Electric cars have a motor just like conventional, internal combustion engine cars. The difference is that the power supply is derived from battery-stored electricity rather than the mechanical power derived from burning gasoline. The batteries used in electric cars vary in design, and include the lead-acid type familiar to all conventional car owners, lithium ion, similar to those used in laptops and mobile phones, but once again much larger, molten salt, zinc-air, and various nickel-based designs. In an electric vehicle the traditional gasoline or diesel engine and fuel tank is replaced with an electric motor, a battery pack and controllers. The vehicle also has a controller that powers the electric motor that uses rechargeable batteries as its energy source. The motor itself can be either AC or DC. The main advantage to electric vehicles is that the motor and battery configuration allows the vehicle to run more fuel-efficiently. DC motor installations tend to be easier and less expensive to build. DC motors also have an overdrive feature which means that for a short period of time the motor will accept more energy and deliver more horsepower as a result. This feature is useful in a vehicle because it can help during acceleration. The motor, however, cannot be run in overdrive too often because the motor will overheat and could malfunction. AC motor installations are more expensive than DC installations. They usually use a three-phase AC motor that allows regenerative braking. This means that during braking the motor acts in reverse as a generator and delivers power back to the batteries. 15% of the energy used for acceleration can be recovered using regenerative braking. This amount is not enough to fully recharge the battery pack, but it will extend the range of the vehicle. In a DC electric car, the controller delivers the power from the batteries to the engine in a controlled way. The controller pulses the power to the engine usually at a frequency of 15,000 times per second. The frequency is outside of normal human hearing which is why the controller and motor is silent. In an AC electric car, the controller needs to create three pseudo-sine waves. The controller takes the DC voltage from the batteries and pulses it to the motor