The voltage of a battery at rest, measured at least 4 hours after the last charge and after no current has been drawn for at least 1 hour.
The resting voltage provides information about the condition of a battery and notes on care.
To avoid damaging your battery, it should always be charged at least 75 %. For a battery with a nominal voltage of 12 V, 100 % charge corresponds to 12.69 - 12.75 V, 75 % corresponds to 12.51 - 12.57 V. If the battery is to be constantly fully charged in order to be available for certain tasks in case of emergency, it should be kept permanently at 0.1 V per cell above the nominal voltage. For a 12 V battery, this corresponds to a total voltage of 13.2 V.
If the open-circuit voltage of your battery is too low by the voltage of one cell, this indicates the plate short, i.e. the defect of a cell. This is probably a warranty case. In this case contact your dealer.