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.
-Lyle
PYS Electrical Technician
Currents PDFs
I believe 10V is considered dead and 13.2V is fully charged. At 11.5V my battery will still start my engine, which is the biggest draw on my boat.
Hi Rondo,
Assuming you battery temperature is 77 degrees Fahrenheit, there are no loads for 24 hours, and this is a 12VDC flooded lead battery, here are some capacity to voltage rating:
10.5 VDC = 0%
12.2 VDC = 50%
12.6 – 12.8 VDC = 100%
13.3 VDC = float charging
14.4 VDC = max bulk voltage
A 12 Volt start battery (different construction than a deep cycle battery) when cranking an engine will experience the biggest draw and hence the voltage will drop to about 11 VDC and even sometimes 10 VDC.
Regards,