Hey Tina, welcome to the forum
Oh man that sucks ! Some dealerships...
You will need to talk to the service manager this time. Tell them you need a new battery under warranty and that they also need to credit you the $98 for a wrong diagnosis. I'm not sure they'll agree to this but you can try. Get them to confirm they won't charge you for diagnostics again before you bring it in.
Chances are you have a defective battery.