I would like to design a battery capacity system for my AGM lead acid battery (12v, 40Ah). My idea is that I have values of the battery voltage and its corresponding capacity, which I will use to plot a curve which I can then interpolate with the current voltage reading. I will be measuring the voltage of the battery with a INA219 chip.
I contacted the company that manufactures the battery and was told that designing such a battery capacity monitoring system is not useful as an aging battery would eventually have reduced capacity despite an unchanged full charged voltage reading. They gave an example where the battery might start of with say 13.2V but may have reduced capacity (by 10Ah) in a few years while still retaining its maximum voltage reading (13.2V). Hence, plotting such a curve may not be useful. Also, I would not want to fully discharge the battery with the intention to regularly update changes to this voltage-capacity curve, since doing so would significantly impair the lifespan of my battery.
Does anyone have any thoughts on alternative methods that I can monitor the capacity of the battery while taking into account capacity changes caused by an aging battery? Otherwise how can I continue using the voltage-capacity plot method while taking into account the changes caused by aging?