question

laurenceh avatar image
laurenceh asked

BMV SOC just does not add up (or subtract) what is wrong here?

Setup: I have a BMV 700 On my boat with 300Ah (@C20) batteries, a pretty constant load current of 1A and an MPPT recharging the batteries when there is enough Sun.

The BMV SOC and Ah consumed values disagree by approximately 50% which is worrying because if the trend is across the full capacity the battery could be empty when the SOC says 50%.

I dont think it is anything to do with the setup (wiring to shunt etc.) as all I am looking at is the BMV data (see graph below) the BMV is reading the current (in/out) and making the calculations on this data so it is just not in agreement with itself. I would be happy to understand where I have gone wrong and learn from my mistakes, here is my reasoning.

I have read the article here.

https://community.victronenergy.com/questions/58699/bmv-700-soc-percantage-does-not-match-to-the-used.html

But I started a new thread as this is new data and I think it might be proof that something is not right (or I dont understand). In that article it says it might be the Peukert exponent but it is set to the Victron default 1.25, charging efficiency default 95%, battery capacity set to 270AH which is the C20 capacity less 10% all as set my my Victron installer. So if I am following Victron recommendations what more can I do?

Considering:

1) The load at 1A is so low that Peukert should not have any effect (which is about inefficiency when drawing high currents).

2) The problem can't be charging efficiency because the SOC is too high even when no charging has happened.

3) it cant be the wrong current reading because the BMV is using the current reading as its only input value for all the calculations. (it could be wrong but it should not be inconsistent)

4) it can't be the battery capacity setting that throw the readings out by 50% and anyway I have only entered the one capacity value so the BMV must base all its calculations on the capacity I have set (270 Ah).

5) Finally it should not be the discharge floor setting (15%) as setting a discharge floor should cause the SOC to be lowered for a given AH consumed value. Anyway according to the documentation discharge floor only impacts the time remaining.

BMV is Firmware version 3.11 - it was installed by an authorised Victron installer.

OBSERVATIONS

At the end of charging yesterday afternoon the BMV reset to 100% SOC and 0Ah used. Then 14 hours later this morning before the MPPT started up the values were.

  • SOC 98%, (actually looking via the VRM it gives the SOC at 97.6%)
  • Consumed 12.9 Ah,
  • Current -0.9A

The current was constant at between 0.9 and 1 A overnight, so I feel I can trust the consumed Ah. But the SOC at 97.6% suggests that 2.4% has been consumed and 2.4% of 270 is 6.5 Ah almost exactly half of the displayed consumed Ah.

The other suggestion in the article above is that it is rounding errors so I looked back through my data, as I took screen shots of the BMV display on my smart phone at intervals over the last 2 months.

Here is what I see:

The orange line is the % consumed calculated based on consumed AH (according to the BMV), the blue line is the corresponding line for 100%-SOC% (I.e. how much the BMV SOC says has been consumed). The lines are amazingly straight and as you can see the SOC is consistently saying that the amount consumed is half of what the consumed AH says.

1616059408144.png

IF anyone want to check here are my battery setting in the BMV.

img-0198.pngMy ultimate worry is that others who use the boat from time to time may rely on the SOC display and run the battery down without realising.

BMV Battery MonitorSOC
1616059408144.png (34.9 KiB)
img-0198.png (357.6 KiB)
2 comments
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klim8skeptic avatar image klim8skeptic ♦ commented ·

TLDR.

Your battery capacity does not match. RU Fudging any other figures?

BMV manual.


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laurenceh avatar image laurenceh klim8skeptic ♦ commented ·

Not sure Understand what you are asking? - (unless I have inserted any typos) all figures are exactly as per the Victron recommendation and as configured by the Victron installer - nothing fudged. the 270Ah is not a fudge it is what I think is recommended (by Victron) (considering that my installed capacity is 300Ah) - to allow for battery capacity fading over time. And as point out should not impact the discrepancy that is observed anyway.

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1 Answer
JohnC avatar image
JohnC answered ·

Hi @LaurenceH

You've made one incorrect assumption, and this is key to what you're observing: "The load at 1A is so low that Peukert should not have any effect (which is about inefficiency when drawing high currents)."

It also works on low currents, and your 1A is akin to drawing at a C270 rate (or C300?). What happens then is that the BMV will compensate by lowering the effective Ah drawn from the batts, and applying the corrected Ah to the SOC. With a high Peukert this will be pronounced.

Now it's all very well to rely on Victron's defaults, but given your low load and dread at the results, do it properly. Download Victron's Peukert Calculator from their website (it came to me as a Win .exe file that you can keep). Find your batt makers C5, C10, C20, C100, (whatever), ratings and feed them into the calculator over the range you commonly use. The result may vary over different ranges, so choose the one most appropriate.

You may then need to adjust your BMV's Charge Efficiency to tune it properly. Now you still won't get a direct Ah-SOC correlation, and never will while running a Peukert above unity, but the SOC should be more reliable and reproducible when set up right.

And if your batts are new, use the real ratings, downgrade when you notice something, hopefully years away..

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laurenceh avatar image laurenceh commented ·

Thanks @JohnC

That is certainly an answer I can accept and understand. I will read up on Pukerts and adjust accordingly. I certainly see the point that discharging at the C270 rate means that the battery has even more capacity than what it says on the label.

To be clear the current load is an exceptionally low one because I am not on the boat and the only things drawing current are my RPi and the GSM router connecting it to the internet. So when we start to use the boat again our load profile will be more normal.

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