iphone 3g battery drains VERY fast with DATA ON.

Hello, im new here, i hope im posting in right section. I have a problem with my old iPhone 3G, i know its a very old unsupported phone, but i still use it. I have a strange problem now, the batter drains VERY fast when i have toggled my 3g and EDGE, its like from 100% to 0% in 40min. When im playing even some better games with airplane mode (or Edge, 3g off), like Plants vs Zombies or Wheres my water it does go normally.. I dont know where the problem could be but i would really like to fix this..