Confused with % of Battery discharge

backroad
backroad Solar Expert Posts: 185 ✭✭
in Advanced Solar Electric Technical Forum #1
Like I said, I'm confused again. I've been thinking that I've been doing great with my 48v battery bank by not letting it go any lower that 46-47 volts on discharge. Now, I've finally found a chart that tells me at that voltage I've actually been bringing my battery bank down to 20% of full. I know this is NOT good, but is this really correct.

Please, no spankings.....already had enough :):):)
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  • jeffkruse
    jeffkruse Solar Expert Posts: 205 ✭✭✭
    #2
    Re: Confused with % of Battery discharge

    Ouch. Thats low.:cry: I think you don't want them to go below 48.5V or higher.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,623 admin
    #3
    Re: Confused with % of Battery discharge

    A 48 Volt Bank at resting voltage at 77F is around 45% state of charge (see generic bank voltage chart from Battery FAQ--12.00 volts).

    However, the actual resting voltage is really dependent on the initial specific gravity of the electrolyte fill...

    So, what was your "full charge" Specific Gravity?

    One thread here
    Another thread here

    So, to pull from earlier posts:

    Well mixed electrolyte is important--so if you notice all cells not coming back to "full charge" level of s.g., then there is a good chance that "mixing" (equalization) will bring the s.g. back up to your logged readings (keep these as your "good batteries" / "fully equalized" numbers.

    The amount of variation you are seeing is not that great (less than 0.030 between all cells). And having "high" s.g. is not always a good thing either.

    I ran across this page on why different types of batteries have different starting s.g. fills... Is pretty interesting:
    Specific Gravity vs Applications
    1.285 Heavily cycled batteries such as for forklifts (traction).
    1.260 Automotive (SLI)
    1.250 UPS – Standby with high momentary discharge current requirement.
    1.215 Geral applications such as power utility and telephone.

    As mentioned earlier, the specific gravity (spgr.) of a fully charged industrial battery, or traction battery, is generally 1.285, depending on the manufacturer and type. Some manufacturers use specific gravities as high as 1.320 in an attempt to gain additional Ah capacity, but at the cost of a shorter cycle life.

    ...

    Higher Gravity = vs Lower Gravity =
    More capacity / Less capacity
    Shorter life / Longer life
    Higher momentary discharge rates / Lower momentary discharge rates
    Less adaptable to "floating: operation / More adaptable to "floating" operation
    More standing loss / Less standing loss
    Also on that page is the formula between cell resting voltage and specific gravity:
    Specific gravity = single-cell open-circuit voltage - 0.845 (example: 2.13v – 0.845 = 1.285)
    Or
    Single-cell open circuit voltage = specific gravity + 0.845.

    So, if you are getting 46 volts after resting, if the formula is correct (and your battery is around 77F) your specific gravity should be:
    • 46v/24cells - 0.845 = 1.072 (dead?)
    • 47v/24cells - 0.845 = 1.113 (~20% state of charge)
    Yuasa states that the temperature correction for SG is:
    • specific gravity changes by +.001 for every -3 degrees Fahrenheit.
    J.R. Buchanan did a nice set of charts on battery SG and temperature based on a 100% charge = 1.265 SG.

    By the way, do you have a Battery Monitor?

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • audredger
    audredger Solar Expert Posts: 272 ✭✭
    #4
    Re: Confused with % of Battery discharge

    learn,Learn,LEARN! Good thread(s). Are you going to let me coast for a while Bill?
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,623 admin
    #5
    Re: Confused with % of Battery discharge

    Please feel free to step in Mike--Maybe I can rest my math/reading skills for a bit. :roll:

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • audredger
    audredger Solar Expert Posts: 272 ✭✭
    #6
    Re: Confused with % of Battery discharge

    Hey, I'm just the student here.

    But, according to Surrette
    % Charge Specifi c Gravity* (SG)
    100........1.255 – 1.275
    75..........1.215 – 1.235
    50..........1.200 – 1.180
    25..........1.165 – 1.155
    0............1.130 – 1.110

    This rather than voltage might be the better way of checking state of charge.?

    Note: The Surrette uses 1.265 electrolyte. Other brands may use electrolite of diferent SG.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,623 admin
    #7
    Re: Confused with % of Battery discharge

    Yes, Specific Gravity is "better" for checking state of charge... But you also need to know the initial SG (or SG when new and fully 100% charged) as the starting point.

    SG measurements are a pain (go to shed, spill acid, drop pliers on battery bank, run like he11, etc.).

    Voltage measurements are OK--if battery has rested for a few hours (no charging, no discharging). Somebody here commented that voltage measurements are less accurate at less than 40% state of charge. :confused:

    And, then you have AGM/Sealed batteries where you cannot measure the specific gravity.

