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Battery Voltage vs. State of Charge

Discussion in 'Ask All Sailors' started by Maine Sail, Jan 22, 2016. Add this thread to a FAQ

  1. Maine Sail

    Maine Sail Moderator

    Joined Feb 6, 1998
    10,824 posts, 633 likes
    Canadian Sailcraft 36T
    US Casco Bay, ME
    I apologize in advance for the quality of the images. I used an intervalometer app on my iPad to take a shot every two minutes and the quality was not quite what I am used to.

    For this illustration I wanted to:
    #1
    Show the relationship of loaded voltage to SOC on a popular lead acid battery.
    #2 Utilize a battery not in perfect health.
    #3 Show why using generic voltage to SOC charts may not work for your battery bank.
    #4 Show the actual rebound voltage after discharging to 50% SOC under controlled conditions.

    After 20 Hour Test = 95.69Ah - Actual Tested Capacity

    [​IMG]


    The image below is an big fail on this boaters part. He found this voltage to SOC chart on the net and I found it on his chart table. He was using it with resting voltages not loaded voltages as this chart was likely developed from.

    If we examine this chart to the 20 hour rate loaded voltages in my test, they line up pretty closely but if using a chart like this for a RESTING VOLTAGE you will be murdering your batteries.

    ___Chart V___Test V
    90% 12.75V > 12.55V

    80% 12.50V > 12.50V

    70% 12.30V > 12.37V

    60% 12.15V > 12.24V

    50% 12.05V > 12.11V

    Some charts on the net are for resting voltages and some of them are for 20 hour rate loaded voltages. If using voltage as an indicator of SOC the rate of discharge can affect your SOC to voltage points. Using the WRONG CHART only exacerbates this and makes your method of murdering your batteries even worse.
    Generic SOC vs. Voltage Chart
    [​IMG]


    The battery in this test is a very popular AGM, a Lifeline GPL-31T. It is rated at 105Ah at the 20 hour rate but this battery, off my brothers boat, was used and only capable of delivering 95.69 Ah's.

    Despite the slightly diminished capacity, 91.3% of factory rated capacity, I kept the discharge rate at 5.25A at a battery temp of about 76.4F. Ideally I would have liked to have spent the time to figure out the new 20 hour discharge rate but this entails numerous tests to 10.5V and was not really that necessary for this experiment.

    The 5.25A discharge rate is slightly higher than what this battery can deliver for 20 hours but close enough to make the points I wanted to make. Perhaps less than .02% of boat owners ever tests their batteries for actual capacity, so using the 20 hour rate on this battery is a bit more real world.

    For a 20 hour test, or to see where your bank stands against how it is rated for capacity, a 20 hour discharge test is performed. The battery is charged to 100% SOC and then discharged at a 0.05C discharge rate. A discharge rate of 0.05C is equal to 5% of the batteries 20 hour rated capacity so 5% of 105Ah is 5.25A.

    On a cruising boat the battery bank would be considerably larger and a 20 hour discharge rate would look like this:

    200 Ah Bank = 10A Discharge Rate = 20 Hour Capacity
    300 Ah Bank = 15A Discharge Rate = 20 Hour Capacity
    400 Ah Bank = 20A Discharge Rate = 20 Hour Capacity
    500 Ah Bank = 25A Discharge Rate = 20 Hour Capacity
    600 Ah Bank = 30A Discharge Rate = 20 Hour Capacity

    95% SOC - 5.25A Load - Volts =12.59V
    [​IMG]


    Here the battery is at 90% SOC with a loaded voltage of 12.55V.

    With a 400Ah house bank this would be a continuous 20A load from 100% SOC in order to see this loaded voltage at 90% SOC.
    90% SOC - 5.25A Load - Volts =12.55V
    [​IMG]


    As cruising boaters the average discharge rate is almost always far lower than the 20 hour rate. A typical boat with a 400Ah bank will be discharging, on average, somewhere in the 5-6A range over a 24 hour period, thus your voltages should not drop this fast with healthy batteries.
    85% SOC - 5.25A Load - Volts =12.52V
    [​IMG]


    Voltage can can be used as a predictor of SOC but you really need to know the voltage performance of your battery bank against a known discharge rate and it takes work to figure out and then it will change on you as the batteries age.

