You should look at the exact verbage of the warranty, but I'm pretty sure your battery capacity needs to be less than 70% of your rated value to qualify for replacement.
Mary Barra, CEO of GM, is fond of saying that GM has *never* done a warranty replacement of a Volt battery due to degradation.
By all means you should take it in for a battery health check before your warranty is up, but the odds of them giving you a new battery are slim to none.