A good healthy lawnmower battery should register between 12.6 to 12.7 volts. Due to way batteries discharge it's very important that you test the battery after it has been sitting (not running) for at least an hour, in order to get the resting voltage of the battery. Testing the battery just after use will give an inaccurate reading. See below, a basic battery charge status voltage readings:
Terminal Voltage: 12.6 - 12.7 V - Fully Charged
Terminal Voltage: 12.4 V - Half Charged
Terminal Voltage: 12.2 V - Flat Battery - Requires immediate charge
Terminal Voltage: 12.0 V and below regarded a deep discharge and may have permanent damage.