    This is why generic *OCV to SOC charts rarely help the average boater and often serve to actually help murder the batteries. *OCV = Open Circuit Voltage Reading
    80% SOC - 5.25A Load - Volts =12.47V
    [​IMG]


    Over the years I have come to learn that most boat owners drastically over discharge their battery banks when using voltage as a predictor.

    In a recent survey right here on SailboatOwners.com there were in excess 1000 responses, 98.4% of them from sailboat owners. The survey was about battery use. Of those 1000+ responses, 1188 to be exact, 72.42% of responders regularly discharged the bank to 12.1V or less. D'oh!!!!!

    The bottom line with lead acid batteries is that even at your average house loads the terminal voltage should not be dipping below 12.1V, if you expect any sort of decent life from them.

    As we can see in this experiment, under a 5% load to capacity discharge, 49.3% SOC is achieved at 12.10V!!!!
    75% SOC - 5.25A Load - Volts =12.42V
    [​IMG]


    Here we are at 70% SOC at 12.37V. Now compare this to the chart from above at 12.30V that was being used as a resting voltage chart. This is not a resting voltage and the battery has been under a constant 5.25A load now for nearly five and a half hours straight.
    70% SOC - 5.25A Load - Volts =12.37V
    [​IMG]


    65% SOC - 5.25A Load - Volts =12.3V

    [​IMG]


    60% SOC - 5.25A Load - Volts =12.24V

    [​IMG]


    55% SOC - 5.25A Load - Volts =12.18V

    [​IMG]


    Here we are at 50% SOC after over 9+ hours of discharging at a 5.25A load and the loaded voltage is still 12.11V.

    If you expect or plan to use voltage as your indicator of SOC and when to re-charge I would strongly urge you to begin recharging at a bare minimum of 12.1V, even when drawing your average house loads. Remember your house loads are a lower average load than the 20 hour rate so you will be better served to begin re-charging at 12.15V -12.2V in order to get the most life from your bank.
    50% SOC - 5.25A Load - Volts =12.11V
    [​IMG]


    Here's why I typically suggest you begin recharging at a loaded voltage of 12.15 -12.2V because at 12.1V this battery is already at 49.3% SOC while being discharged at 5.25A at 75F.
    49.3% SOC - 5.25A Load - Volts =12.10V
    [​IMG]


    In this image we can see, at 49% SOC, the resting open circuit voltage has rebounded to 12.32V. If you had been using my customers chart as resting OCV you would have mistaken 49% SOC for 70% SOC.

    With this it's easy to see how battery assassins commit murder without even being aware of it. Use voltage carefully if you plan to use it to predict SOC.

    Resting Voltage - Approx 16 Hours @ 49.3% SOC = 12.32V
    [​IMG]
     


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  2. Stu Jackson

    Stu Jackson

    Joined Feb 26, 2004
    20,258 posts, 780 likes
    Catalina 34
    224 CA Maple Bay, BC, Canada
    Finally, a lucid response to "the battery voltage is all you need" crowd.

    Thanks.
     


  3. Maine Sail

    Maine Sail Moderator

    Joined Feb 6, 1998
    10,824 posts, 633 likes
    Canadian Sailcraft 36T
    US Casco Bay, ME
    Battery voltage can certainly be used, I personally used it with lead acid more than my Coulomb counter, because my voltage display was bigger and I understood my bank in relation to temp and loads. Most boaters don't and won't understand their bank this well...

    How people most often use voltage for SOC is usually incorrect, misinterpreted or does not apply to their bank at its state of health or its Peukert/discharge rate. I find most folks using voltage drastically over-discharge rather than erring on the side of caution and doing shallower discharges. As the bank ages the way current effects voltage also changes.

