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: 你可以很容易地从一英寸左右的高度将AA/AAA电池垂直地摔在桌子上，来估计它们的电量。新电池会静止不动地落下，而放了电的电池会像球一样反弹。