Ring2 not charging

I installed a ring 2 on an existing electro-mechanical doorbell system with a 16 vac transformer. It worked great for over 5 weeks, charge level staying between 98% and 100%. Now the charge goes down 5% per day requiring me to remove the battery pack to charge it. The battery is not being charged internally. Voltage at the transformer is 17.1 volts. Voltage at the Ring2 rear terminals is 17.1 vac. Lack of a voltage drop at the ring terminals means that there is no charge current flowing into the ring2. The ring2 is not charging even though there is 17.1 vac at the terminals. I think there must be an internal failure of the charging circuit