If something is on charge, you are putting an amount of electricity into it.

Is it all right to leave/put the battery on charge overnight? 让电池充一夜电可以吗？

She asked me to put her battery on charge for her.

You shouldn't leave appliances on charge unnecessarily.

She went into her bedroom to put her phone on charge.

I used a manual toothbrush because my electric one was on charge.

See also: charge noun (ENERGY)