Battery Monitors and Shunt's

Blackcherry04
Blackcherry04 Solar Expert Posts: 2,490 ✭✭✭
in Off Grid Solar & Battery Systems #1
Battery Monitors and Shunt's has come up in another thread. If I set up and calibrate two different shunts and Monitors from two different manufacturers I get different results. Talk about your results and how much you find in the results over different charge and discharge schemes. I find over a week of them not being reset to 100% full charge they can be as much as 20 - 30 % apart. What about the different calibration perimeters that you can set to specifics of your system. I sometimes find the information from them to be no more then I can get with the SG level and a DVM.

I am now using the Outback FNdc set up with a 3 shunt configuration with 2 different charging sources plus a Inverter / Charger. Does Outback know something that others don't ? How much error is acceptable ? If I use SG's as the gold standard, then the multiple shunt's gives better results when totalized. Is it the flow of electrons on common buss? Is it about what the batteries actually receive and store ?
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Attachment not found.

Think of the input shunts as breaks or junctions in the DC (-) conductor. The current is conducted
as though it were a continuous conductor, but the shunts allow the user to monitor the current,
providing information for more accurate battery recharging.
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  • vtmaps
    vtmaps Solar Expert Posts: 3,741 ✭✭✭✭
    #2
    Re: Battery Monitors and Shunt's
    Blackcherry04 wrote: »
    Does Outback know something that others don't ? How much error is acceptable ? If I use SG's as the gold standard, then the multiple shunt's gives better results when totalized. Is it the flow of electrons on common buss? Is it about what the batteries actually receive and store ?

    Outback's system uses three shunts to get the total battery current. Other systems use a single shunt. A single shunt that measures the total current into the battery will give the same results as dividing the current into three parts and measuring each part with a separate shunt (Kirchhoff's law).

    If your two monitors give you different SOC, there are several reasons. Some monitors account for temperature and Peukert factor. Some monitors use vary the charge efficiency based on the SOC.

    There are many possible reasons for the discrepancy, but i doubt it is because of the number of shunts. If each charging source or load has its own shunt, then the sum of the shunts is the battery current.

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
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  • karrak
    karrak Solar Expert Posts: 326 ✭✭✭✭
    #3
    Re: Battery Monitors and Shunt's
    Blackcherry04 wrote: »
    Battery Monitors and Shunt's has come up in another thread. If I set up and calibrate two different shunts and Monitors from two different manufacturers I get different results. Talk about your results and how much you find in the results over different charge and discharge schemes. I find over a week of them not being reset to 100% full charge they can be as much as 20 - 30 % apart. What about the different calibration perimeters that you can set to specifics of your system. I sometimes find the information from them to be no more then I can get with the SG level and a DVM.

    I would need more information to make an informed opinion on this. I would think the main variable here is the coulomb/current efficiency of the battery. Was either reading close to your gold standard, did either of these devices try to take into account the efficiency of the battery?

    With my LiFePO4 battery I have no problems measuring the SOC accurately with fairly basic cheap equipment due to the >99.5% efficiency of these batteries. Only ever gets out by a few % over a week.
    I am now using the Outback FNdc set up with a 3 shunt configuration with 2 different charging sources plus a Inverter / Charger. Does Outback know something that others don't ? How much error is acceptable ? If I use SG's as the gold standard, then the multiple shunt's gives better results when totalized. Is it the flow of electrons on common buss? Is it about what the batteries actually receive and store ?

    Maybe Outback make an estimate of battery efficiency and use this to correct their SOC reading. Would be very impressive if they can make this estimate with any accuracy.!
    Off-Grid with LFP (LiFePO4) battery, battery Installed April 2013
    32x90Ah Winston cells 2p16s (48V), MPP Solar PIP5048MS 5kW Inverter/80A MPPT controller/60A charger, 1900W of Solar Panels
    modified BMS based on TI bq769x0 cell monitors.
    Homemade overall system monitoring and power management  https://github.com/simat/BatteryMonitor
     

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  • Blackcherry04
    Blackcherry04 Solar Expert Posts: 2,490 ✭✭✭
    #4
    Re: Battery Monitors and Shunt's
    karrak wrote: »
    I would need more information to make an informed opinion on this. I would think the main variable here is the coulomb/current efficiency of the battery. Was either reading close to your gold standard, did either of these devices try to take into account the efficiency of the battery?

