Apple

I bought a 7 series 45mm from a friend of mine, the battery health was %100, it went down to %88 in a matter of 2 months, is this normal? I don't really wear the watch a lot nor have features/complications/work outs on I just have a normal nike digital watch and check my notifications on it, and I don't usually stress dead batteries but I hear the battery can't be changed in those right? It's still under the warranty tho