A battery charger is an electrical device used to provide energy to a rechargeable laptop battery or a secondary cell by forcing an electric current through it, and the protocol depends on the type and size of the battery being charged. Some batteries can be charged by connection to a constant current source or a voltage source, while others cannot withstand extended high-rate over-charging. These chargers come with voltage or temperature-sensing circuits and a microchip controller to adjust the current. Slow chargers may take hours to complete a charge, while fast ones can do the task within an hour.
All of the battery chargers have one thing in common: they function by feeding current via batteries for the time being, expecting the inside cells to hold on to some energy passing through them. Some of the best adapter chargers that you can buy today are Dell Internal SD Dual Module for PowerEdge R720, R620, HP Power Converter for Proliant DL560, and Dell AC Charger for ThinkPad.