that is just my take on it. We have one by Solar Converters and a Samlex.
    Northern NM, 624 watts PV, The Kid CC, GC-2 batteries @ 24 VDC, Outback VFX3524M
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,439 admin
    #4
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    Re: Charge 12 volt battery from 24 volt battery bank

    I moved to your own thread so you can guide the discussions towards your needs.

    Regarding the MorningStar TS-45 controller--Is that a MPPT (maximum power point tracking) or a PWM (pulse width modulation) type.

    The MPPT type can take a higher voltage battery and down convert to a lower voltage battery just fine (you should always confirm with the manual/mfg. to be sure).

    A PWM controller cannot.

    Modern solar charge controllers simply reduce the average current, or turn off completely, the current from the solar array when the battery bank is full. There is no reason for the controller to "dump" or dissipate any excess energy from the solar array (standard solar panels can be turned on/off without any issues).

    There are dump controllers too--They are typically used for horizontal axis wind turbines--This turbines will over speed in high winds unless they have a permanently connected battery bank/load to prevent the the blades from turning too fast (no load) which can self destruct the turbine). The dump controller is used to dissipate the unwanted excess energy from the turbine.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • Klayman
    Klayman Registered Users Posts: 3
    #5
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    Re: Klayman: 24v to 12v DC system

    Thanks for the replies to my question.

    Bill, thanks for questioning my statement that I have a TS-45 MPPT controller. It is not a MPPT, but rather a PWM. It does not state that anywhere but the manual talks about PWM functions. I bought it years ago when I did not know the difference. So that brings up a new concern. Would a more modern controller, say like the Midnight Solar KID, be more efficient or is the TS-45 doing just about as well as any new equipment.

    Don, good thoughts. I also have the Solar Converter EQ 12/24-20, a great device but it seems to struggle to keep up with my loads. I tried using it as an equalizer to tap the center poles of four 6 volt T-105 batteries but the center poles were always a volt or so lower. I didn't like that. So now I have added four more T-105's and am using them as a 12 volt bank while using the EQ 12/24 -20 to charge the 12 volt bank, but my loads are a bit more than it is cut out for and it only keeps the bank at barely 12 volts. I also don't like that those batteries are not being conditioned or 'equalized'. I also have the Samlex SDC-15 24VDC-to-12VDC converter and tried it hooked up to the 24v bank. If I had 3 or 4 lights on and the radio on when the forced air furnace kicked on (in a camping trailer) the radio would cut out and the lights dimmed. I have a volt meter hooked up in the trailer and it shows the SDC-15 is putting out 13.5 volts, but it can not handle the surges.

    vtMaps,
    That sounds like wise advice, too many MPPT's would confuse each other if they were set up as I was proposing. It may be better, and cheaper in the long run to spend the money for the big 24VDC-to-12VDC step-down converter and keep all eight T-105's as a 24 volt bank.

    I sure wish Northern Arizona Wind and Sun carried the bigger step-down converters. A lot of folks going off-grid nowadays, and most appliances and such are all still 12 volt.

    Thanks all for the advice, it feels like I have a settled plan now.
    -Klayton
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,439 admin
    #6
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    Re: Klayman: 24v to 12v DC system

    PWM controllers work fine with existing/well designed systems...

    When you get to larger systems, then MPPT controllers can make more sense. The controllers are much more expensive, but the "GT type" solar panels (Grid Tied solar panels are not "12 volt" or "24 volt" panels for batteries, but typically around 30 volts Vmp) and need a MPPT type controller to take the high voltage/low current of the array and efficiently "down convert" to the low voltage/high current needed to charge the battery bank (think a little bit like the DC version of a variable AC transformer).

    Also, because you can run the array at ~90 VDC and the battery bank at 12-48 volts, the wiring from the array can be much smaller diameter vs a PWM charge controller (really nice if your array is quite a distance from the charge controller/battery shed).

    There are limited times where an MPPT controller can give you 10-15% more power vs a PWM controller (both on well designed systems)--But that usually only happens in sub freezing temperatures (Vmp rises as temperatures drop).

