Sorry this is specific but might apply to any. I have Nikon EN-EL15 batteries, One, no longer how long I put it in the charger for won't charge past about 45% and the other 93%. The Nikon age thing is showing them as 4 year old. Is this usual for batteries to get to a point where they only hold a % of full capacity? can they be reconditioned or best to just let them go?