After downloading google maps battery gets discharged quickly why?

The battery is most likely being drained quickly because the phones GPS is locating your precise location. GPS as well as other apps such as email, social networking apps and others can drain your battery quickly. This is due to the apps trying to update or sync information. Some apps you can control how often they try to sync information by getting into the settings menu in that specific app and changing how often they should sync or update. You can also disable GPS when it's not needed which will extend the battery life before needing charged again.