Batt Bluetooth SOC won’t sync-up with Victron BMV SOC

Bunkysdad
Bunkysdad Registered Users Posts: 38 ✭✭
in Caravan, Recreational Vehicle, and Marine Power Systems #1
Hi, I have a single 200a/h LiFePO4 (with built-in smart phone SOC bluetooth), and a separate Victron BMV-12 SOC meter.

Problem is that with the Victron meter set at actual 200a/h battery capacity, as the battery discharges (say from a FULL charge), the two SOC meter readings get waaay outta wack… In fact the battery’s BMS has cut-out when the Victron was still reading 25% SOC…A more recent example is that while the battery’s bluetooth was reading 15% SOC, the Victron was reading 31%…BTW, per the manual, the Victron is set-up for use battery with a LiFePO4 chemistry, yet the Victron seems to be considerably overstating the SOC…I do get that over time, meter drift can occur, but this happens from the outset starting with a full charge…Any ideas are much appreciated! : )


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  • BB.
    BB. Super Moderators, Administrators Posts: 33,613 admin
    #2
    Welcome to the forum Bunkysdad,

    First question is: The current shunt (precision resistor) is correctly wired... One end to the battery (no other wires/loads). The other end of the shunt all loads/return wires/chassis ground are connected to that end of the shunt.

    Battery negative=====Shunt====load/ground connection

    If there are other "wires" (loads/chargers/etc.) connected to the Battery Negative, then those "currents" will be "invisible" to the shunt (but will be visible to the Battery's BMS).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • Bunkysdad
    Bunkysdad Registered Users Posts: 38 ✭✭
    #3
    Thanks Bill, that’s a great point you make, so to clarify the Victron shunt is wired correctly as you’ve pointed out, that is to say that ALL in & out ‘current activity’ passes through the Victron shunt…Actually, I was not even aware of this issue until I just swapped out the former LiFePO4 battery (of like-kind amperage) with the new one that has built-in smart phone bluetooth metering…

    My next test might be to fully charge the battery, then (with a known and timed amperage load) deplete the battery (until BMS cut-out) to see if the battery's amp/hr rating is reliable and compare to both SOC meters…
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  • Bunkysdad
    Bunkysdad Registered Users Posts: 38 ✭✭
    #4
    Ok, my plan is to do a continuous load test of the battery by using the air conditioner blower motor (fan only, not the compressor) which uses about 24.5 amps when on high speed…Theoretically, I believe that the fully charged 200a/hr battery should last nearly 8 hours at this rate…

    Does anybody see any flaw in this methodology??

    Thanks




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  • Horsefly
    Horsefly Registered Users Posts: 476 ✭✭✭✭
    #5
    @Bunkysdad - I responded to your newer post about the SoC before I saw this one. 

    When you say you have a Victron BMV-12, I assume you mean the BMV-712 battery monitor. Correct? That has a shunt, and must be the one you are talking about. So you don't have a separate Victron Smart Shunt, right?

    I don't know what BMS your battery has, but generally speaking the SoC it will report is much more of a guess, mostly based on battery voltage, although some do have a tiny (but not very accurate) shunt on-board.  So I would believe the BMV-712 before I would believe what the Bluetooth / BMS says, assuming you have the BMV-712 correctly configured for your battery.
    Off-grid cabin: 6 x Canadian Solar CSK-280M PV panels, Schneider XW-MPPT60-150 Charge Controller, Schneider CSW4024 Inverter/Charger, Schneider SCP, 8S (25.6V), 230Ah Eve LiFePO4 battery in a custom insulated and heated case.
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  • Bunkysdad
    Bunkysdad Registered Users Posts: 38 ✭✭
    #6
    Sorry for the (2 post) confusion, yes I meant a shunt-based Victron BMV-712 rather than their smart-shunt model.
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