1. Rest the bank first
Turn off charging and all loads and leave the bank for at least four hours (overnight is better). Surface charge makes a tired battery look healthy.
2. Read resting voltage
Set the meter to DC volts and measure across the bank terminals. For a 12 V lead-acid bank: 12.7 V is 100%, 12.5 V is 75%, 12.2 V is 50% and 12.0 V or below is effectively discharged.
Anything that will not rest above 12.4 V after a full charge is losing capacity.
3. Load test
Switch on a known load such as the anchor windlass or a cabin heater and watch the voltage. A healthy bank sags a little and recovers; a failing one collapses below 11.5 V and stays there.
Test each battery individually — one bad cell drags the whole bank down and will kill its neighbours.
4. Check charging
Run the engine and measure again. You should see roughly 13.8-14.4 V at the bank. Much lower means a failing alternator, corroded connections or undersized cable; much higher risks cooking the batteries.
Maintenance routine
Clean and grease terminals annually, keep flooded cells topped with distilled water, and log resting voltage each month so you can see decline before it strands you.