    My personal opinion, many times it makes more sense to run Energy Star (modern energy efficient 120 VAC appliances) on a good quality AC inverter (TSW--true sine wave recommended) vs using XX to 12 VDC down converters. Both DC down converters and AC inverters have similar losses, and it is much easier to send 120 VAC power much longer distances vs 12 VDC power.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • Klayman
    Klayman Registered Users Posts: 3
    #7
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    Re: Klayman: 24v to 12v DC system

    Bill, thanks for the discussion. I have the GT type panels so I think I will switch to a MPPT and run the panels in series for a high voltage and lessen my losses there. I did not know that DC-DC converters consumed power like the inverters. It will make more sense to use an inverter when I build a house and will use the Energy Star appliances. For now, in the camping trailer, A larger DC-DC will have to suffice.

    -Klayton
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  • CDN_VT
    CDN_VT Solar Expert Posts: 492 ✭✭✭
    #8
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    Re: Klayman: 24v to 12v DC system
    Klayman wrote: »
    Thanks for the replies to my question.
    spend the money for the big 24VDC-to-12VDC step-down converter and keep all eight T-105's as a 24 volt bank.

    I sure wish Northern Arizona Wind and Sun carried the bigger step-down converters. A lot of folks going off-grid nowadays, and most appliances and such are all still 12 volt.

    Thanks all for the advice, it feels like I have a settled plan now.
    -Klayton

    NAWS has done special drop shipping to my WA house also.
    Here is the converter page & they show a 50 amp unit : http://www.solarsolutions.ca/component/virtuemart/power-systems/solar-converter-dc-autotransformer-eq-1224-20-detail

    Give NAWS all the info you find & call them up . They helped me get some funky stuff.
    Sale pricing on there deals (July 4th & black fridays ) they don't count for funky not-normally stocked items..
    Last sale I did well on Kids & classic's
    JFI

    VT
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  • vtmaps
    vtmaps Solar Expert Posts: 3,741 ✭✭✭✭
    #9
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    Re: Klayman: 24v to 12v DC system
    Klayman wrote: »
    vtMaps,
    That sounds like wise advice, too many MPPT's would confuse each other if they were set up as I was proposing.
    Actually, multiple MPPT controllers usually work well together to charge a single battery. My point was that you shouldn't try to use a charge controller
    to charge one battery from another. The problem is that the controller will try to load the input (usually PV) to find the maximum power point. Trying to load a large battery bank usually does not have a happy ending.
    Klayman wrote: »
    I sure wish Northern Arizona Wind and Sun carried the bigger step-down converters. A lot of folks going off-grid nowadays, and most appliances and such are all still 12 volt.

    Consider converting your 12 volt stuff to 120 volt. Where you can't convert, it may be more practical to use a 12 volt DC power supply at the point of use, than to use a DC to Dc converter. Depends on the 12 volt loads and how far apart they are.

    For example, I use a 12 volt RV type water pump. I could have bought the 24 volt version and run a 70 ft cable from my battery, but I chose to run 120 volts (from my inverter) to a 12 volt power supply near the pump. I also have a small 12 volt computer fan that ventilates the basement. It runs from a little wall wart type of 12 volt power supply.

    Modern switching power supplies are incredibly efficient. Almost all residential ventilation fans run on 120 volts. Some of the most energy-star efficient of them use brushless DC motors and have built-in DC power supplies. I have one of these fans and my power goes from battery (DC) to inverter (AC) to power supply (DC).

    All sorts of electronics runs on DC. Some folks have modified their equipment to run directly on battery DC. It seems like a lot of bother for little gain. Even LED lightbulbs run on DC. You can buy DC powered LEDs, but you have a better selection and better prices if you buy a 120 volt LED bulb that makes its own DC.

    Face up to the fact that 120 volt AC is the universal currency of power transmission in homes today. Make your DC where you need it... at the pump, the radio, the LED lightbulb, etc.
    Klayman wrote: »
    Bill, thanks for the discussion. I have the GT type panels so I think I will switch to a MPPT and run the panels in series for a high voltage and lessen my losses there.

    Be careful.... run the design by us here on the forum. Running MPPT controllers at higher input voltages makes them less efficient and makes them run hotter. You need to strike the right balance between cable losses and controller losses.

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
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