Electrical engineering already figured out that the voltage level vary with the charging level. It's accounted for when designing a circuit. The voltage specif

Electrical engineering already figured out that the voltage level vary with the charging level. It's accounted for when designing a circuit. The voltage specification you see written on batteries is actually the discharged voltage. For instance a car battery is known as 12V. That's the uncharged voltage, it's 14V at full charge.