Ring 2 batteries take longer and longer to charge

I have a Ring 2 doorbell hardwired but during the winter months when below freezing, the battery do not charge so I bought an extra one to charge inside while the other is being discharged outside running the doorbell. Thing is, this year, although I wait a day to recharge once the battery is inside to start charging so it goes to room temperature, I find it takes longer and longer for the orange light to disappear. Like now, that battery has been charging for several days and as yet to finish charging! Both batteries behave the same. I've switched charging cable as well as adapter and outlet, same result. What could be causing this? The original battery is two years old and the other one is just a year old.