Linear battery chargers are constructed of a transformer that converts the mains voltage to charge voltage. A simple linear battery charger uses a resistor to limit the current. The battery chargers get so hot that they cannot have a tight housing, which is why they are only suitable for wet lead-acid batteries for indoor use. They need to be monitored to avoid gassing. This is the most common type of battery charger that is frequently sold at petrol stations. Simple and inexpensive - but damages the battery. No control of the charge voltage causes gassing.
Controlled linear battery chargers have a large transformer that converts the mains voltage to charge voltage. The charge voltage is controlled via a regulator in combination with a rectifier. This method offers simple control of the charging process by stopping when the set voltage is reached. Large, heavy and bulky. Fan cooled High current and voltage ripple that creates noise from the battery charger and interference to peripheral equipment. High energy consumption. Sensitive to variations in mains voltage, which impacts directly on the charge voltage. High EMC interference.
The solution to the problems above is the switch mode charger.
The linear transformer is replaced by a rectifier and an electronic switch. A smoothing condenser accumulates the energy as a high direct current (at around 300V). The high frequency transformer (50,000Hz) converts the high voltage to the exact charge voltage. The transformer is more efficient and produces less heat. An electronic switch controlled by a regulator provides exactly the right current and voltage to the battery. The switch mode charger can handle fluctuations in the mains voltage, providing the exact charge voltage giving a fully charged battery with the longest life for a battery. The ripple is negligible,it is small and effective and is cooled by natural convection. A switch mode charger can be made water resistant and dust proof (IP65).
