If your boat has a battery monitor which reads the amp hours used and the voltage - a rough method to calculate the state of your batteries is to discharge your battery bank to 50% voltage - then read how many amps have been used and double the amount to have a rough guide of battery condition.
Example = If you find that it takes 100 amp hours to bring the voltage to 12.1 then the battery capacity is twice that i.e. 200 amp hours.
If the battery bank started out as 400 amp hours – then they only have half the original capacity.
At one point we could only get 80 amp hours to reach 12.1 volts from Ti Gitu’s 460 amp hour bank – definitely time for a new set!!!
State of Charge (approx.)
12 Volt Battery
The approximate battery voltages for various states of charge.
The exact measurement will depend on the manufacture of the battery.