Amp-hours and charging current
Amp-hours describe electric charge, while amps describe its transfer rate. Multiply capacity by the charge-level increase, then divide by average current and charge acceptance efficiency. This efficiency concerns restored charge, not wall-to-battery energy conversion.
hours = Ah × (target% − start%) ÷ 100 ÷ average A ÷ efficiency fraction
Example with a 60 Ah battery
Charging a 60 Ah battery from 50% to 100% restores 30 Ah. At 5 A average current and 90% charge acceptance, estimated time is 6.67 hours. If tapering reduces average current to 3 A, the estimate rises to 11.11 hours.
What to confirm with the manufacturer
Use a charger and mode approved for the battery chemistry. The charger maximum is not necessarily the average for the session. Absorption and finishing stages, temperature and battery condition may increase time. Resting voltage alone does not give a precise charge percentage for every battery.