I think you meant the unused balance.
"Your unused balance will remain active and carries over into the next month so long as you continue to refill your account with a minimum of $10* a month every 30 days, before your next anniversary date. If you don’t refill your account before your anniversary date, any remaining balance in your account will expire and be lost. You’ll no longer be able to use your phone except for emergency 911 calls. However, you can restore your expired balance if you refill within 7 days after your anniversary date, as we understand life can get busy."
Taken from here.