Battery estimates are based on past usage patterns, they change constantly as you use your phone. They are like snowflakes in that each user is going to generate their own unique pattern. If you change your usage pattern for an extended period the estimate changes. Then it takes a while for new patterns to affect the estimate.
That's why it is an estimated time. Your actual use may vary.
Example: My 1520 gives me tremendous battery life day to day. I recently was on an extended road trip. I used the snot out of my phone taking pics all day, internet, etc. I also used Nav a LOT. Frequently I was plugging the phone in to charge while I drove. I skewed the heck out of my charge and drain cycle for 30+ days. Now my estimates are off. It will take some time to get back to normal.
That is an extreme example.
Having to charge a 520 each evening does not sound so bad unless you are a very light user. Not experiencing any abrupt shutdowns due to low battery? Then there really is no issue.
You could but a new battery off eBay or Amazon. They are plentiful and not too badly priced.
FWIW I like this Battery App and the info it gives:
https://forums.windowscentral.com/e...Fapps%2Fbattery%2F9wzdncrfjc2z&token=8xpTThZT
The charge and discharge rates, along with the usage history graphs are very helpful.