12V vs. 24V

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BenB
BenB Registered Users Posts: 2
in Solar Beginners Corner #1
Hello,

I am designing a system now that will use a 24V display and a 12V alarm. I have figured out that I will use two AGM 100AH batteries and I will need about 400-500 watts from my solar panels. I will have the two batteries wired in series to get the 24V and I am wondering if it makes more sense to get 24V panels and use a single charge controller for the two batteries or to get 12V panels and have a separate charge controller for each battery. I think I would rather go with 24V panels, but I am having a hard time figuring out which panels will work. I keep seeing rated voltages of 30.7-34.0 for the panels, but I thought I was looking at 12V panels. I'm new at this so any advise would be great.

Thank you,
BenB
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #2
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    Re: 12V vs. 24V

    Welcome to the forum Ben.

    To tell the truth your initial description doesn't make a lot of sense. A 24 Volt display of what and a 12 Volt alarm of which? What exactly are you trying to do? As a rule, mixed Voltages don't play well together.

    If you need 24 Volts for something and 12 Volts for another thing and put together a 24 Volt battery bank then tap half of it for 12 Volts (I think this is what you are going for) you can be asking for trouble. There will be a difference in the amount of power drained from the two halves of the battery bank. How much trouble will depend on the amount of current drawn off one half for the 12 Volt equipment. More current = more trouble.

    The best way around it is to use a DC to DC converter to supply the 12 Volts from the whole 24 Volt battery bank. Something like this: http://www.solar-electric.com/12to24or24to.html

    If the draw is not too large then charging the two batteries as separate 12 Volt entities can be done, but it is tricky because you have to keep the charging equipment entirely isolated between halves. The greatest risk here is a common connection of negatives, which effectively shorts out one half of the whole thing. Ground connections can cause this problem because normally negative is connected to ground.

    The panels you see with the 30-ish Vmp are grid tie panels and cannot be used on a 24 Volt system without being put in series and connected through an MPPT type controller. Normal Vmp for a 24 Volt system is 35-36. 12 Volt panels tend to be only in smaller sizes these days: 140 Watts or less. 100 Amp hours only needs about 10 Amps to charge with, and really a couple of 140 Watt panels in series at about 7.9 Amps each would work.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,439 admin
    #3
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    Re: 12V vs. 24V

    Welcome to the forum Ben.

    First, it is usually very bad news to draw 12 and 24 volts from a single battery bank. The 12 volt portion will be be continuously under charged and the upper 12 volt bank will be over charged.

    Ideally, a pure 12 volt or 24 volt battery bank with a 12:24 volt converter would be the better option.

    In some cases, people will have a 24 volt bank and use a 24:12 volt charger and run a second small 12 volt battery (not ideal, but does work).

    Your idea of using two charge controllers (one for each 12 volt bank) to allow you to draw 12 and 24 volts from the single bank can work. But you could run into "issues" with the "high side" solar charge controller if you plan on using computer monitoring--The "high bank" charge controller will have a "different" ground voltage and can cause communication bus issues (i.e., if you short the two controller grounds together, you will get 12 volt high current flow through your "two grounds").

    There are also special balancing chargers that can keep a 12/24 volt bank "balanced"... These are typically used in RV Bus conversions where you have a 24 volt vehicle chassis and a 12 volt house power usage. Can work fine, adds cost/complexity.

    Depending on what your loads are--I usually suggest picking 120 VAC as your "standard power" for running devices. The AC inverter is usually needed anyway for some loads, and the VAC to VDC converters are generally pretty efficient these days--Plus 120 VAC wiring/power strips are easy/cheap to use vs making one or two DC power distribution systems (and car lighter plugs are bulky/unreliable). Plus a true off grid batter bank will run from 10.5 to 15.5 volts or so--And many "12 volt car" adapters (and other devices like HAM radios) really do not like that wide 12 volt range--They are designed to run at 13.8 to 14.4 volts or so typically and will run less efficiently, or even shut down/fail, at the 12 volt bus margins.

    Regarding panel voltages--Yes, you are correct here. For a 12/24 volt battery bus, you need ~17.5/35 VDC Vmp rated panels (minimum) to properly recharge the batteries (all losses/temperature deratings).

    The typical (best bank for the buck) solar panel is designed for Grid Tied AC power systems and has Vmp~30 volts--That is two low of voltage for a 24 volt battery bank (a hot panel will drop Vmp to ~24 volts, and AGM batteries need ~28.8 volts to recharge at 77F).

    You would put two panels in series for Vmp-array~60 volts, and use a MPPT (maximum power point tracking) charge controller to efficiently take "high voltage/low current" from the array and down convert to low voltage/high current used by the battery bank. MPPT controllers are much more expensive than "simple" PWM (pulse width modulation) controllers--So another reason not to use a pair of 12 volt arrays on a 12/24 split power battery bank (two "very expensive" mppt controllers).

