The term 'battery units' can be broken down into four crossword solutions: VOLTS (5 letters), tests (5 letters), ANODE (5 letters), and ACID (4 letters). VOLTS measure the electrical potential difference between two points in a circuit, and are a unit of electrical power. Tests measure the capacity of a battery, and are used to determine its performance. ANODE is the positive terminal of a battery, and is where electrons enter the battery. ACID is a type of electrolyte used in batteries, and is responsible for the chemical reaction that produces electricity.