A battery can only be charged using direct current (DC). Truck accessories also are powered by DC. In order for the alternator to provide the electricity required, the AC it produces must be converted, or rectified, to DC. While generators accomplish this using a mechanical commutator and brushes, alternators achieve this electronically with diodes. A diode can be thought of as an electrical check valve. It permits current to flow in only one direction—positive or negative. When a diode is placed in a simple AC circuit, one-half of the current is blocked. In other words, the current can flow from X to Y, but it cannot flow from Y to X. When the voltage reverses at the start of the next rotor revolution, the current again can pass from X to Y, but not back. This type of output is not efficient because current would be available only half of the time. Because only 50 percent of the AC voltage produced by the alternator is being converted to DC, this is known as half-wave rectification.