question

@codefoster

The battery monitor would only count energy in and out of the battery. So really if it is the same load everyday then energy returned, it would look right all the time. But as batteries age they loose capacity.

So really you need to work out if you still have the same capacity. And to work out capacity is to do a full discharge test at its rated c rating and then charge it up again at its rated c rating and see how much was put back in, then enter that as your new battery capacity in the BMV.

The chart only applies when the battery has been completely at rest, no charge or discharge, for several hours.

Tracking is tricky as temperature is also a factor. You'd basically have to track, voltage, SoC, current and temperature for correlation. It's a mess and is often inconclusive.

I periodically run a reserve capacity, RC, test. This is the number of minutes a battery can provide 25A and stay above 10.5V. In most cases, this is notably higher than the C20 rate, so you're not actually testing them down to 0%. Some batteries have other capacity ratings for higher currents. Testing to these values also help you avoid taking them down to 0%.

If the RC is 100 minutes, and I get 90 minutes, I assume the batteries meet 90% of rated capacity. I then adjust my capacity in the BMV to reflect the tested capacity.

There are also other times where I might do a deeper than normal discharge. If I can leave the system offline for a few hours to allow the voltage to rebound, I'll use the voltage to SoC charge for correlation.

Another thing to watch for with batteries in series is that the battery voltages stay identical. If they aren't perfectly balanced and matched, one battery can get over charged and the other under charged.

I doubt 6-7 year old batteries meet capacity unless they've been very lightly used and always maintained at a good float voltage, but they could be at a very high state of health.