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Specific Gravity vs. Actual Ah Capacity

Discussion in 'Musings With Maine Sail' started by Maine Sail, May 18, 2017. Add this thread to a FAQ

  1. Maine Sail

    Maine Sail Moderator

    Joined Feb 6, 1998
    10,686 posts, 518 likes
    Canadian Sailcraft 36T
    US Casco Bay, ME
    I hear it repeated endlessly on boating forums that the only "good measure" of a flooded batteries health is to take a specific gravity reading (SG). A battery showing a 100% SOC reading will have a temp correct specific gravity of between 1.265 and 1.280. While SG can certainly help diagnose an inter-cell battery issue, all it does is tell you the state of charge of each individual cell, as related to the others, and create the potential for holes in your clothes.

    The best use of SG testing is to determine if each cell in the battery is at the same state of charge (SOC). Obviously AGM and GEL batteries can't have their SG measured so an OCV test (open circuit voltage) is really the only predictor that can be easily used.

    An OCV test is the sum of all cells in the battery. For example, in a healthy rested 12V battery, each cell would be delivering a 100% SOC voltage of approx 2.12V. There are 6 cells in a 12V battery so...

    2.12V X 6 Cells = *12.72V or 100% SOC (*Trojan Data for OCV)

    However, in a flooded battery that has become out of balance, due to partial state of charge use (PSOC), sulfation, age, over or under charging, or infrequent full charges with absorption voltage held long enough, the cells can get out of balance and the low cell causes the normal cells to begin getting over charged.

    12V Battery Under Charge With 1 Bad Cell:

    *2.51V + 2.51V + 2.51V + *2.05V + 2.51V + 2.51V = 14.6V

    *A cell at 2.51V is the 12V equivalent of regularly charging the battery at over 15V.

    *A cell at 2.05V is the 12V equivalent of being charged at just 12.3V. As once cell gets low the decline becomes more rapid due to chronic under charging of that cell.

    For a 14.6V absorption charge, the voltage of each cell, in a healthy battery, should be: 2.43V X 6 = 14.6V.

    With this example, taking an SG measurement can show you what is going on between cells.

    SG = SOC of each cell in the battery - Can show you a cell imbalance
    OCV = SOC of all six cells in the battery - Can't show you a cell imbalance

    The Problem:

    What OCV and SG can't tell you is the state of health of the battery as related to Ah capacity. Actual Ah capacity and SOC are entirely different measurements. A dead battery, by capacity standards, can still easily reach 100% full.

    Example 100Ah Battery:

    New Capacity & SG = 100Ah & 1.280 - 100% SOC
    Used Capacity & SG = 55Ah & 1.280 - 100% SOC

    In both cases the battery attained 100% SOC yet the stored capacity is telling us the battery is dead.

    Even a severely unhealthy battery can deliver an SG reading of 1.265 - 1.280 or a 100% SOC resting voltage. It is often repeated, incorrectly, that an unhealthy battery will never attain 100% SOC or that SG will never be 1.265 - 1.280 with a battery that has diminished capacity. These points are simply not true.

    Considering SG readings for a failed battery means a failed battery will have a cell imbalance of .050 SG difference between cells. With SG differences of .030 or less corrective equalization's can help but is not necessarily a cure. With a cell imbalance this is the only way SG can dictate SOH and it is really only a pass or fail..

    Myths I've heard repeatedly:

    "An unhealthy battery will never deliver a good SG reading." = FALSE
    "A battery that has lost capacity will never reach 100% SOC." = FALSE
    "Specific gravity can tell you the capacity of the battery." = FALSE


    "Specific gravity cell tell you the cell to cell balance within the battery." = TRUE

    An example from two weeks ago where SG & OCV fails to show any SOH:

    Trojan SCS-225 (Group 31) - May 2015 Date Code
    Rated Ah capacity = 130Ah
    In service Date June 17, 2015
    Months of Actual use 8.5 Months
    Actual Deep Cycles to 50% DOD = 45
    (approx - owners data)
    Lab Rated Cycles to 50% = 600
    Boat Type = Sail / On Mooring
    Ah Capacity Test Date = May 5, 2017
    Tested Ah Capacity @ 76F = 81.77 Ah
    (6.5A constant load to 10.5V)
    Percentage of Rated Capacity = 62.9% / FAIL
    SG Readings = 1.280 - 1.283
    (All six cells - using sight refractometer and hydrometer)
    Factory Rated 100% SOC SG = 1.280
    Resting Voltage Day 5 @ 72F Battery Temp = *12.803V
    Factory Rated 100% SOC to OCV = 12.72V

    * 24 hours rest to remove "surface charge" is often no where near enough to attain a "rested voltage", especially if the battery temp is below 80F. This one was at 72F. The 12.803V OCV was taken at day 5 with a NIST calibrated Fluke 289. This battery is considered at 100% SOC, as defined by Trojan Battery, at 12.72V. Even at 5 days after the last charge it was still sitting/resting +0.083V above "100% SOC" and this is with a DEAD battery confirmed by Ah capacity testing..

