Is this something that you have started to experience very recently, for example after updating your device, or has it been going on for a while?
How long would you say it takes to charge the device to full when using the wall mounted charger that was included together with your device? According to the environmental document, a full charge when the device is powered off takes around 4,5 hours.
There are a few things that you can try and see if there is any change. To start with, try charging the device from a very low percentage to full when it's powered off, so we can see how long that takes.
You can also try and charge your device while it's powered on, but when the device is restarted in safe mode, so we can make sure that no downloaded applications are the cause of this.