If the readings range between 12 and 12.3 volts, your battery is operating at ¾ of its capacity and it should take you sometime before you’ll need to replace it. Lastly, if your battery is operating in critical condition and is at around 11.8 volts, this means that it is operating at around ¼ or 25% of its full capacity.
For readings that are over 12.5 volts, it means that your battery is fully charged and it is in good condition. If the readings range between 12 and 12.3 volts, your battery is operating at ¾ of its capacity and it should take you sometime before you’ll need to replace it.
Using modern precision chargers allows both a fast charge and safe floating voltages, allowing them to be left on the battery continuously. 6V batteries need to stay below 7.1V to avoid gassing, and typical charge voltages are 6.9V (float) to 7.5V (bulk charge).
Those batteries that are used in deep discharge cycling mode can be charged up to 2.45 volts/cell (14.7V for a 12V battery) to get the highest charge rate, as long as the voltage is dropped to the float voltage when the charge is complete. Voltage table for cyclic use charging.
A battery's capacity is the amount of electric charge it can deliver at a voltage that does not drop below the specified terminal voltage. The more electrode material contained in the cell the greater its capacity. A small cell has less capacity than a larger cell with the same chemistry, although they develop the same open-circuit voltage. [ 49 ]
6V batteries need to stay below 7.1V to avoid gassing, and typical charge voltages are 6.9V (float) to 7.5V (bulk charge). The basic lead acid battery is ancient and a lot of different charge methods have been used.