    And BTW there is very large percentage crowd out there using Coulomb counters incorrectly and ending up at a similar result to the voltage only crowd.....
     


  4. Stu Jackson

    Stu Jackson

    Joined Feb 26, 2004
    20,258 posts, 780 likes
    Catalina 34
    224 CA Maple Bay, BC, Canada
    Yup, why you've written your programming topic and why I did my Gotcha.
     


  5. Bill Roosa

    Bill Roosa

    Joined Jun 6, 2006
    6,983 posts, 145 likes
    Hunter 40.5
    US Harrington Harbor North, MD
    So what your are saying MS is that you actually HAVE to know what you are doing when you play with batteries. Sounds like the old story one more time. sail trim, engine mechanics, battery charging, anchoring......

    Sailing is not "easy" when you do it right but it is still mostly fun even if you do it wrong.

    I personally would recommend the charts that have several Voltage VS SOC load (C/load) graphs to avoid being able to read a two column chart incorrectly.
     


  6. Maine Sail

    Maine Sail Moderator

    Joined Feb 6, 1998
    10,824 posts, 633 likes
    Canadian Sailcraft 36T
    US Casco Bay, ME
    As a follow up to this article the second half of this was to determine the effect of charge rate on the charging time from 50% SOC to 100% SOC.

    I conducted two back to back tests discharging to 50% SOC then recharging at either .4C (40% of rated AH capacity) and .2C (20% of rated Ah Capacity) to 100% SOC. A full charge of 100% SOC was deemed at 0.525A at 14.4V per Lifeline batteries 0.5%.

    I will try and get the data together in the next few days but who wants to takes some stabs at how long it takes to charge a fairly healthy 105Ah rated Lifeline AGM battery from 50% SOC to 100% SOC at .4C / 42A and at .2C / 21A?

    Questions:

    At approx what SOC % did the 42A charge rate reach 14.4V / absorption voltage and begin limiting charge current?

    How many minutes were spent in bulk (constant current charging) at .4C / 42A?

    How many hours and minutes to reach 100% SOC at .4C / 42A charge rate?

    At approx what SOC % did the 21A charge rate reach 14.4V / absorption voltage and begin limiting charge current?

    How many minutes were spent in bulk (constant current charging) at .2C / 21A?

    How many hours and minutes to reach 100% SOC at the .2C / 21A charge rate?

    Here is the link to the finished article:

    How Fast Can an AGM Battery be Charged? (LINK)
     


  7. RussC

    RussC

    Joined Sep 11, 2015
    1,205 posts, 536 likes
    Macgregor 25
    US Oregon lakes
    Lots of good info there Main Sail. thanks for putting it out there.
    Curious if you actually use a 42A charge rate on a regular basis?? I thought as a general rule the slower the charge rate the better, for battery health. not so????

    Thanks,
    Russ
     


  8. Charles Erwin

    Charles Erwin

    Joined Jan 30, 2012
    1,020 posts, 144 likes
    Nor'Sea 27 - "Kiwanda"
    US Portland/Anacortes
    I think 42A is an arbitrary figure. The idea here is to make a comparison between how long it takes to get to 100% charged if maximum charge current is limited to 40% of C/20 (40% of 105=42A) as opposed to 20% of C/20 (20% of 105 = 21A).

    As to the question presented and since we are asked to guess - which is all I can muster anyhow - my guess is the lower rate (20%) is the winner because the lower the rate the better the charging efficiency will be.


    Charles
     


    Last edited: Jan 26, 2016
  9. Maine Sail

    Maine Sail Moderator

    Joined Feb 6, 1998
    10,824 posts, 633 likes
    Canadian Sailcraft 36T
    US Casco Bay, ME

    Russ,

    For flooded batteries they ideally like about 10% but in cycling applications they can be fast charged at up to 20-25%. Some deep cycle flooded batteries, like Rolls batteries, do not have the additives to sustain high charge rates so staying below .2C is best with those.

