Not necessarily Rojash, Dell laptops are funny things. Mine worked fine when plugged in but failed to charge the battery. As soon as you unplugged, it turned off. I replaced the power supply and hey presto, everything was fine again and the battery charged. Apparently there is a data-feed as well as power coming in from the power supply and the laptop will only charge the battery if it gets a signal saying "I'm a genuine Dell power supply". It's this data-feed going wrong that stops batteries charging even though it works fine on AC power.