Lead Acid batteries like Odyssey (and any other that have lead and acid in them) generally have 3 charging phases. These are more commonly know these days with the marketers pushing number of stages or switch modes in their chargers. Of coarse bigger or more are considered to be better but it really comes down to just 3 phases. Bulk charge, Absorption charge and Float Charge which is sometimes referred to as maintenance charge.

These different charge phases have 3 important factors, current, voltage and time. For this example, we'll use the Odyssey manufacturer's recommendation of 25% of the batteries capacity (to recover a fully discharged battery). So putting some numbers to this we'll use the Odyssey PC2150 battery with 100Ah capacity at 11.6 volts. This battery needs 100 amps plus 10% for losses in charging. So even with a 25A charger this battery will require 4 hours on a bulk charge at 14.7 volts before the current will want to taper so that the voltage doesn't go over 14.7 volts. Then the battery will require approx 6 hours at 14.7 volts reducing the current down to 5% of capacity before the charger will change phases again to float mode at 13.6 volts. At which time it's safe to say the battery will be fully charged.

This information below is produced by the manufacturer to highlight how quickly the batteries will charge when using the correct voltages as a one-off cycle. But if you were to discharge repeatedly you should consider leaving your charger on and connected for over 12 hours to ensure a full charge is always achieved.

Suggested charge times for ODYSSEY^®batteries

Note that the charge times recommended in Table II are based on an assumption that the ODYSSEY^® battery is fully discharged with an open circuit voltage of 11.6 volts. If the battery is only partially discharged the charge times should be appropriately reduced.