Basically the camera is just guessing at the % left, they have no real way of knowing!
I would guess that Nikon camera's have a way of counting the discharge/recharge cycles. What that is I don't know. I've never seen it published. My friend's vintage 2008 D300 still shows 100% battery life. She only has one battery. Bad guess? I think so.
My 300s (2 batteries) and D800 (3 batteries), all batteries show 100%. I don't believe it. This may be because I try to recharge at 40-60% charge remaining.
Dave Hartman
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General practice I think is when a battery can store only about 50% of what it did when new it's time to replace it.
Paid work, better be more careful than above and carry at least a couple of spares or at least 2x what one thinks they will need. Is this careful enough?
Dave