Here is a simple way to check the condition of a 12-volt outboard marine battery in your boat with a simple battery checker.
The motor is off:
1. Connect the battery checker to the battery terminals. The red connector goes to the positive terminal and the black connector goes to the negative terminal.
2. The light that illuminates under the "battery condition" will indicate the battery's approximate voltage charge.
3. A fully charged and healthy battery should read 12.6V. A partially discharged battery that needs recharging would have a 12.1V to 12.5V level.
The motor is running:
1. Connect the battery checker to the battery terminals. The red connector goes to the positive terminal and the black connector goes to the negative terminal. Make sure it is safe to run the motor.
2. The light that illuminates under the "charger output" will indicate the stator's approximate voltage charge.
3. A healthy charging system should read 13.2V to 14.5V. A charging system that may need attention might read under 13.1V or over 14.6V at idle.
Please note, as a reference, a 12-volt lead acid battery will deliver about 12.7 volts when fully charged, but only about 11.6 volts at 20% capacity. Meanwhile, a lithium battery will deliver 13.6 volts when fully charged, and 12.9 volts at 20% capacity.