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Gari Williams avatar image

BMV-712 Predictable SOC Drift

I have a BMV-712 which monitors the state of charge of an 800 Ah lithium pack. This is installed on a liveaboard boat and is charged purely from solar. Day to day the soc remains within the range 30-80% and is in continuous use. I recognise the issue of drift and as a result each month I bring the soc up to calibrate the BMV back to 100%. I have the reset conditions quite tightly controlled, the BMV needs 13.7v (absorption at 13.8v) and a 2% tail current for 6 minutes. This reset always occurs ‘prematurely’ ie at an indicated soc of about 92%. I know I cannot avoid soc drift; however, for it to be consistently in one direction makes me believe the situation can be improved by changing my settings. Since, at the end of the month the BMV is underestimating the true soc I felt that I need to increase the charge efficiency factor. I already have this set to 99% and changes are only available in whole numbers. Could I ask that you confirm my logic and if I am correct is there any chance we could get a software update to introduce more precise settings for the charge efficiency factor?

BMV Battery Monitor
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JohnC avatar image

Hi @Gari Williams

Look also to your Peukert settings, which is like charge efficiency, but covers discharge efficiency. Victron suggest 1.05 I think for Li batts, and if it's set higher then you could try that. Or even go lower.

Unity for both those settings implies a perfect world.

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Hi John, my Peukert coefficient is indeed set to 1.05. I had considered using a smaller number but given that my discharge rates vary dramatically, my intuition for fiddling in that area was a bit grey. On the other hand the charge efficiency is much easier to grasp so that is where I directed my attention. At the same time this appeared to be an area where the software was failing the customer so I hoped to shine a spotlight on that.

JohnC avatar image JohnC ♦ Gari Williams ·

Yeh, it's grey. How best to work both factors in practically is maybe only possible with an onboard batt bms SOC to compare against.

Without that it's a bit of a guess. But you still have a half dozen adjustments left to find that elusive neutral or upward drift. Tis why I mentioned unity. If you get that far without satisfaction you have deeper issues.

Have a go..