Best way to determine battery SOC at the end of bulk charge?

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AceNZ
AceNZ Solar Expert Posts: 104 ✭✭✭✭
in Advanced Solar Electric Technical Forum #1
What's the best way to determine the SOC at the end of bulk charge?

I need to specify this number to my inverter/charger. It's used to re-sync the internal battery meter.

Unfortunately, the internal meter is showing very different numbers from the external meter, so I need something more than just the meter.

I realize that I could just measure SG at the end of bulk. However, the electrolyte isn't well-mixed at that time, so the SG would be low, indicating they are less charged than they really are.

My understanding is that SG is only a good measure of SOC after absorption (or equalization). So, can I back into the end-of-bulk SOC somehow from there? Maybe use the external meter to measure the change in AH or SOC from the end of bulk to the end of absorption, and subtract the difference from the measured charge based on SG?
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  • vtmaps
    vtmaps Solar Expert Posts: 3,741 ✭✭✭✭
    #2
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    Re: Best way to determine battery SOC at the end of bulk charge?
    AceNZ wrote: »
    What's the best way to determine the SOC at the end of bulk charge?

    I need to specify this number to my inverter/charger. It's used to re-sync the internal battery meter.

    That's a really good question... something most of wouldn't think to ask unless we have Victron chargers. Your proposed approach makes sense, but assumes that the AH you are measuring (during absorb) correlates linearly with SOC. It doesn't... the charging process become less and less efficient during absorb. The efficiency of the charging process during absorb may vary depending upon the depth of the discharge before bulk.

    I suppose the only way to know for sure would be to do a load/capacity test on the battery after bulk. Or turn on your electrolyte recirculation pump and judge SOC by SG of the stirred electrolyte 8)

    One other thing... I'm not too sure that the SOC is the same (at the end of bulk) every day. It may vary depending on how deep and long lasting he discharge has been.

    Tell us all more about the Victron charging protocols... the europeans are doing some interesting things with charging... some european chargers won't charge a battery on a sunny day if its SOC starts out too high.

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
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  • AceNZ
    AceNZ Solar Expert Posts: 104 ✭✭✭✭
    #3
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    Re: Best way to determine battery SOC at the end of bulk charge?
    vtmaps wrote: »
    Your proposed approach makes sense, but assumes that the AH you are measuring (during absorb) correlates linearly with SOC. It doesn't... the charging process become less and less efficient during absorb. The efficiency of the charging process during absorb may vary depending upon the depth of the discharge before bulk.

    Good point.

    In addition, the external meter displays AH used vs. maximum / full-charge, and it often hits 0 AH / 100% SOC before absorption completes.
    vtmaps wrote: »
    One other thing... I'm not too sure that the SOC is the same (at the end of bulk) every day. It may vary depending on how deep and long lasting he discharge has been.

    True. Another issue is that the charger sometimes goes into bulk mode when the SOC is already relatively high.

    Hmm.
    vtmaps wrote: »
    Tell us all more about the Victron charging protocols... the europeans are doing some interesting things with charging... some european chargers won't charge a battery on a sunny day if its SOC starts out too high.

    Victron's charging curve looks like this:

    Attachment not found.

    The process is:

    1. Start in Bulk, allowing the battery to take as much current as it can, up to a specified maximum, while allowing voltage to rise.
    2. If "Battery Safe" mode is enabled, absorption starts at 57.6V, regardless of the configured absorption voltage, and voltage is allowed to increase with a fixed ramp until it reaches absorption voltage.
    3. If "Adaptive" charging mode is selected, the charger stays in Absorption for a time that's proportional to the time spent in Bulk. If "Fixed" mode is selected, the time spent in Absorption is fixed.
    4. If "Tubular plate traction battery" mode is selected, when the charge current reaches 10% of the Bulk current, then it's kept at that level until either voltage rises to 68V (not configurable) or Absorption ends.
    5. After Absorption, the charger goes into Float. The Float voltage used here is configurable, and is usually set a little above the manufacturer's recommendations -- say 55.2V instead of 52.8V. The reasoning is that a slightly higher float will cause a little gassing, which will allow the electrolyte to mix more thoroughly, so the battery can reach full charge.
    6. After 24 hrs in Float, the charger changes to Storage mode, and drops the voltage to 52.8V.
    7. Once every few days (configurable), the charger goes back into absorption mode for an hour or two, to help reduce stratification and to keep the battery at a true full charge.

    The normal charging process doesn't use SOC. There is an internal battery meter, but it's only used to trigger various configurable switches -- or, in my case, as part of their self-consumption algorithm / system.
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  • Blackcherry04
    Blackcherry04 Solar Expert Posts: 2,490 ✭✭✭
    #4
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    Re: Best way to determine battery SOC at the end of bulk charge?

    I hate to tell you, but all those monitors are a waste of time and are a estimate at best. There are only 3 things you need, Voltage, Ending Amps and the SG level at completion of the charge. I have been watching monitors for 30 years and all you do is spend your time trying to calibrate them day after day. Peukert's law requires a factor, that factor is a moving target that you'll never get 100% correct, if you ever did, it would change, Temperature, Current and charge Over Voltage all effect it, as does Battery type and Condition.
    Another issue is that the charger sometimes goes into bulk mode when the SOC is already relatively high.
    It goes into Bulk because it's at or below the Voltage point that re-sets the charge cycle, that usually 12.5, 25 or 50V depending on the system. Sometimes a Inverter / Charger will have a auto re-set that will do a force charge when reconnected to AC.
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  • AceNZ
    AceNZ Solar Expert Posts: 104 ✭✭✭✭
    #5
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    Re: Best way to determine battery SOC at the end of bulk charge?
    Blackcherry04 wrote: »
    I hate to tell you, but all those monitors are a waste of time and are a estimate at best. There are only 3 things you need, Voltage, Ending Amps and the SG level at completion of the charge. I have been watching monitors for 30 years and all you do is spend your time trying to calibrate them day after day. Peukert's law requires a factor, that factor is a moving target that you'll never get 100% correct, if you ever did, it would change, Temperature, Current and charge Over Voltage all effect it, as does Battery type and Condition.

    I agree that the meters are estimates.

    Nonetheless, one is necessary for my application; it's an automated system, and I have to live with the firmware. I'm not looking for perfection, but I'd like to improve the quality of the estimate to the extent I can.
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  • Blackcherry04
    Blackcherry04 Solar Expert Posts: 2,490 ✭✭✭
    #6
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    Re: Best way to determine battery SOC at the end of bulk charge?
    AceNZ wrote: »
    I agree that the meters are estimates.

    Nonetheless, one is necessary for my application; it's an automated system, and I have to live with the firmware. I'm not looking for perfection, but I'd like to improve the quality of the estimate to the extent I can.
    From my observations the higher the C/ *rate, the lower the SOC will be at the trigger Voltage for absorb. The larger current will push up the charging voltage higher and cause a early transition to Absorb. So if you use the rule of thumb of 70%-85% at the end of Bulk, the higher the current the lower the soc will be. I can charge at 250 amps and drop 125 amps and the Voltage will drop immediately at least .50 volt + and the bulk will continue, so that tells me the soc would be lower as it's a voltage trigger.
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