YSK: You could easily estimate the charge of AA/AAA battery cells by dropping them vertically on a table from a height of around an inch. Fresh batteries will drop still, while discharged ones will bounce like a ball.
from /r/YouShouldKnow
YSK: 你可以很容易地從一英寸左右的高度將AA/AAA電池垂直地摔在桌子上，來估計它們的電量。新電池會靜止不動地落下，而放了電的電池會像球一樣反彈。