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I am looking to calibrate a new battery monitor for an off-grid battery bank that's ~5 years old, since the previous battery monitor stopped working recently. I'm not sure how to go about it given the age of the batteries and the fact that I am not currently able to charge them to 100%.

The setup is a 17280w battery bank (24 lead-acid units of 2V each, rated at 360Ah at C5, in series for a total of 48V) that is charged by solar panels via a Victron SmartSolar MPPT 150/70 [1], and supplies AC current via a Victron Multiplus 48 / 3000 / 35-16 Inverter [2]. I had a Victron BMV-700 battery monitor [3] that has worked well for the past 4 years and 11 months but recently broke. While I try to get the warranty activated I want to use this cheap AiLi monitor [4] to tell me the State of Charge (SOC) (and eventually replace the broken BMV-700 with a repaired one).

So I need to calibrate the battery monitor. How would you suggest going about it? A few options / questions:

  1. pre-program

    (A) I could pre-program it for a capacity of 360Ah. This is what I originally did with the BMV-700. I think it worked well, because the batteries still seem to function well, although I don't have any empirical measurement of this beyond "we generally have had power when we needed it".

    (B) pre-program it for a reduced capacity, e.g. 330Ah, to account for the batteries' reduced capacity over 5 years of use. What reduced capacity would be appropriate in this case? FWIW the batteries have rarely discharged down below 60% (as shown by the BMV-700, whatever that's worth) and only twice below 50%.

  1. measure the SOC using a hydrometer (??) and calibrate accordingly? I believe something of this nature is possible, though I don't know exactly how. What are all the factors I'd need to account for? E.g. temperature, water level inside the batteries, initial chemical composition of the liquid? And would I need to measure each of the 24 batteries and then average them out? How do I convert the measurement into an SOC or capacity estimate?

  2. Other options?

The instructions for the AiLi monitor suggest calibrating it by telling it when the batteries are at 100%. But due to cloudy weather recently and short days in winter (I'm in northern Italy) I cannot know when the batteries are at 100%.

Many thanks in advance!

[1] https://www.victronenergy.com/solar-charge-controllers/smartsolar-mppt-ve.can

[2] https://www.off-grid-europe.com/victron-multiplus-48-3000-35-16-inverter-charger

[3] https://www.victronenergy.com/battery-monitors/bmv-700

[4] https://www.amazon.com/AiLi-Battery-Monitor-Voltmeter-Motorhome/dp/B07CTKYFTG?th=1

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I ended up calibrating it to 310Ah, which corresponds to a 3% loss each year (360Ah * 0.97 ^ 5). I also had to tell the monitor when the batteries were at 100%, which I simply did not have a good answer for. I did it after a stretch of 4 sunny days during which I made very little use of the batteries. In the summer I'll re-calibrate.

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