If I remember correctly, I think the gaurdians blocked him from accessing his ring's power. Not that it mattered since he killed a bunch of corps members and took their rings instead but they all did run out of charge eventually since he destroyed the main power battery on Oa.

Emerald Twilight, damn that’s a great name, I wish I thought of that.

Hal's ring ran out of power after he made a green construct replica of Coast City. When a Guardian showed up in a hologram, Hal stole the energy from that hologram and headed straight to Oa. On his way, he stole other Green Lanterns' rings. He ditched those rings in the end to have a fair fight with Sinestro.