    Your best bet is to do several paper designs before you buy any hardware.... "Cheap" non-standard (for battery system) panels + expensive MPPT controller vs the typically more costly Vmp~17.5 volt panels and cheap PWM controllers typically almost come out even in costs (i.e., more than you want to spend).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • BenB
    BenB Registered Users Posts: 2
    #4
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    Re: 12V vs. 24V

    Thank you for the answers. Let me give you my revised idea based on your suggestions and see what you think.

    All this system is doing is powering a 24V display that draws .5A and when there is need for an alarm, a 12V alarm will turn on that draws .67A. The alarm will rarely run and when it does it will be tended to by an on-site work as quickly as possible.

    If I set up a purely 12V system that could power the alarm and alarm switch, I could then use a 12:24V converter for the display.

    I will use 6V 100AH batteries in series to give me my 12V. Then if I use an 100 watt 12V panel, with 4.5hrs of daylight on average I could charge up to 30AH per day using the rule of thumb of 15W = 1AH of charge per hour of sunlight. This should be plenty enough to handle what I am trying to do.

    I guess my final questions are, 1. Does this all make sense? and 2. Which panels/charge controllers/12:24V converters would you recommend for this application?

    Thank you,
    Ben
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,439 admin
    #5
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    Re: 12V vs. 24V

    Well--Assuming that most of your draw is the 24 volt x 24 hours @ 0.5 amps--And you will be using some sort of converter (dc to dc, or DC to AC to DC) with 80% efficiency:
    • 29 volts charging * 0.5 amps * 24 hours per day * 1/0.80 eff = 435 WH per day (sort of worst case)
    • 435 WH per day * 1/0.80 conversion eff * 1/24 volt battery bank * 2 days of storage * 1/0.50 max discharge = 91 @ 24 volt battery bank nominal
    Sizing the solar array based purely on battery bank capacity:
    • 91 AH battery * 29 volts charging * 1/0.77 panel+controller derating * 0.05 rate of charge = 171 Watt array minimum
    • 91 AH battery * 29 volts charging * 1/0.77 panel+controller derating * 0.10 rate of charge = 343 Watt array nominal
    • 91 AH battery * 29 volts charging * 1/0.77 panel+controller derating * 0.13 rate of charge = 446 Watt array "cost effective maximum"
    Then there is sizing the array based on amount of power you use and amount of sun you have. Typically for much of the US, around 4 hours of sun per day for 9 months of the year... You can go down to 3-2 hours or less per day in places with poor sun. Pick 3 hours for now:
    435 WH per day * 1/0.80 battery eff * 1/0.77 panel+controller eff * 1/3 hours of sun per day = 235 Watt array minimum (based on 3 hours minimum sun per day)

    So, your recommended system would be 235 to 446 watt array with 343 Watt being a healthy nominal value based on a 91 AH @ 24 volt battery bank (use a 24:12 volt converter for the alarm--You can look at a separate 12 volt battery to justify an alarm if the 24 volt system "goes dead").

    Lots of guesses and fairly conservative windage factors... Make sense to you? You pick the sun/storage/system that makes sense for your needs (do you want to run a genset to recharge during bad weather, exchange a battery pack 2-3 times per week, etc.).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #6
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    Re: 12V vs. 24V

    So the 24 Volt 0.5 Amp draw is constant and the 12 Volt 0.67 Amp draw will be "only if something goes wrong"? In that case I'd go for 24 Volt and not worry about the possible imbalance because if the alarm never sounds there won't be any.

    An alternative, depending on just what the alarm is, would be to build a simple Voltage drop circuit for the alarm. With these small draws it doesn't make sense to use a complex and expensive DC to DC converter in either direction.

    If you are going to use 100 Amp hour 12 Volt battery you would need more than 15 Watts of panel to charge it. The <1 Amp of current it could provide would about handle the self-discharge rate of the battery. In case you're wondering: 15 Watts / 17.5 Vmp = 0.85 Amps (remember you have a 0.5 Amp draw while charging too). But you could use 'minimal panel' for such an application. Since 30 Amp hours is 30% of 100 you'd want that 5% or roughly 85 Watts producing about 5 Amps.

    An 85 Watt panel is about $200. A 140 Watt panel about $100 more (can be less if you look for bargain new or used) and will give you a couple extra Amps advantage. Small charge controller like a SunSaver 10 Amp and you're good.

    But I would do this at 24 Volts (same Wattage just half the current @ twice the Voltage) and drop down for the alarm.
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