Whenever a current flows in a conductor, a potential difference is developed across it. I x R is known as Ohm's Law . The unit of resistance is the OHM . Every source of electromotive force has some resistance within it which limits the amount of current that can be drawn from it. This is called its internal resistance .
Internal resistance increases with age and also as the battery's energy is used up.
The value for a new lead-acid car battery is of the order of 0.02 W . When a battery is being discharged, part of the electrical energy is converted into heat within the internal resistance. This is called the short-circuit current. The maximum power output of a battery is inversely proportional to its internal resistance. The smaller the internal resistance, the large is the maximum available power.