If your battery goes below 80% charge, generally when you start charging at the maximum recommended charge current, the voltage at the battery terminals will be below float voltage level (13.3V to 14.7V depending on battery type and manufacturer). At the point where the battery charges sufficiently to allow the terminals to reach the float voltage level, the rule of thumb is that the battery is 80% charged. Usually, with the proper sized charger, this takes 20% of the total charge time (again rule of thumb). The remaining 20% charge usually takes and additional 12 to 18 hours.
If you do not discharge the battery below the 80% level, it will take shorter to charge the battery to full. Do not rely on the “Full” indication on most battery chargers, It is usually better to charge the battery for a prolong time to insure minimizing sulfation. Ideally a battery should be on a maintenance (float) charger continuously unless in use.
Also, do not rely on cell voltages unless you know the temperature of your electrolyte and the battery is clean with no load. The 10.5V level mentioned above is for a battery under load. Open cell voltages for charge state are different.
The attached table is for wet cell, lead/lead or lead/calcium batteries. The voltage is open cell voltages, no connections. As you can see, the voltages vary greatly over temperature. If you must use open cell voltages to monitor state of charge, use this table taking the temperature into consideration.
There are two types of fools...
One says this is old, and therefore good..
The other says this is new, and therefore better...