iPhone battery drops by 20% suddenly while not using.

Hello to whom it may concern,

I've noticed a weird behavior in my battery recently when my battery reaches to 50-60 % range it suddenly drops by 20% but my battery drain routine is normal and my battery health is at 100%.

P.S: I use 18-watt fast adaptor to charge my phone, I have a refurbished iPhone 1.

P.S2: I disabled location services, background app refresh, and all the possible thing that can drain your battery note that my battery doesn't drain fast it drops by 20% suddenly