What each of the battery icons indicatesUpdated 6 months ago

Note - if you have a Yoto Mini the icons will be the same as the Yoto Player (below)

The player battery is charging 
(displays for 3 seconds as you charge the player; to check if it's still charging, short-press the power button)

Battery fully charged

Battery level ok 

Battery low

Battery very low

Battery empty 

(the player will automatically turn off and won't turn on until charged)

Yoto player (3rd Generation) displays finer granularity battery level icons when charging and discharging.