LBX (Low Battery Transfer) Mode Settings in Xantrex SW4024 Inverter

AlanRT
AlanRT Solar Expert Posts: 57 ✭✭✭✭
in Grid Tie and Grid Interactive Systems #1
Can anyone explain the follow passage from page 46 in the Xantrex SW manual regarding correct settings for LBX mode?
SET LOW BATTERY CUT IN VDC - This setting controls when the inverter turns back on once it has shut off after the battery reached the LOW BATTERY CUT OUT VDC setting. It is also used to control when the system resumes powering the AC loads from the inverter when LBX mode is being used. In LBX mode, the best performance will often be achieved if this setting is higher than the BULK and FLOAT VOLTS DC setting in order to reduce cycling of the system. The DC charging sources (wind, solar etc.) must then cause the battery voltage to rise above the charger settings before the system resumes inverter mode operation. This setting is not temperature compensated.
Are they referring to the BULK and FLOAT volts settings in the Inverter, the charge controller, or both? In a 24 volt system, would this mean that you need to set the LOW BATTERY CUT IN voltage somewhere around 30 volts (FLA batteries) to put it above the bulk voltage setting? Will the batteries ever get that high in order to disable the LBX function and return the inverter to service?
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #2
    Re: LBX (Low Battery Transfer) Mode Settings in Xantrex SW4024 Inverter

    Looks like bad writing to me. :roll:

    They're worried that if the cut-in Voltage is set to, say 24 VDC, the loads may draw the batteries back down below cut-out level, causing the whole process to begin again: panels bring the Voltage up, inverter turns back on, battery Voltage drops, et cetera.

    So the intent is to allow the charging system(s) to fully recharge the batteries before reactivating the inverter. In the case of a 24 Volt system with an Absorb level of 28.4 - 28.8 this is where you'd have the inverter turn on again. If you have enough panel it wouldn't be a problem at lower levels (with known loads).

    I'm extrapolating from Outback's similar function. The term "Bulk Voltage" is a mystery to me; I can only assume they mean the Absorb level (Bulk to Absorb switch-over V point).

    Sometimes I think the qualification for writing IB's is to be functionally illiterate in three languages. :p
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  • RCinFLA
    RCinFLA Solar Expert Posts: 1,485 ✭✭✭✭
    #3
    Re: LBX (Low Battery Transfer) Mode Settings in Xantrex SW4024 Inverter

    The LBX mode is intended to run on batteries instead of grid when possible. If you have a strong PV power supplied to batteries it may never go to grid for battery charging if battery capacity is large enough to make it through the night.

    Once batteries have reached a LBCO (low battery cut out) there is not much choice if grid is available to use it for recharging. You may also have PV charging also going in parallel with grid charging once grid charging starts, just make sure you don't go beyond a max safe charge rate (20-25% AH) with the two combined.

    Trace used the term Bulk for what most of us call absorb voltage.

    The SW bulk current charging begins to taper off as it approached bulk (absorb) voltage setting. The best you can get to on recharge is setting Low battery Cut In to Bulk (absorb) voltage. This will yield about 85% SOC on recharge before ACout goes back on battery sourced power.

    It states on page 46, if you set LBcut in above bulk (absorb) voltage you must have another battery charging source that takes the battery voltage above the bulk voltage on SW to get inverter to take power from batteries again.

    I really question this. I believe the SW will suck of any PV or wind power that tries to raise the battery above its bulk voltage setting and supply it to ACout load or push it to grid preventing battery voltage from rising beyond SW set point. Pushing excess to grid may depend on software rev. Old software 4.01 or earlier will push it back to grid. Newer software will do it for a short time then shut off inverter to avoid pushing to grid (this assumes your ACout loads don't consume PV excess).

    I would set LB cut in slightly below bulk (absorb voltage).

    (actually I would not use the LBX mode since battery wear and tear for this mode usually cost more then the grid power)
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  • AlanRT
    AlanRT Solar Expert Posts: 57 ✭✭✭✭
    #4
    Re: LBX (Low Battery Transfer) Mode Settings in Xantrex SW4024 Inverter
    RCinFLA wrote: »
    I would set LB cut in slightly below bulk (absorb voltage).

    (actually I would not use the LBX mode since battery wear and tear for this mode usually cost more then the grid power)
    Interesting. Here is what I originally had in mind for settings (US Battery, 24 volt system):

    (9) INVERTER SETUP
    SET GRID USAGE > LBX
    LOW BATTERY CUT OUT VDC > 23.0 VOLTS
    SET LBCO DELAY MINUTES > 15
    SET LOW BATTERY CUT IN VDC. > 29.8
    (10) BATTERY CHARGING
    SET BULK VOLTS DC > 29.4
    SET FLOAT VOLTS DC > 26.2
    (16) LOW BATTERY TRANSFER MENU
    LOW BATTERY TRANSFER VDC. > 23.5 VOLTS
    So you're saying to set the LBCI at perhaps 29?

    I should add that this system was installed in Baja California Sur, Mexico about 7 to 8 years ago. The inverter was manufactured in May, 2002. The customer just recently was able to get street power. The Mexican utility, CFE, has introduced grid tie metering but the permitting process takes forever. We wanted to use LBX in the meantime. Also, wasn't sure if we can feed 120 volts back up just one line of the 240 volt grid? (1 inverter)
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  • RCinFLA
    RCinFLA Solar Expert Posts: 1,485 ✭✭✭✭
    #5
    Re: LBX (Low Battery Transfer) Mode Settings in Xantrex SW4024 Inverter

    Try to get as close to Bulk voltage as you can to get the max charge into batteries. If inverter never switches over to supplying from batteries then lower the LBX cut in voltage.

    As to pushing 120vac, (single sided) its okay as long as power company allows it and their power meter registers it. Keep in mind if the other side consumes more power then pushes up inverter side the meter will still spin as grid supplying power.
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  • AlanRT
    AlanRT Solar Expert Posts: 57 ✭✭✭✭
    #6
    Re: LBX (Low Battery Transfer) Mode Settings in Xantrex SW4024 Inverter

    On further research, I think we'll just defeat battery charging from the inverter altogether and let the solar system do the catchup. The idea of an Inverter charger circuit overcharging the batteries makes me nervous.
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  • RCinFLA
    RCinFLA Solar Expert Posts: 1,485 ✭✭✭✭
    #7
    Re: LBX (Low Battery Transfer) Mode Settings in Xantrex SW4024 Inverter

    The SW4024 will not allow the battery to go over its float or bulk voltage setting levels (which ever of the two modes is in progress).

    It will act as a voltage limiter. It will try to suck off any excess current injected from PV into batteries and push AC power to ACout node.

    Newer software may shut down inverter if ACout loads cannot consume the excess. This is due to changes to prevent excess power being pushed to grid to avoid issues with UL1741 compliance. Older software will just push excess power to ACout loads or grid.

    SW inverter will charge batteries in parallel with PV when battery voltage is less then SW charge set voltages (bulk or float). This is why you set PV controller above SW voltage setting to ensure PV controller alway keeps pumping out power. SW will set the battery voltage regulation.
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