Low Voltage Cut Out default to 21 volts, why?

yeahgofigure
yeahgofigure Registered Users Posts: 7 ✭✭
edited December 2017 in Off Grid Solar & Battery Systems #1
Just installed a Schneider Electric SW4024 inverter in my 24V off grid system and don't want to damage my batteries (24 6V flooded 232 amp batteries) so checked inverter user manual and it says low batt cut out has default value of 21V. Why so low?? Isn't 21V dead in a 24V system given 10.5V is dead in a 12V battery.

Yes under load the voltage may drop some but wouldn't think could hit 21V as if that's dead don't see how inverter would have any power to even decide to cut off power at that point. My old inverter was Xantrex XW4024 (Schneider took them over) and had same setting and I've seen it successfully cut off power before so I must be missing something here.

I must install Schneider's System Control Panel just to change this setting so would like to avoid doing so if the default setting is ok. Discovered the setting on page 4-7 of their user manual https://41j5tc3akbrn3uezx5av0jj1bgm-wpengine.netdna-ssl.com/wp-content/uploads/2015/09/Conext-SW-NA-Owners-Guide-120-240VAC-975-0638-01-01_Rev-E_ENG.pdf
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,613 admin
    #2
    Low voltage cutoffs for AC inverters is really there to "save" the inverter. Remember that inverters are "constant power" devices. I.e., if you are driving a 1,000 Watt load, and Power=Voltage*Current, you see that falling battery bus voltage causes higher DC input current to the inverter:
    • P=V*I; I=P/V
    • 1,000 Watts / 14.5 battery bus voltage = 69 amps
    • 1,000 Watts / 10.5 battery bus voltage = 95 amps

    Also, remember that there is voltage drop too... Having 0.5 to 1.0 volts of wiring drop is not unusual for a 12 VDC inverter under full load (heavy/short battery bus cabling needed to keep voltage drop low).

    Lastly, remember that a typical "good quality" AC inverter can surge about 2x current for starting motors and such... That is double the DC bus current and double the voltage drop (V=I*R).

    Is 10.5 volts going to "save your batteries"--Probably not. You can try 11.5 volts for LV shutdown (23 volts for 24 VDC bank). Some higher end inverters allow timed current settings (i.e., 11.5 volts ~10 minutes; 10.5 volts for 10 seconds, etc.). Battery bus voltage is also affected by temperature--Using battery bus voltage for an accurate estimate of battery state of charge is not really very accurate.

    -Bill

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • Estragon
    Estragon Registered Users Posts: 4,496 ✭✭✭✭✭
    #3
    IMHO, 21v is low for off-grid LBCO if there's a reasonable (eg 30sec) delay. As Bill noted, the default is set to protect the inverter, not so much your batteries. Looks like default delay is 10secs, which should cover most starting load sags. If possible though, I'd try to borrow a panel to set it to ~23v and ~60secs.

    I would likely end up buying one, as I'd want to be able to adjust generator input/charging current limits for different generators, etc., but your situation may be different.
    Off-grid.  
    Main daytime system ~4kw panels into 2xMNClassic150 370ah 48v bank 2xOutback 3548 inverter 120v + 240v autotransformer
    Night system ~1kw panels into 1xMNClassic150 700ah 12v bank morningstar 300w inverter
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  • Dave Angelini
    Dave Angelini Solar Expert Posts: 6,894 ✭✭✭✭✭✭
    #4
    Estragon said:
    IMHO, 21v is low for off-grid LBCO if there's a reasonable (eg 30sec) delay. As Bill noted, the default is set to protect the inverter, not so much your batteries. Looks like default delay is 10secs, which should cover most starting load sags. If possible though, I'd try to borrow a panel to set it to ~23v and ~60secs.

    I would likely end up buying one, as I'd want to be able to adjust generator input/charging current limits for different generators, etc., but your situation may be different.
    Definitely the OP needs to be able to adjust the setting as they are there for him with an SCP or combox. There is also a hysteresis setting to tweak how the LBCO behaves.

    22V was always the standard with Outback and Schneider but I think a few women complained about the hair dryer being the most important thing offgrid, much more important than protecting an inverter :)
    "we go where power lines don't" Sierra Nevada mountain area
       htps://offgridsolar1.com/
    E-mail offgridsolar@sti.net

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  • yeahgofigure
    yeahgofigure Registered Users Posts: 7 ✭✭
    #5
    Yes it does have a delay timer which is defaulted to 10 seconds. Aw nuts looks like I'll need to get a system control panel just I'm in process of selling the place and didn't want to spend any $$ that didn't need to. Kinda silly that there's no way to set the settings directly on the inverter.
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  • Dave Angelini
    Dave Angelini Solar Expert Posts: 6,894 ✭✭✭✭✭✭
    #6
    Your old XW did not have them on the inverter either. It is the way Outback and Schneider chose to run their business.
    Stop by after work and you can borrow one ;)
    "we go where power lines don't" Sierra Nevada mountain area
       htps://offgridsolar1.com/
    E-mail offgridsolar@sti.net

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