    For AGM batteries generally the more current the better for longevity. Higher charge rates in AGM batteries actually yields longer life. Dave V. the lead engineer at Lifeline did a study on this years ago presented at the IEEE Power Sources Symposium. Odyssey battery, thin plate pure lead AGM, wants to see a minimum of .4C and Lifeline a minimum of .2C...

    This is from the conclusion section of Dave V's study:

    "In order to achieve the maximum cycle life from sealed lead acid batteries, (AGM) not only should the DOD be kept as low as possible, but the charge current limit should be as high as possible."

    It then goes on to suggest that a balance needs to be met between equipment and optimal cycle life. Today Dave suggests a minimum charge current of .2C for Lifeline AGM's.

    It is a pretty rare sailboat boat that can muster a .4C charge rate. On a 450Ah bank that would be an alternator that can put out 180A when hot....
     


  10. Bill Roosa

    Bill Roosa

    Joined Jun 6, 2006
    6,983 posts, 145 likes
    Hunter 40.5
    US Harrington Harbor North, MD
    see below my gestimations

    At approx what SOC % did the 42A charge rate reach 14.4V / absorption voltage and begin limiting charge current?
    84% SOC using my graphs

    How many minutes were spent in bulk (constant current charging) at .4C / 42A?
    4 minutes max

    How many hours and minutes to reach 100% SOC at .4C / 42A charge rate?
    3 hours and 25 minutes

    At approx what SOC % did the 21A charge rate reach 14.4V / absorption voltage and begin limiting charge current?
    92% SOC again using my wet cell graphs

    How many minutes were spent in bulk (constant current charging) at .2C / 21A?

    How many hours and minutes to reach 100% SOC at the .2C / 21A charge rate?
     


  11. Maine Sail

    Maine Sail Moderator

    Joined Feb 6, 1998
    10,824 posts, 633 likes
    Canadian Sailcraft 36T
    US Casco Bay, ME
    Remember the charge rate determines the length of bulk. Bulk/CC charging ends when absorption/CV charging begins. Bulk or constant current charging is also nearly 100% efficient (usually 98% or better in a battery like this) in the energy being put in being stored,.

    4 minutes is 0.07 hours. If the battery started at 49.3% SOC then:

    42A X 0.07 = 2.94Ah returned in 4 minutes

    Battery capacity was 95.69Ah and we removed 48.54Ah placing the battery at 49.3% SOC. Remaining capacity was 47.15Ah

    47.15 + 2.94 = 50.09

    50.09Ah's is 52.4% SOC where absorption was attained if we think it lasted 4 minutes in bulk.

    That would be amazing.
     


  12. NICK1951

    NICK1951

    Joined Aug 3, 2014
    68 posts, 12 likes
    CATALINA, BENETEAU OCEANIS 36, 400
    US Grosse Ile, Mi and Fajardo, PR
    Hello all,

    I am sitting on the hook in Culebra PR enjoying the thread. Curious, the thread suggests when to charge, I want to understand when to quit charging. Currently, I quit charging when the 400 amp hour agm battery bank draws 20 amps.

    What do you suggest?

    Nick
     


    Last edited: Jan 30, 2016
  13. Maine Sail

    Maine Sail Moderator

    Joined Feb 6, 1998
    10,824 posts, 633 likes
    Canadian Sailcraft 36T
    US Casco Bay, ME
    20A at what terminal voltage? Your batteries can be considered full at 2A or less at 14.4V +..

    With AGM getting to full as often as possible is best. 20A on a 400Ah AGM bank is quite a ways from full but may give you the energy you need. Without knowing more it is hard to suggest anything other than get back to 100% SOC as often as possible..
     


