Resistors are used to limit voltage and/or current in a circuit and can have a fixed or variable resistance.  Variable resistors (often called potentiometers or rheostats) are used when dynamic control over the voltage/current in a circuit is needed, for example, in a light dimmer or volume control.

A diode allows current to pass easily in one direction and blocks current in the other direction.

Connecting the 6 batteries in series multiplies their voltage while keeping their current the same yielding a 36V 15A configuration. Connecting the 6 batteries in parallel multiplies their current while keeping their voltage the same yieleding a 6V 90A configuration. Using a series-parallel connection, 3 batteries can be connected in series and 3 can be connected in parallel resulting in a 18V 45A configuration.

Current is the rate of flow of electrons per unit time and is measured in amperes (A). A coulomb (C) is the quantity of electricity conveyed in one second by a current of one ampere.

Resistors can be chosen in a wide variety of values to control the voltage in a circuit, limit the current, or moderate the heat produced by the components in the circuit.