Victron BMV settings not right?

blentz · November 2023

I have 2 Renogy 12v 200amp hr batteries in parallel with Victron MPPT charge controller. My load on the system is about 1amp per hour (Off grid cabin with security camera and radio internet bridge and switch) running 24 x 7) - no inverting, all devices are powered by 12v.

What i don't understand is in the morning after starting the previous evening with 12.7v battery full charge, my battery voltage is 12.3v which is about 70% state of charge, My BMV 712 states that battery bank is at 97% state of charge - something is not right.

Questions:
1) How can a 1amp per hour draw take the battery bank down to 70% state of charge over a 14 hour period?
2) Why would the BMV think we are at 97% state of charge when the battery bank voltage is at 12.3v ?

Thanks in advance for your thoughts!

Bob

Horsefly · November 2023

The BMV does not use the battery voltage to determine the state of charge (SoC) of the battery. Rather, it uses Coulomb counting to measure the Amp hours (Ah) going into and coming out of the battery. You just need to make sure you have the settings of your BMV correct for your battery: Battery capacity, charge voltage, Peukert exponent, charge efficiency, discharge floor, and tail current. It used to be that the BMV came pre-configured for lead-acid, so you may not need to do anything with the peukert, or efficiency.

blentz · November 2023

So you are saying that the voltage as displayed on the color controller is incorrect? if the voltage is correct at 12.3, that would be 70% SOC right? Voltage does not lie so even if i make sure the battery parameters are correct in the 712, i should then see 70% SOC, not 97% - which brings me back to how can i loose 30% of battery over a 14 hour period only drawing 16w or 1amp ?

littleharbor2 · November 2023

The only time voltage is somewhat accurate is when the battery has been sitting with no load and no charging for maybe 4 or more hours. As Horsefly said accurate parameters need to be entered into the BVM before you can trust its readings. Also as batteries age they lose capacity and with that lost capacity goes the accuracy of the BVM.
Question, are ALL loads on your battery running through the shunt? If not, it's Soc.% is no good.

Horsefly · November 2023

Lead acid battery voltage as an indicator of SoC is only a rough approximation. Beyond that I really don't have any answer to your question. I think @littleharbor2 has a pretty good hint to follow.

My lead-acid experience is a bit dated now (and I not a pleasant memory). The systems I've built and/or helped with are all running LiFePO4 batteries, which allow SoC to be measured very accurately using a shunt-based system such as the BMV-712. I would never use voltage to estimate SoC, but my attitude is a bit biased by my tilt toward LiFePO4.

blentz · November 2023

I understand - here is my problem - this cabin is at 9000ft where we have many days in a row of no sun. I have a battery protector installed that cuts off all loads at 12.0v to protect the battery from over-discharging during this period. I guess i will need to manually check the battery voltage 1st thing in the AM before any charging takes place to compare it to the BMV.

All my loads are run through the shunt.

Horsefly · November 2023

For lead acid (you didn't say, so I'm still just assuming) 12.0V is a good cutoff to use on your battery protect, as most LA batteries are at about 50% around 12V.

How late in the evening is the 12.7V measurement? I'm having trouble remembering my lead acid battery voltages, but with only 1A of loads and assuming your battery got fully charged during the day, it doesn't seem like 400Ah of battery would have dropped down that far. I think mine (230Ah of 24V AGMs) were generally around 26V when we went to bed.

Edit to add: My system and one of the others I helped put in are both in the mountains at about 9,000 ft elevation, so we at least have that in common!

Victron BMV settings not right?

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