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