High battery drain after fully charged

I bought a forerunner 965 a few days ago. It came with 79% battery and I haven't charge it for a few days. Today when it reached ~ 50% I charged it to 100% (BTW: it was relatively fast to reach 99%, and then took lot of time to reach 100%) and the battery is draining very fast now. It lost 4% in the 1st hour. It looks like it's getting better with time (6% in 4 hours). On the other hand the built-in battery widget predicts 23 days. Is this something that will gett better after a few times I'll charge it?