Battery Internal Resistance
The sum of the plate, electrolyte and connection resistances inside the battery itself. It is on the order of milliohms and, multiplied by the current drawn, produces a drop in the terminal voltage. It increases in ageing or cold batteries; it is the fundamental reason for voltage sag under load.
Internal resistance is the quantity showing that a battery is not an ideal voltage source. The terminal voltage falls below the open-circuit voltage by current × internal resistance. If the internal resistance of a 12 V battery is 10 mΩ, a loss of about 0.5 V appears at the terminal when 50 A is drawn.
This resistance grows with three things: ageing, a low state of charge and low temperature. That is why a half-empty, cold battery drops noticeably more voltage at the same load than when it is fully charged and warm. Sulphated plates also raise the resistance permanently.
In the field the most visible consequence of this is with motor loads. The KTS200 jacks run on a DC motor and the torque of a DC motor is proportional to the applied voltage. If the battery internal resistance is high, the voltage sags as the movement starts, the jacks slow down, and under heavy load they may not move at all. In complaints of slow jacks, the state of charge of the battery and the behaviour of the terminal voltage under load are the first things measured.
The context in which this term is used: RV automatic levelling guide