    With my LiFePO4 battery I have no problems measuring the SOC accurately with fairly basic cheap equipment due to the >99.5% efficiency of these batteries. Only ever gets out by a few % over a week.



    Maybe Outback make an estimate of battery efficiency and use this to correct their SOC reading. Would be very impressive if they can make this estimate with any accuracy.!
    Here is the Battery Factor.

    Battery Charge Factor (BCF)
    The BCF compensates for recharging inefficiency from gassing and heating losses to
    assure the battery state-of-charge (SOC) display is accurate by scaling it; the BCF is
    programmable from 65% to 100% (see page 29).

    If the SOC percentage displays full before the batteries’ user-determined charge settings
    are met, decrease the BCF percentage; increase it if the SOC is low.
    The default setting is 94%, meaning 94% of the charging current is being used for
    recharging and 6% is lost. OutBack suggests maintaining the default setting until the
    FLEXNet DC has monitored the system through a few charge/discharge cycles.
    Setting the BCF to 100% effectively disables any BCF display changes.
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  • Blackcherry04
    Blackcherry04 Solar Expert Posts: 2,490 ✭✭✭
    #5
    Re: Battery Monitors and Shunt's

    If I move one of my charging source shunt's and parallel that sources output directly to the Inverter inputs will my total show more or less going into the batteries and stored ?? Is any shunt is a resistor ??
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  • vtmaps
    vtmaps Solar Expert Posts: 3,741 ✭✭✭✭
    #6
    Re: Battery Monitors and Shunt's
    Blackcherry04 wrote: »
    The BCF compensates for recharging inefficiency from gassing and heating losses
    <snip>
    The default setting is 94%, meaning 94% of the charging current is being used for
    recharging and 6% is lost.

    Trimetric also uses 94% as the default.

    The problem is that the BCF varies with the SOC... it is well understood that a battery charges more efficiently in bulk (no gassing) than in absorb or equalization. A more sophisticated monitor would start out at a higher BCF and adjust downwards as the battery SOC rises. The BCF should be 0% when the SOC is 100%.

    If you take a discharged battery and hook it up to a charger set to only float, most battery monitors will count the amphours and ultimately show that the battery is 100% (it might take days). Of course, the battery is not fully charged despite what the monitor shows.

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
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  • Blackcherry04
    Blackcherry04 Solar Expert Posts: 2,490 ✭✭✭
    #7
    Re: Battery Monitors and Shunt's
    vtmaps wrote: »
    Trimetric also uses 94% as the default.

    The problem is that the BCF varies with the SOC... it is well understood that a battery charges more efficiently in bulk (no gassing) than in absorb or equalization. A more sophisticated monitor would start out at a higher BCF and adjust downwards as the battery SOC rises. The BCF should be 0% when the SOC is 100%.

    If you take a discharged battery and hook it up to a charger set to only float, most battery monitors will count the amphours and ultimately show that the battery is 100% (it might take days). Of course, the battery is not fully charged despite what the monitor shows.

    --vtMaps
    Yeah, your right , there lots of variables, thats why I am always suspicious about any data i see on them. It's hard beat a hydrometer and a DVM. The temperature here has changed about 15° drop and everything has changed.
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  • zoneblue
    zoneblue Solar Expert Posts: 1,220 ✭✭✭✭
    #8
    Re: Battery Monitors and Shunt's

    There are probably 10s of thousand of thesis pages written on lead SOC modelling, with few real conclusions. And hence an 8bit micro, with 32K ram, having to deal with untold user and battery combinations, is never going to get better than a rough approximation.

    As vt said, the coloumb efficiency varies with SOC, and this is taken into account by none of the retail monitors. So where in the charge stage you run it, compared to the"average" efficiency, will make the results vary.
    1.8kWp CSUN, 10kWh AGM, Midnite Classic 150, Outback VFX3024E,
    http://zoneblue.org/cms/page.php?view=off-grid-solar


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