homeassistant

What kind of battery did you put in ? A standard alkaline battery has different voltage discharge curve compared to the recommended rechargeable NiMH batteries (example)

You can't really measure the capacity of a battery directly, all you can measure externally is the voltage, so the logic has to make some assumptions about the type of battery being used when calculating percentage. Since they recommend the LADDA rechargeable batteries, which are NiMH, they probably set the 100% threshold lower - perhaps as low as 1.2V, so as long as whatever battery you are using is above this voltage, it will show up as 100% - it might take longer for an Alkaline battery compared to NiMH rechargeable, but later you might have the opposite problem, when the reported percentage will start dropping faster for Alkaline vs NiMH, due to the different discharge curve characteristics.

More sophisticated devices can sometimes have a learning function, where they observe the voltage curve over a full discharge cycle of a battery, and then adjust the % reporting to more closely match, but since every battery can be different, this is still not always accurate. The discharge curve can be affected by temperature as well, further complicating the issue.

Since these sensors are supposed to last up to 15 months on single charge, and it actually wouldn't be too unusual for a device like this to report 100% battery for up to the first 1/3 of its battery usable life (especially if using Alkaline batteries), 3 months is probably still within reason. There is often some intentional padding as well by manufacturers, as they don't want customers to see big drop in battery percentage early on, as this might cause people to make ultimately incorrect predictions about total battery life, and complain or return the product.