Also, if you are NOT using the OEM charger, then its possible you're not receiving the proper charge to the phone. After market chargers work, but some, though advertised compatible, are not!
As well as the charger you should also be careful of the cable too - a cheap cable can deliver in a portion of what a better quality cable would. When I tested cables with the same charger I got between 230mA and 1010mA.
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I had the same problem with running the SatNav on my phone in the car - it would drain quicker than I could charge it. As well as changing the car charger I also upgraded to a better cable and that resolved the problem. I would recommend using the Ampere app to check what your drain and charge amount is.