24 v battery bank low voltage question

rrroaerrroae Solar Expert Posts: 46 ✭✭
What should the low voltage setting on a 24 volt battery bank be set at? I thought it should be 2x the setting of a 12 volt bank of 12.1 to keep from going below 50% so I set it at 24.2.


My battery bank is 4 Crown 395amphr 6 volts wired together for 24 volt and I thought this should give me at least 180 amp hours of electricity bet we're closer to 70! Batteries are stored in basement and we have a 2" pvc vent pipe going into the enclosure. Battery temp monitor shows temp usually around 55' degrees this winter


I checked the specific gravity late today and all cells were in the 1.265 - 1.275 range.



Am I doing something wrong?

Comments

  • BB.BB. Super Moderators Posts: 26,837 admin
    Re: 24 v battery bank low voltage question
    rrroae wrote: »
    What should the low voltage setting on a 24 volt battery bank be set at? I thought it should be 2x the setting of a 12 volt bank of 12.1 to keep from going below 50% so I set it at 24.2.
    Works for me--if the battery is warm and/or under heavier loads--you may have problems with early "cut-off".
    My battery bank is 4 Crown 395amphr 6 volts wired together for 24 volt and I thought this should give me at least 180 amp hours of electricity bet we're closer to 70! Batteries are stored in basement and we have a 2" pvc vent pipe going into the enclosure. Battery temp monitor shows temp usually around 55' degrees this winter
    How are you determining the 70 AH capacity?
    • Voltage measurements?
    • Temperature corrected Hydrometer?
    • Amp*Hours / Watt*Hour / Battery Monitor?
    • Battery Tester?
    I checked the specific gravity late today and all cells were in the 1.265 - 1.275 range.
    The spread in specific gravity seems normal (not large).

    How are the absolute values vs when the battery has just been equalized?

    Is the above reading temperature compensated for 55F vs 77F?

    You can make some temperature/charge compensated tables for your specific gravity like this gentleman. [note: I looked closer at the charts, and currently they have the SG temperature offset applied backwards--SG gets higher, denser, as temperature falls, not as temperature increases--I have sent an email to the person a while ago--so I don't know when/if this will be fixed]

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • rrroaerrroae Solar Expert Posts: 46 ✭✭
    Re: 24 v battery bank low voltage question
    BB. wrote: »
    How are you determining the 70 AH capacity?
    Magnum battery monitor. I'm going off Amp hours out when my voltage hits 24.2(under load)

    Heaviest daily load might be 12 amp.
    How are the absolute values vs when the battery has just been equalized?
    This was the first time I measured specific gravity. Will log values when I equalize and at low voltage.
    Is the above reading temperature compensated for 55F vs 77F?
    Not sure. Thought it might be as I seem to recall that the Magnum 4024 120/240 split does temperature compensation.
  • BB.BB. Super Moderators Posts: 26,837 admin
    Re: 24 v battery bank low voltage question

    Since you have a battery monitor--take a look at the Amp*Hours out--that is more accurate than measuring 24.2 volts under load (wait a least an hour for accurate battery "resting voltage" to estimate state of charge).

    Regarding temperature compensation--I was asking about Specific Gravity Temperature compensation... From this chart, a 55F battery reading should be compensated by:
    • 1.265 at 55F - 0.010 = 1.255 at 80F (Temperature corrected SG reading to 77/80F standard condition)
    Close to full charge--but without knowing your battery's actual starting specific gravity--don't know for sure.

    Next time you have a chance, equalize (until the SG stops rising after 1 hour) and use those as your "maximum" SG levels to compare with for 100% charge.

    And don't feel you have to hit 100% every day--hitting 90% every few days is fine. Trying for 100% charge every day will eventually cause damage to the battery (too much electrolysis/equalization drives oxygen into the positive grid causing corrosion).

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