    -Bill "pushing audredger down hill" B.
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • audredger
    audredger Solar Expert Posts: 272 ✭✭
    #8
    Re: Confused with % of Battery discharge

    Mr Bill, You'r too quick!
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  • Ralph Day
    Ralph Day Solar Expert Posts: 1,022 ✭✭✭✭
    #9
    Re: Confused with % of Battery discharge

    Yes Mike, SG is the only way to determine state of charge...if you have flooded lead acid batteries. I have a Xantrex T500 remote meter and it's calculated state of charge is always reading higher than the hydrometer measured SOC.

    I keep a pilot cell measured most days just to know when I last had a full SOC. A piece of paper with the date and sg value is hanging by the battery box. It's in the basement, so access is fast and easy, not to mention convenient.

    Look on a map, hanging down into Lake Ontario is Prince Edward County. We don't have any plans to succeed, but have the geography of it nailed already!

    Ralph
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  • higgins
    higgins Solar Expert Posts: 56 ✭✭✭✭
    #10
    Re: Confused with % of Battery discharge
    audredger wrote: »
    Hey, I'm just the student here.

    But, according to Surrette
    % Charge Specifi c Gravity* (SG)
    100........1.255 – 1.275
    75..........1.215 – 1.235
    50..........1.200 – 1.180
    25..........1.165 – 1.155
    0............1.130 – 1.110

    This rather than voltage might be the better way of checking state of charge.?

    Note: The Surrette uses 1.265 electrolyte. Other brands may use electrolite of diferent SG.
    Does the SG correspond to voltage i.e i saw a chart which shows voltage readings
    and percentage of dischrage and it had similar values as audregger's readings,if you move the decimal point one place to right it would show actual voltage can this be applied all the time?
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,623 admin
    #11
    Re: Confused with % of Battery discharge

    Actually, the formula for Battery Cell Voltage vs Specific Gravity (at 77F) is:

    Specific gravity = single-cell open-circuit voltage - 0.845
    (example: 2.13v – 0.845 = 1.285)
    Or
    Single-cell open circuit voltage = specific gravity + 0.845

    Somebody else here (sorry, I don't remember who) said that the equation is less accurate below 40% state of charge.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • bryanl
    bryanl Solar Expert Posts: 175 ✭✭✭✭✭
    #12
    Re: Confused with % of Battery discharge

    I was going to note the fact that the link above did have the formula relating specific gravity to voltage but Bill beat me to it! Both of these measures are primary measures of state of charge.

    That link also notes that both specific gravity and voltage need proper care in measurement to assure reliable results.

    Specific gravity measurements are HAZMAT territory and also provide a risk for cell contamination. With modern DVM's, you can often avoid the hazards unless you really need information about a specific cell.

    The question I have is that the graph provided at the link does not seem to match its formula and other comments. That may be due to range problems. It should be interesting to investigate.
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  • higgins
    higgins Solar Expert Posts: 56 ✭✭✭✭
    #13
    Re: Confused with % of Battery discharge

    how would go about checking cells or measuring sealed type batteries?
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,623 admin
    #14
    Re: Confused with % of Battery discharge

    Here is a thread about "rejuvenating" AGM batteries by adding new caps with catalyst and even adding a bit of water to the cell.

    It is not easy... Larger cells you can measure each cell's voltage (with an accurate DVM). For smaller batteries without access to individual cell bus connections--I don't know...

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    #15
    Re: Confused with % of Battery discharge
    higgins wrote: »
    how would go about checking cells or measuring sealed type batteries?

    you don't. any attempt to do so on a sealed battery with multiple cells could 1> void a warranty and 2> ruin the battery. you could average the voltage by taking the overall voltage and divide by the number of cells, but this won't indicate individual cell problems very well until it gets much worse.
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  • tallgirl
    tallgirl Solar Expert Posts: 413 ✭✭
    #16
    Re: Confused with % of Battery discharge
    BB. wrote: »

    Somebody else here (sorry, I don't remember who) said that the equation is less accurate below 40% state of charge.

    -Bill

    It's also less accurate at higher states of charge. The voltage-versus-SG curve is S-shaped. Sort of. For very stretched out values of "S".
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  • RCinFLA
    RCinFLA Solar Expert Posts: 1,485 ✭✭✭✭
    #17
    Re: Confused with % of Battery discharge

    Attached is the chart of voltage vs. SG from battery handbook.

    Rested state is referred to as kinetic equilibrium.

    During current movement, either charge or discharge, there is some voltage potential needed to create the movements of ions, and chemical movement throughout the battery. This includes conversion of sulfuric acid into and back to water, lead/lead dioxide to and from lead sulfate and movement of water and sulphuric acid around within the battery. This is called plate polarization voltage. Some of the total is for positive plate and some for negative plate and they are different, you see only the sum result in discharge voltage drop or extra voltage requirement during recharge.

    Unfortunately, the amount of the polarization depends on condition of battery and its type. For example, AGM in good condition requires lower plate voltage delta polarization to get the kinetic movements accomplished. An old battery generally requires more plate polarization voltage delta then a new battery. This makes telling state of charge by battery voltage during an actual current flow very difficult.
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