Here is a general overview of how the voltage and current change during the charging process of lithium-ion batteries: Voltage Rise and Current Decrease: When you start charging a lithium-ion battery, the voltage initially rises slowly, and the charging current gradually decreases. This initial phase is characterized by a gentle voltage increase.
Yes, it does get lower. The effect you see is called internal resistance: A practical electrical power source which is a linear electric circuit may <...> be represented as an ideal voltage source in series with an impedance. This impedance is termed the internal resistance of the source. Put simply, a battery is not an ideal voltage source.
When the battery is open you are measuring an open cell voltage. When the battery is in the system it's closed cell voltage under load. You are dropping some voltage across the internal impedance of the battery because your system is drawing current when the measurement is being made (so at the terminals the voltage is indeed lower).
This initial phase is characterized by a gentle voltage increase. Steady Voltage and Declining Current: As the battery charges, it reaches a point where its voltage levels off at approximately 4.2V (for many lithium-ion batteries). At this stage, the battery voltage remains relatively constant, while the charging current continues to decrease.
So, the voltage drop is real — the measured voltage is what your load gets. The more current it draws from the battery, the lower is voltage it gets. When the battery is open you are measuring an open cell voltage. When the battery is in the system it's closed cell voltage under load.
Going below this voltage can damage the battery. Charging Stages: Lithium-ion battery charging involves four stages: trickle charging (low-voltage pre-charging), constant current charging, constant voltage charging, and charging termination. Charging Current: This parameter represents the current delivered to the battery during charging.