There is no fixed charge time for every boat battery. For a rough ideal calculation, divide the amp-hours to replace by the charger’s output current. For example, replacing 100Ah at a constant 10A would take 10 hours in ideal conditions. That is arithmetic, not a promise that the battery will be full after 10 hours.
Why can the actual time be longer?
A lead-acid battery’s charge current changes as it fills, and charging losses, temperature, battery condition, depth of discharge and onboard loads affect the result. A battery that is only partly discharged needs less charge than a fully depleted one. The charger may also reduce current during the later part of charging.
How do I know when a marine battery is charged?
Use a charger with a profile approved for the exact battery chemistry and follow the battery and charger manuals. Rely on the charger’s completion indication where the maker specifies it; do not leave a battery charging past its instructions. Do not apply this lead-acid example to a lithium battery.
Sources: U.S. Department of Energy: Handbook 1084-95 — Lead-Acid Storage Batteries
