I noticed this problem long time ago and I just remember about this battery problem in the game.
At least some of ONI game copy owners is real electrician or know a little or more about battery.
It is very strange to see how fully charged battery behave when connected to an empty newly built secondary battery.
The only type of electricity used in real life power banks and industrial grade batteries is DC voltage and not AC voltage.
DC voltage is used to charge battery while AC voltage is for industrial / household appliances converted via Transformer (in short Trafo).
General rule of Direct Current is higher voltage always flow to lower voltage.
Unlike in this video footage,
or briefly in the picture below, which a fully charged battery will not fill an empty battery connected in a circuit.
Which actually in real life it will; if anybody wants to build one.
By looking at the red & black alligator clip above the battery I can tell it uses DC voltage to operate because of its design & color.
battery A has little or FULL juice in it,
battery B is empty,
both connected with wires,
electricity must flow from battery A to battery B
if both batteries Joule is equal,
then stop electricity flow
Voila~ Now understanding made simple!
Steps to Reproduce
This issue has not been confirmed by a developer yet.