Replaced stator but battery went nearly flat

I recently had my stator replaced because there was no spark, and that fixed it.

I charged my battery (it’s a gel battery) with a charger and it showed it was fully charged, I rode to work and when I got on it to go home it was nearly flat and didn’t have enough power to start the engine.

What’s the problem? Even if the new stator is bad, should the battery have gone flat already?