Oddly enough there is data regarding internal self discharge.  For an
empirical test, charge an old battery that has had distilled water added to
cover the tops of the plates to some known voltage after the "float" or
surface charge effect has been run down.  Use a 12V light to draw a minimum
current until the battery reads ~13 VDC on a good meter.

Even in an isolated environment with a clean case there will be a slow but
discernable discharge that will continue until the battery is well below the
nominal 12VDC.  Lower temperatures can slow the process slightly but it will
continue.

There is a compound that can be added to the battery electrolyte that will
also greatly retard the rate of discharge but not stop it entirely.

pensioner (slave to entropy)