iPhone 13 Battery Health Dropped 3% in Five Days

I have been using an iPhone 13 that I purchased about one and a half years ago through the online marketplace N11, and I have recently experienced a serious issue with the battery. Over the past week, the battery health has dropped very rapidly, decreasing by approximately 3% in just five days, which I believe is abnormal for a device of this age and usage level. When I contacted support about this issue, I was told that this could be considered normal, but based on my experience and the sudden decline in such a short period of time, I do not find this explanation satisfactory and believe there may be a problem with the battery. I request that the battery of my iPhone 13 be thoroughly inspected and that a free battery replacement be provided if any defect or abnormal condition is identified, as I do not believe this rapid loss of battery health falls within normal limits.