Batteries are the energy source that provides energy to the vehicle for the first movement. Batteries used in motor vehicles nowadays are usually lead-acid batteries. This paper introduces the basic components of lead-acid batteries and describes the concept of a battery capacity test. The general definition of the standard for testing, EN50342, and the required conditions are presented. Furthermore, a cost-effective capacity tester for heavy-duty lead-acid batteries in compliance with the standard was developed and introduced.