OCV, CCV and Internal Resistance

OCV = Open Circiut Voltage
CCV = Closed Circuit Voltage

There are 2 ways of measuring battery voltage. There are OCV and CCV.
Voltage measured when no load connected to battery is called OCV and voltage measured when load is connected to battery is CCV.
CCV is always lower than OCV because battery can maintain higher pressure (voltage) when no current flow from it.
The OCV of a 12v Lead-Acid battery is about 13.2V (for Battery rated 12V). When even a small load apply to the battery, the CCV will measure near 12v.
The CCV of a batteryis the function of the load applied and the state of charge of the battery. If the battery is connected to heavy load, the CCV will lower than if the battery connected to the light load.
If the battery is totally near its discharge state, the CCV is at its lowest. OCV typically not affected until the battery is near complete discharge state.

INTERNAL RESISTANCE(IR)

The opposition or restriction of direct current flow within the cell or battery. Its value may vary with the current, state of charge, temperature and age.
With an heavy load applied such as engine starting, the battery voltage may drop to a certain value because of the internal resistance in the cells. The cell that has near discharge has higher internal resistance than fully charged cell. So, nearly discharge cell/battery will have higher voltage drop.
The IR is determine by the load applied and state of charge of the battery. A battery IR is equal to difference between OCV and CCV, divide by applied load.

IR = (OCV - CCV)/I load