Off Grid With and Without Batteries

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Diver
Diver Registered Users Posts: 9 ✭
in Off Grid Solar & Battery Systems #1
What would happen if you connect 24V modules to a 24V PWM charge controller (with combiner box & circuit breakers) and a 24V inverter with a 12V battery system? Besides the power loss, would it harm the batteries from overvoltage?

What would happen if you connect 24V modules to a 24V PWM charge controller (with combiner box and circuit breakers) and a 24V inverter without batteries? Is there a possibility of damaging the inverter?
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #2
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    Re: Off Grid With and Without Batteries
    Diver wrote: »
    What would happen if you connect 24V modules to a 24V PWM charge controller (with combiner box & circuit breakers) and a 24V inverter with a 12V battery system? Besides the power loss, would it harm the batteries from overvoltage?

    No, because the charge controller would be set for a 12 Volt system not a 24 Volt. So when the batteries reach the appropriate Voltage, the controller will shift from Bulk to Absorb and then to Float as normal. PWM's pass whatever current is available from the panels, which are a current (not Voltage) source. The panel Vmp will be "pulled down" to battery level. This is assuming that the current potential of the panels isn't too much for the batteries (like charging a 35 Amp hour battery from w 3500 Watt array) in which case the excess current will heat the batteries unduly, with possibly catastrophic results. If you leav out he charge controller, the panels will try to push the battery Voltage too high and boil off the electrolyte.
    What would happen if you connect 24V modules to a 24V PWM charge controller (with combiner box and circuit breakers) and a 24V inverter without batteries? Is there a possibility of damaging the inverter?

    Nothing will happen. The controller will detect no batteries and not function. The inverter will detect no Voltage and not function. If you have a cheap controller that will pass the power anyway, the inverter would try to start upon detecting Voltage and then its draw will pull the Voltage down again causing it to turn off. Cycle, cycle, cycle. How bad this would be depends on the ratio between the panel power and the inverter demand.

    In short; off-grid no batteries means no power. This is actually a frequently asked question, although its usually people with expensive GT systems asking because they want to find some way to make their inverters provide power when the utility goes down.
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  • Photowhit
    Photowhit Solar Expert Posts: 6,003 ✭✭✭✭✭
    #3
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    Re: Off Grid With and Without Batteries
    Diver wrote: »
    What would happen if you connect 24V modules to a 24V PWM charge controller (with combiner box & circuit breakers) and a 24V inverter with a 12V battery system? Besides the power loss, would it harm the batteries from overvoltage?
    If you set the charge controller to charge 12 volt battery the battery would be fine, but the inverter would never have enough voltage to work....
    Diver wrote: »
    What would happen if you connect 24V modules to a 24V PWM charge controller (with combiner box and circuit breakers) and a 24V inverter without batteries? Is there a possibility of damaging the inverter?
    No, but likely you'd never be able to use any meaningful draw from the inverter.
    Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Magnum MS4024, Prosine 1800(now backup) and Exeltech 1100(former backup...lol), 660 ah 24v Forklift battery(now 10 years old). Off grid for 20 years (if I include 8 months on a bicycle).
    - Assorted other systems, pieces and to many panels in the closet to not do more projects.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,439 admin
    #4
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    Re: Off Grid With and Without Batteries

    I will say that it is certainly possible to damage an off grid inverter by attaching the inverter directly to the panels...

    Remember a "24 volt array" has Vmp~35 volts and Voc-stc~40-44+ volts (even more if colder). A typical 24 volt inverter will shut down at around 30-32 volts input. How high you can go above that without damage--your guess is as good as mine.

    In an off-grid power system--The battery bank is really the voltage regulation. The rest of the "charge controllers" are there to limit current into the battery to reduce possible damage from over charging the battery--not really there to "regulate" voltage (i.e., a 12 volt car alternator may damage the vehicle's electrical system from over voltage if the battery is disconnected when the car is running).

    -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 ✭✭✭
    #5
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    Re: Off Grid With and Without Batteries

    To expand on what Bill said, it's an issue of which array and which inverter. Certainly some inverters would "see" Voc, scream "HIGH VOLTAGE!" and shut down. They may not be able to scream in time, though. Others may pull the array Voltage down before they detect it being too high and try to start but without success. Then the next time they try to start (caps charged, no start-up load to pull the Voltage down) scream "high voltage" and quit. Or fry.

    The thing is, PV's try to put out steady current and let the Voltage fluctuate whereas inverters want a steady Voltage and vary their current demands. It takes batteries in between to mitigate the differences.
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  • Photowhit
    Photowhit Solar Expert Posts: 6,003 ✭✭✭✭✭
    #6
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    Re: Off Grid With and Without Batteries

    Thank goodness we have moderators, I stand corrected, you might well fry you inveter! I didn't think of the applied voltage!
    Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Magnum MS4024, Prosine 1800(now backup) and Exeltech 1100(former backup...lol), 660 ah 24v Forklift battery(now 10 years old). Off grid for 20 years (if I include 8 months on a bicycle).
    - Assorted other systems, pieces and to many panels in the closet to not do more projects.
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  • Diver
    Diver Registered Users Posts: 9 ✭
    #7
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    Re: Off Grid With and Without Batteries

    Thanks everyone for your educational answers.
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #8
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    Re: Off Grid With and Without Batteries
    Photowhit wrote: »
    Thank goodness we have moderators, I stand corrected, you might well fry you inveter! I didn't think of the applied voltage!

    Well we're not going to say there isn't an inverter out there somewhere that wouldn't fry but ...
    Not Recommended Procedure. :roll:
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