  14. NICK1951

    NICK1951

    Joined Aug 3, 2014
    68 posts, 12 likes
    CATALINA, BENETEAU OCEANIS 36, 400
    US Grosse Ile, Mi and Fajardo, PR
    Thanks Maine,
    I quit charging when the MC 14 reaches absorbtion, terminal voltage is 14.1. I start charging when terminal voltage is 12.4 to 12.5. There is a 4 amp load at the 12.4 to 12.5 terminal voltage.
    Nick





    Thanks.

    Nick
     


    Last edited: Jan 30, 2016
  15. Maine Sail

    Maine Sail Moderator

    Joined Feb 6, 1998
    10,824 posts, 633 likes
    Canadian Sailcraft 36T
    US Casco Bay, ME
    What brand of AGM or are they Gel? 14.1 is correct for GEL but too low for AGM...
     


  16. Bill Roosa

    Bill Roosa

    Joined Jun 6, 2006
    6,983 posts, 145 likes
    Hunter 40.5
    US Harrington Harbor North, MD
    Who ever quits charging the batteries? I leave mine on trickle charge (leave is probaly misleading as the charger goes to trickle automagicly)
    As for the sharp shooting of my "gustemates" I was only responding to your question MS. You asked for a guess and that's what I gave you. Thought it was going to be a "see who guesses closetes with out going over" sort of thing. Clearly I have misunderstood the intent.
    My defense of my numbers is that this is my experience. I personnaly don't think the concept of bulk to absorption change over (and absorption to trickle) and times and actual Ah delivered to the battery is a well documented event.
    Is is
    a) actual Ah delivered
    or
    b)is it when the batteries get to some specifice voltage while being charged
    or
    c)is it when the current drops to some specific A=x/capacity value (and what x is too)
    I've seen all three described as "the definition of bulk to absorption change over". I beleive this is due to manufacturers having various ways of selling us battery charging devices/controlers/regulators. Those with sophisticated instriments (read expensive) have both current and voltage sensors so the toute that "Ah delivered" is the correct way to measure, the voltage only and amps only units toute simmilar things because that is the way they "do it".
    I've never seen an industry standard for battery charging that can be used by all.
     


  17. Bill Roosa

    Bill Roosa

    Joined Jun 6, 2006
    6,983 posts, 145 likes
    Hunter 40.5
    US Harrington Harbor North, MD
    BTW these are the charts I use. You have to know:
    the capacity of the bank (and yea we are all guessing here as I'll never test to determine)
    the voltage of the bank
    the current flowing into or out of the bank (not necessicarily the current on your nav panel as the solar panels may be "producing" xx amps but the reefer is consuming xx-yy and only the differance is going to the batteries
    C is the battery capacity in Ah, so "C/20" for a 400 Ah bank would be 400/20 = 20 amps.
    To use look at the ammeter that shows current flowing into/out of the battery.
    Note if you are charging or discharging the battery and pick the correct chart
    divide the bank capacity by the current flowing into/out of the bank (bank capacity/current flow=the number under the "C" in the "C/xx" labeled lines).
    read down the curvy "c/xx" line (probably have to interpolate between two lines)(note that the relationship is not a liniar one between the lines!!!!!!) till you get to the loaded battery voltage and then read down to get SOC.
    example: 400 Ah bank being discharged a 5 amps with a voltage of 12.5
    400/5=80, so we are looking for the "C/80" line. Dang it there isn't one. There are "C/20" and a "C/100" lines though so have to read between them. Using your sailors interpolation binos and noting the "way" the rest of the lines read (20-100 is a really small vertical space and the 3-5 space is much larger) I'm going to read much closer to the C/100 line than the C/20 line. Moving down the imagined C/80 line till it crosses the horizontal 12.5 volts line and then down to get the SOC I read (in the discharge chart) 57% SOC.
    A very conservative value based on what MS is teaching in his 12.1 volts discussion. image_3535.png image_3536.png
     


