"Most battery chargers charge at a constant rate. This is often way below the rating of the battery and can take at least 10 hours or more. This is to reduce the possibility of overcharging a battery and is why most batteries are charged overnight. The circuitry of a battery maintainer is able to charge the battery at much higher rate while regulating the current to prevent overcharging. This can shorten charging time to just a few hours."
http://www.differencebetween.net/technology/hardware-technol...