For comparison, w/no SIM installed, and using wifi, I streamed an internet radio feed for 10hrs. yesterday, and still had 29% charge left. Try getting another battery. Over time, batteries lose capacity as their crystalline structure breaks down.
FWIW, if it's not too bad off, I've had some moderate success freezing my poorly performing batteries, letting them fully thaw/dry, and having at least partially restoring them to original performance. YMMV.