  18. Bill Roosa

    Bill Roosa

    Joined Jun 6, 2006
    6,983 posts, 145 likes
    Hunter 40.5
    US Harrington Harbor North, MD
    Don't know what is going on with the multipel pictures in my previous post. There are only 2 charts.
    Also if you where to watch the volt meter (and ammetere too) as the reefer compressor cycles on and off (5 amps and 0 amps drawn from the bank you can refferance the "at rest" line in the charging chart to get a "second opinion" noting that that "resting voltage" is not true untill about 1/2 hour and the observed voltage is going to be lower than that shown on the chart. FWIW
     


  19. Maine Sail

    Maine Sail Moderator

    Joined Feb 6, 1998
    10,824 posts, 633 likes
    Canadian Sailcraft 36T
    US Casco Bay, ME
    People who cruise often need to know when it is no longer being "beneficial" to run the engine or gen set for charging...


    Was not meant as a sharp shoot but I saw a big disparity in two of your answers that I thought warranted an explanation. Your 4 minutes was a close guess but charge current effects when the bank will transition from bulk to absorption and the 4 minutes and 92% were just out of sync..

    Bulk to absorption is simply the transition from CC to CV. If the absorption voltage is 14.4V then you exit bulk (CC) when the battery hits 14.4V and begins absorption charging. CV or absorption charging is where current begins declining. The higher the charge rate the lower in the SOC curve this transition from CC to CV will occur. The lower the charge rate the higher in the SOC curve this will occur.

    The answer is B. In the marine market all charge sources we use operate on CC/CV charging. So bulk/CC/Max charge current ends when the battery becomes voltage limited by controller, regulator or charger.
     


  20. Bill Roosa

    Bill Roosa

    Joined Jun 6, 2006
    6,983 posts, 145 likes
    Hunter 40.5
    US Harrington Harbor North, MD
    So when crusing you use the solar/wind/tarrif rail trickle charging with infrequent engine charging and that is where we need to understand that solar/wind/tarrif rail is not meeting the demand.

    "Was not emant as a sharp shoot but....." I did not take offense just trying to explain I did not do a very through annalysis, and yes the numbers you quoted are part of the issue at hand, bulk-absorption-trickel changeover points.
    "The higher the charge rate the lower in the SOC curve this will occur. The lower the charge rate the higher in the SOC curve this will occur." Exactly my point, there is no "formula" for it. ie when the charging rate is xx what formula do I use vs when it is lower than xx?

    "
    The answer is B. In the marine market all charge sources we use operate on CC/CV charging so bulk ends when the battery becomes voltage limited by controller, regulator or charger." So your charging source is looking at charge current and supplying the needed voltage so the current stays at a constant value during bulk charging???? Pretty sure this is not how it works electrically. My understanding is the charge controler "does magical things" and determines that it needs to bulk charge and sets the voltage at the predetermined bulk charge voltage (probably 14.4). That in no way insures that charge current is constant. My experience is that the bank rapidly becomes (12 minutes) voltage limited and the current falls of mightly. The charge controler stops charging momentairly and does more "magical things" (like measure the "resting" voltage) and determines what charge regime to do for the next time period. It is really a puso resting voltage assessment of the SOC. If it really was CC charging then the regulator would need to know what the charging current was going into the batteries would it not? They are using fixed voltage points when not charging to assess SOC and then determing which regime to do for the next time period.
    Which brings up the observation that since they are using resting voltage to assess SOC and that is highly dependant on the state of health of the batteries (with old batteries taking a lot longer to settel down to resting voltage), a lot of the problem is that the charge controler is set up to over charge older batteries by its very (limited) design.
    Every charge controler I've seen does it this way. I cannot say that I've seen as many as you MS so correct me if I'm wrong but when I notice the lights slightly dimming and then brightning every 15 minutes or so while using the engine and this coorisponding to the engine sounding like it is taking up a load when the lights brighten and loosing load when they dim.......I'm pretty sure the controler is checking the "resting" battery voltage to determine what it needs to do next.
    Thoughts?
     



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