The best way to determine remaining battery capacity is by using a self-learning battery monitor with a percentage state of charge function (SOC). However, not every boat is equipped with that type of device nor does every boat owner have such a device in their budget. If the only means of estimating remaining battery capacity is a voltage meter, then a reasonable approximation can be made based on an accurate digital voltage measurement with a battery utilizing lead/acid chemistry.
A standard lead/acid cell has a voltage range from 2VDC-2.12VDC; so a 12VDC battery (six cells) will have a nominal range of 12.0VDC-12.72VDC. These numbers are resting potentials and slightly higher values will be seen due to “surface charge”. For estimating purposes, 12.6VDC to 12.7VDC would be 100% charged. A battery at 12VDC is essentially discharged and a battery at 12.2VDC is 50% charged. It is a slim 0.2VDC between the desired recharge point of 50% capacity, and the undesirable fully discharged state that separates good battery management from poor battery management.
Although it is possible to estimate battery capacity using a voltage reading, it is far easier and more accurate to use a battery monitor.
PYS Electrical Technician