24v Battery Bank

Zakarume
Zakarume Solar Expert Posts: 143 ✭✭
in Solar Beginners Corner #1
When using a 24v Battery Bank how low can you go when under a load?
1460 Watts Solar @24v. 675 AH Battery Bank using 12 6v Trojan T-105. 1 Midnite Classic 150. 1500 Watt 24v Samlex Pure Sine Inverter
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  • Photowhit
    Photowhit Solar Expert Posts: 6,006 ✭✭✭✭✭
    #2
    Re: 24v Battery Bank

    It will depend many things, what inverter you have will limit the low voltage disconnect, what type of battery will limit the lowest % of discharge you will want to go before damaging the battery, what amount of load will create different voltage sags, what size of wires will vary the amount of voltage sag, what temp your batteries are at will vary the available capacity, what time of day will effect the amount of assistance the array is providing, what the future weather will be like will effect the possibility of recharging hence how low you can take down your battery while being able to replacing the energy without damaging the battery. The age of the batteries, as batteries age the have less capacity, when young they have less capacity...

    Sorry, I'm not up yet, I'm sure I can give you a few hundred more...
    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: 33,642 admin
    #3
    Re: 24v Battery Bank

    In general, with lead acid batteries, we don't like to go to 50% state of charge or lower too many times--Pretty stressful on the battery bank. Plus it takes a lot of energy (days of sun) to get them back charged again.

    The absolute maximum discharge would be ~20% state of charge. If you go below that, a weak cell in your bank could go dead and actually begin to "reverse charge" (battery cell goes from ~+2.0 volts to ~-2.0 volts charging. In pretty much all rechargeable battery chemistries, "reverse biasing/charging" a cell will kill them.

    Voltage wise, lead acid battery bank state of charge is difficult to estimate by purely measuring voltage. The bank voltage depends on SOC, temperature, charging/discharging current levels, age, etc... So it is difficult to set a specific cut-off voltage.

    For a 24 volt battery bank, setting a warning voltage at ~23.0 volts is a good start. Under moderate loads, ~23.0 volts would be ~50% state of charge. It could dip lower under heavier loads for a few seconds or minutes (some systems let you set different voltage set points at different times--~1 hour average, ~5 minute average, ~1 minute average voltage, etc..).

    To give you an idea:
    New poster "leaf" has a really nice set of charts that compare battery voltage against different rates of discharging and charging (as well as resting voltage readings).
    leaf wrote: »
    Am trying to upload the charts I am using...

    Attachment not found.Attachment not found.

    attachment.php?attachmentid=3655

    attachment.php?attachmentid=3654

    [note charts are from: I believe those charts are from Home Power #36, August- September 1993. Lead-Acid Battery State of Charge vs. Voltage ©1993 Richard Perez.
    Here is a link: http://www.scubaengineer.com/documen...ing_graphs.pdf

    vtMaps    ]

    I don't quite a agree with the resting voltage line (I think the voltage is a bit low)--But it shows how to estimate a battery's state of charge while operating.

    Note, where the charts "flatten out"--the room for error estimating state of charge is pretty high.

    -Bill

    There are also "maximum" current draw from your battery bank. For general flooded cell lead acid batteries, using the 20 Hour Rate/capacity (i.e., 225 AH capacity):

    225 AH * 1/20 hour rate = ~11 amp draw (20 hours to take battery bank dead)--Good average current draw (efficient)
    225 AH * 1/8 hour rate = 28 Amp draw--Maximum recommended continuous current draw
    225 AH * 1/5 hour rate = 45 Amp draw--Maximum draw for an hour or so
    225 AH * 1/2.5 hour rate = 90 Amp draw--Maximum surge (seconds to minute or so--i.e., starting pump, etc.).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • Photowhit
    Photowhit Solar Expert Posts: 6,006 ✭✭✭✭✭
    #4
    Re: 24v Battery Bank

    Bill, I know this is the second time I've seen these charts, and I still think either the legend in wrong or they haven't been done correctly. Most flooded lead acid batteries have a resting voltage at 50% SOC of near 12 volts. Yet the first chart as I read it says a 12 volt battery under a 10% of capacity load will have a voltage of 12 volts. I would guess normally it would be closer to the c/5 or 20% load...

    Perhaps I'm reading it wrong or missed something?
    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: 33,642 admin
    #5
    Re: 24v Battery Bank

    Photowhit,

    You are probably correct that the discharge curves seem like they should be a few tenths of a volt lower (maybe even -0.5 volts in some cases)... These are supposed to be measured values by one person on one battery bank at one time.

    In any case, use them as a guide to understand what is happening in "your" system.

    Battery voltage is a poor estimate of state of charge (in general).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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