    As can be seen above specific gravity tells us nothing about SOH in this case, even though the battery is testing at just 62.9% of its rating, and has failed. By industry standards a flooded deep cycle battery is considered dead when it can no longer deliver 80% of its factory rating. Even for coastal use 63% of rated capacity is simply not worth leaving the mooring with..

    The only true way to know your deep cycle batteries actual SOH, as related to stored capacity, is to conduct a 20 hour capacity test. Specific gravity can be entirely misleading and in some cases does little more than ruin clothes.

  2. JamesG161


    Joined Feb 14, 2014
    2,917 posts, 850 likes
    Hunter 430
    US Waveland, MS
    Specific Gravity (SG) measures only dissolved SO4 + other minerals in water.

    It represents gross Chemical Potential to make electricity, which is only one side of the electrochemical equation.

    Available power can never be determined by SG alone.

    PS: Before cheap Digital Voltmeters it was a crude way to check a wet cell status.

  3. Charles Erwin

    Charles Erwin

    Joined Jan 30, 2012
    961 posts, 104 likes
    Nor'Sea 27 - "Kiwanda"
    US Portland/Anacortes
    To find imbalances - assume you put one probe of the volt meter on the negative post touch the other probe into the liquid in each fill well. Wouldn't that disclose cell imbalances - and preserve clothing?


    JamesG161 likes this.
  4. SFS


    Joined Aug 18, 2015
    1,461 posts, 434 likes
    Hunter 31
    US Tampa Bay
    ^ I hope the answer is yes, as this would be a LOT easier than dealing with a hydrometer.

  5. JamesG161


    Joined Feb 14, 2014
    2,917 posts, 850 likes
    Hunter 430
    US Waveland, MS
    Make sure your probe metal will take the H2SO4.;)

  6. Maine Sail

    Maine Sail Moderator

    Joined Feb 6, 1998
    10,686 posts, 518 likes
    Canadian Sailcraft 36T
    US Casco Bay, ME
    Yes a DVM could be used to work your way down the battery however the probes need to be acid proof and made of a material that will not contaminate the cell when the acid begins to eat them, which it will. It takes very little contamination to ruin a battery. The probes would need to be very long to reach the plates in some batteries and simply probing the electrolyte can give false readings. Touching probes to used plates can cause them to chip, break or crumble. If not careful with probe placement the tips can risk poking through a separator, not good... To get individual cell voltages you probe positive post then cell 1, then cell 1 to cell 2 and so on. If you just probe at an end terminal and then probe the cells with the other lead you would get the cell sum voltages and need to do math to obtain individual cell voltages..

    Summary, I do not recommend this practice!

    A sight refractomter only requires one drop of electrolyte.

  7. JamesG161


    Joined Feb 14, 2014
    2,917 posts, 850 likes
    Hunter 430
    US Waveland, MS
    I checked for a test metal that would not generate significant electrochemical reaction and do @Charles Erwin very clever "poor man's" "once in a while", check.

    Stainless Steel

    I would suggest a very thin SS wire attached to the probe. Small surface area for the thin wire, quickly dip in the cell, record actual cell voltage, move to next cell ( no cross contamination worries), and wipe wire clean and rinse after last cell.

    @Maine Sail , A refractometer is just another indirect SG measurement using light refraction for minerals dissolved water.

    PS: Charles should get a few "likes" on this "out of the box" direct voltage test idea.

  8. JamesG161


    Joined Feb 14, 2014
    2,917 posts, 850 likes
    Hunter 430
    US Waveland, MS
    It takes big exposure TIME to put metal ion in the acid.
    But I don't do this for a living. For those who want to not be liable for contamination, they can use this metal...

    LEAD Wire :)

    The high shunting resistance of a DVM and the short time to test the cell, Lead will NOT interfere with the voltage measurement.:waycool:


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