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abeness avatar image

BMV-712 capacity discrepancies

Hello, Group. I'm using a BMV-712 with a Relion 100AHr LiFePO4 battery that I purchased a couple of months ago. Both are in an emergency power battery box I built as a pandemic project. I mostly power computers, phones, and other electronics from it. I also recently acquired a Blue Smart IP67 12/25 charger to charge at 25A, and previously used a West Mountain Radio Epic PWRgate to charge at 10A.

The BMV-712 is configured per recommended LiFePO4 settings: 100Ah, Peukert exponent 1.05, charge efficiency 99%, charged voltage 14.2. What I'm confused by is the following:

Last discharge I hit 11% SOC, and voltage was 11.24 with 4.75A draw. Consumed Ah was reported as 94.5A, however. 5.5A remaining is certainly not 11% of 100Ah. From the low voltage, I'm inclined to believe that 5.5Ah was left, so why is the SOC saying 11%, and how can I align that with actual capacity?

Also, on recharge, SOC reached 100% some time (maybe 20 mins) before the charger output dropped down below tail current. The Blue Smart charger was still charging like gangbusters at 24.37A when SOC hit 100%, with voltage only at 13.92. I think there was around a 3A draw at the time. How can I set this up so SOC is more accurate?

Thanks,

Abe

SOCbattery capacity
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dgnevans answered ·

Just to confirm your Battery capacity on the BMV is set to 100 ah. Charged voltage should be correct at 14.2v Your Peukert is correct at 1.05 your charge efficiency at 99%.
What is your Tail current set to?

What is your Discharge floor set at?

By the looks of it your system was not synchronizing properly on the old charger and as a result when you charged with the new charger it reached 100% based on how much charge has passed through the BMV to the battery before the battery had actually charged. Your BMV should have synced now that the battery reached full charge. You should notice improved accuracy with a few charge discharge cycles.

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Hi, dgnevans. Yes, battery capacity set correctly at 100Ah. Tail current set to 0.50% (=0.5A here). Discharge floor is 0%, but as I understand that from the manual, this only affects relay operation, which is not applicable in my current installation.

I don't quite understand "when you charged with the new charger it reached 100% based on how much charge has passed through the BMV to the battery before the battery had actually charged." Isn't SOC supposed to be reset to 100% when BOTH charged V and tail current are met? I haven't exactly understood how this description of SOC reset is supposed to interact with the BMV-712 counting amps passing through and reaching 100% that way.

This was the second full charge with the new charger. Guess I'll see how it shakes out,