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I stumbled upon an interesting table which allows you to judge the charging state of your battery based on the exact voltage of the battery NOT under load.
Car battery - Wikipedia, the free encyclopedia
(scroll down to see the table)
Terminal voltage
The open circuit voltage, is measured when the engine is off and no loads are connected. It can be approximately related to the charge of the battery by:
Open Circuit Voltage (12 V) / Open Circuit Voltage (6 V) / Approximate charge / Relative acid density
12.65 V / 6.3 V / 100% / 1.265 g/cm3
12.45 V / 6.2 V / 75% / 1.225 g/cm3
12.24 V / 6.1 V / 50% / 1.190 g/cm3
12.06 V / 6.0 V / 25% / 1.155 g/cm3
11.89 V / 6.0 V / 0% / 1.120 g/cm3
That's why I permanently installed a V-meter on my bike, so I know when I need to charge the battery.
Car battery - Wikipedia, the free encyclopedia
(scroll down to see the table)
Terminal voltage
The open circuit voltage, is measured when the engine is off and no loads are connected. It can be approximately related to the charge of the battery by:
Open Circuit Voltage (12 V) / Open Circuit Voltage (6 V) / Approximate charge / Relative acid density
12.65 V / 6.3 V / 100% / 1.265 g/cm3
12.45 V / 6.2 V / 75% / 1.225 g/cm3
12.24 V / 6.1 V / 50% / 1.190 g/cm3
12.06 V / 6.0 V / 25% / 1.155 g/cm3
11.89 V / 6.0 V / 0% / 1.120 g/cm3
That's why I permanently installed a V-meter on my bike, so I know when I need to charge the battery.