Solar up and running

Ah, that could be useful for sure. I guess you would record a series of voltages as a battery is under load, graph it. Then do it again in a month or year and see if there is a change.

I tried to find a graph of voltage vs time on a resting battery but haven't found one. I know, I should do it myself with my batteries.

I did find this comment at battery university:

http wrote:
Voltage Method

Measuring state-of-charge by voltage is simple, but it can be inaccurate because cell materials and temperature affect the voltage. The most blatant error of the voltage-based SoC occurs when disturbing a battery with a charge or discharge. The resulting agitation distorts the voltage and it no longer represents a correct SoC reference. To get accurate readings, the battery needs to rest in the open circuit state for at least four hours; battery manufacturers recommend 24 hours for lead acid. This makes the voltage-based SoC method impractical for a battery in active duty.

Battery university