battery

Try fully charging your battery, then check voltage a couple times a day to see how long before it stabilizes. My newer batteries will read well over 13 volts resting for a month. Perhaps drawing some current from it would remove the “surface charge” so a reasonably accurate state of charge can be found with a voltmeter.

The instrument of choice is the battery monitor that counts the amp hours charging and discharging so it can display the per cent of full charge all the time.
https://forums.goodsamclub.com/index.cfm/fuseaction/thread/tid/29798122.cfm